The Trout and Salmon Encyclopedia

Abundance

The abundance of salmonids like salmon, trout, and char, is a critical metric for evaluating their population health and sustainability. The National Oceanic and Atmospheric Administration (NOAA) extends this concept beyond mere numbers, examining attributes like productivity, life history, genetic diversity, and spatial structure of salmonid populations. NOAA’s approach incorporates factors affecting fish health, such as habitat quality, hydropower, hatchery, and harvest impacts.

Adult abundance in salmonids is measured by the number of adult fish returning to spawn. A critical distinction is made between hatchery-origin and natural-origin fish. Typically, wild spawners are the primary focus for recovery goals. 

Juvenile abundance is another key indicator that reflects the health and productivity of river ecosystems. This measure varies with habitat quality, indicating that superior habitat conditions could influence the number of juveniles in a river.

Overall, salmon and trout abundance encompasses a broad spectrum of factors, from fish counts to comprehensive ecosystem assessments. 

Adipose Fin

The adipose fin is a small, fleshy fin located between the dorsal and caudal fins. It’s only found on a few types of fish, like salmonids and catfish. Initially, it was deemed vestigial (non-functional) because it was thought to contain fat tissue. This has since proven to be false. Newer studies suggest it aids in sensing movement and sound, enhancing maneuverability in turbulent waters. This discovery raises concerns about the common practice of clipping this fin in hatchery salmon to differentiate them from their wild counterparts.

Adfluvial

Adfluvial salmonids, often termed lacustrine, have a distinct life history where adults inhabit lakes and migrate to tributary streams for spawning. After hatching, they rear in these streams before migrating to a lake, where they achieve maturity. 

This pattern is observed in rainbow, cutthroat, bull trout, and many other species of char and trout. Landlocked Atlantic salmon also exhibit an adfluvial life history. 

Adfluvial salmonids uniquely exploit the abundant resources of lake environments for growth and development while relying on the tributary streams for reproduction.

Adriatic Trout

The Adriatic trout (Salmo obtusirostris) is a salmonid native to the Western Balkans in southeastern Europe. Common names include softmouth trout and Adriatic salmon. This rare trout spawns in early spring and is a popular game fish.

The Adriatic trout has an elongated snout, a small and fleshy mouth, relatively large scales, and a deep body. Its body color typically varies but typically displays a green hue with red and black dots.

The species is currently threatened by excessive damming, hybridization with introduced species, and overfishing. Despite overlapping spawning times with brown trout, natural hybridization is not widespread due to their different spawning times.

Alevin

Alevin are salmon and trout that have emerged from eggs and have a yolk sac still attached. This yolk sac is rich in proteins, sugars, minerals, and vitamins and sustains them for about 2-3 weeks until fully absorbed. Once depleted, alevin venture from their gravel nests to forage, marking their progression to the fry stage. 

Alluvial Rivers

Alluvial rivers consist of mobile sediment and soil. Their channels are dynamically shaped by flood-driven sediment transport and ecological interactions. These rivers exhibit diverse morphologies, including straight, meandering, braided, or anastomosing patterns, adapting to environmental changes. Their varied channels and floodplains play crucial roles in supporting distinct ecosystems and geomorphic features.

Amago Salmon

The amago, or red-spotted masu salmon (Oncorhynchus masou macrostomus), is a subspecies of Asia’s masu salmon. It’s unique for its red or vermilion spots alongside black ones. It inhabits the Pacific side of Honshu, Shikoku, and the Inland Sea side of Kyushu. The amago salmon reaches up to 20 inches (51cm) in length. It has both anadromous and stream-dwelling populations and is a focus of aquaculture.

Anadromy

Anadromy is a life history pattern characterized by spawning in freshwater and maturing in the ocean. Most salmon, trout, and char species have anadromous life history variants. In the case of Pacific salmon, all but the masu and sockeye are strictly anadromous.

These fish are born in freshwater, where they initially develop. Once mature enough, they migrate to the ocean, undergoing physiological changes to adapt to saltwater. Anadromous salmonids spend a significant portion of their life in the ocean. They gain size and strength due to abundant marine food sources. As mature adults, they return to their freshwater birthplaces to spawn, completing their lifecycle. 

This pattern provides them with the advantages of rich marine feeding grounds and protected freshwater spawning sites.

Apache Trout

The Apache trout (Oncorhynchus apache) is native to Arizona’s White Mountains, where it inhabits clear and cool headwater streams. It’s a close relative of the Gila trout and some classify it as a subspecies (Oncorhynchus gilae apache).

This rare trout has a yellowish-gold color and distinctive dark spots. The maximum size of Apache trout is around 24 inches (61 cm) and up to 6 pounds (2.7 kg). 

Apache trout are spring spawners, typically from March to June. Like most trout, they are opportunistic feeders with a diet of terrestrial and aquatic insects in streams. In lakes, they also feed on small fish and zooplankton. Conservation efforts are crucial for this critically endangered species, threatened by hybridization and climate change.

Aquatic Insects

Aquatic insects, particularly mayflies, stoneflies, caddisflies, and true flies form the core diet of salmonids in streams. They are also important sources of food in lakes and ponds. Aquatic insects undergo multiple life stages, from larvae or nymphs in water to adults emerging into the air. Other significant aquatic insects in the diet of salmonids in streams include salmonflies, water boatmen, damselflies, and dragonflies.

Amphipods

Amphipods are malacostracan crustaceans. They are characterized by their laterally compressed bodies and the absence of a carapace. Amphipods are predominantly detritivores or scavengers. They inhabit various aquatic environments, from marine to freshwater, and are crucial prey for both juvenile and adult salmonids. Common names for freshwater amphipods include freshwater shrimp, scuds, and sideswimmers.

Anal Fin

The anal fin is on the ventral side near the anus. It stabilizes salmonids alongside the dorsal fin, preventing rolling in water. Its shape and ray count can also serve as a diagnostic feature for identifying salmonid species.

Arctic Char

Arctic char (Salvelinus alpinus) are native to alpine lakes and Arctic/subarctic coastal waters. These adaptable char have varied habitats, spawning in freshwater with lacustrine, resident, adfluvial, and anadromous life histories. 

Arctic char are distinguished by their elongated snout, large scales, deep bodies, and their brilliant color. This color is often green with red and black dots to dark dorsal sides with a red, yellow, or white ventral area. 

The size of Arctic char varies significantly, with dwarfs averaging 3 inches (8 cm) and larger specimens maxing out around 18 inches (46 cm). Exceptional fish may reach up to 20 pounds (9 kg). 

This char species displays notable morphological variations, with different morphs existing sympatrically within habitats, demonstrating diverse feeding behaviors and ecological niches. 

Arctic char are a significant subject of aquaculture, as well. They’re valued for their unique flavor and texture and are the target of commercial and subsistence fisheries in circumpolar regions.

Atka Mackerel

Atka mackerel (Pleurogrammus monopterygius) inhabit North Pacific and Bering Sea shelf regions from Asia to North America. These colorful fish are crucial prey for various marine species, including most Pacific salmon, steelhead, and the endangered Steller sea lion.

Atlantic Salmon

Atlantic salmon (Salmo salar) are found in the northern Atlantic Ocean and rivers flowing into it. Most populations are anadromous, but some are landlocked. Unlike most species of Pacific salmon, Atlantic salmon can survive spawning (iteroparous).

With high culinary and recreational value, Atlantic salmon are a significant target of fishing and conservation efforts. Their size varies, growing up to a meter in length. Distinctive for its color changes during spawning, the species is characterized by blue and red spots in youth, transitioning to a silver-blue sheen as adults. During reproduction, males may display green or red hues. 

Atlantic salmon face challenges from overfishing, habitat destruction, poor ocean conditions, and competition with farmed salmon. 

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Baggot

A baggot is a female Atlantic salmon that either failed to spawn or only partially completed the spawning process. They often arrive too late to the spawning grounds. Baggots are distinguishable by their soft underbellies, darker heads, and enlarged anal fins. 

Bald Eagles 

Bald eagles (Haliaeetus leucocephalus), as predators and scavengers, play a crucial role in the ecology of salmonids. They prey on live salmon and trout in freshwater and estuaries. Bald eagles also scavenge salmon carcasses, which help distribute salmon nutrients into riparian zones, enhancing ecosystem fertility. Research indicates eagles’ distribution varies with salmon carcass availability.

Beardslee Trout

The Beardslee trout is a local form of coastal rainbow trout unique to Lake Crescent, Washington. This rare fish spawns exclusively in the Lyre River. Known as bluebacks, they’re distinguished from Lake Crescent cutthroat trout mainly by their spawning location. 

Facing threats from siltation, hybridization, and habitat degradation, their critically low numbers prompted catch and release regulations for protection.

Bears

Brown bears (Ursus arctos) are key predators of salmonids in freshwater. They typically target spawning salmon or schools on their spawning run. Brown bears also feed on cutthroat trout during their spawning migrations. Bears play a vital ecological role by scavenging carcasses and distributing them into riparian zones, enriching terrestrial and aquatic ecosystems. This interaction underscores the intricate connections within food webs where salmonids are present.

Brown bears aren’t the only species to prey on salmonids. Polar bears (Ursus maritimus) have been observed on occasion feeding on arctic char. American black bears (Ursus americanus) are also known to target spawning salmon where possible. 

Beaver Dam Analog – BDA

Beaver Dam Analogs (BDAs) are manmade structures that emulate natural beaver dams to restore stream habitats. They’re constructed using onsite materials like willow and aspen, mirroring materials used by North American beavers. 

By backing up water in stream channels, BDAs slow water flow and raise water levels. This process creates fish habitats, supports wetland vegetation, increases stream complexity, and traps sediment. 

Building a BDA involves selecting a site with abundant natural resources and favorable conditions. Installation requires layering large woody debris, brushy vegetation, and mud in the stream channel, perpendicular to water flow. The structure is reinforced with wooden posts. BDAs help form backwaters, benefiting species like coho salmon and cutthroat trout, enhancing biodiversity, and mitigating stream channel incision. 

This technique, alongside post-assisted log structures (PALS), offers a cost-effective, low-tech approach to stream restoration, particularly effective in areas like Great Basin National Park.

Beavers

Beavers play a complex role in the ecosystems of cool-water fish like salmon and trout. Both the North American beaver (Castor canadensis) and Euroasian beaver (Castor fiber) evolved alongside salmonids. While their dams can sometimes challenge migrations of some species, the habitat and hydrology created by dams offer numerous benefits to salmon and trout. Beaver ponds are especially important as habitat for juvenile salmonids. They also increase flow during drier weather and cool the temperature of streams. 

Benthic Zone

The benthic zone is the lowest level of a water body, encompassing the sediment surface and some sub-surface layers. Inhabited by organisms known as benthos, including microorganisms and larger invertebrates, it varies greatly in depth, light penetration, and pressure. 

In the ocean, the benthic zone spans from the shoreline down the continental shelf to deep ocean floors, including abyssal plains and hadal zones. This zone plays a crucial role in nutrient flux, with marine snow often being the primary food source. Benthic habitats are diverse, ranging from shallow continental shelves to the deep, pressurized areas of the ocean. 

The streambed is the benthic zone in rivers and streams. 

Benthic macroinvertebrates serve essential ecological functions, influencing aquatic ecosystems’ nutrient and energy flows. 

Biwa Salmon

Biwa trout are a subspecies of masu salmon (Oncorhynchus masou rhodurus), endemic to Japan’s Lake Biwa. They have a varied diet, which includes zooplankton, aquatic insects, crustaceans, and small fish. 

Blueback Salmon

Blueback is a common name for sockeye salmon of the Columbia River drainage of Washington, Oregon, and Idaho. 

Brook Trout

Brook trout (Salvelinus fontinalis) are a species of char native to Eastern North America. Many populations in their native range have declined or disappeared. However, they’ve also been widely introduced across Western North America and the rest of the globe, where they often thrive. 

Brook trout are sometimes called speckled trout or brookies. They exhibit a variety of life histories, including the revered coaster and salter (anadromous) forms. 

This species is recognized for its distinct marbled pattern and red spots. Brook trout hybridize with a number of species like lake trout (splake), bull trout, and brown trout (tiger trout). 

Brown Trout

Brown trout (Salmo trutta) are endemic to Europe, West Asia, and North Africa. This species has been extensively introduced worldwide. Brown trout have remarkable adaptability, evolving into various ecotypes such as riverine, lacustrine, and anadromous (sea trout) forms.

Brown trout display a range of colors from silver to a brassy brown, adorned with dark and red spots. Sizes vary significantly, with some individuals reaching up to 44 pounds (20 kg). Most fish are smaller, especially in smaller streams.

As a game fish, brown trout are highly valued worldwide. However, their popularity and widespread introduction have led to challenges, including habitat degradation and competition with native species. 

Bull Trout

Bull trout (Salvelinus confluentus) are apex predators native to cold, clean waters of western North America. Misidentified for years as Dolly Varden, they’ve been recognized as a distinct species only relatively recently. These fish exhibit diverse life history forms, including anadromy, and are critically dependent on pristine habitats. 

Unfortunately, bull trout have been eliminated from much of their former range. Threats include habitat degradation, climate change, and competition and hybridization from introduced species. Currently listed as Threatened in the US, bull trout have been extirpated from many watersheds. 

Learn more about bull trout!

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Caddis Flies

Caddisflies (Trichoptera) are a crucial prey for many salmonids. Their larvae often construct protective cases from silk and environmental debris. These aquatic insects, closely related to moths and butterflies, display varied feeding strategies, from predation to detritus feeding.

Carcasses 

Salmon carcasses, enriched with ocean-derived nutrients, are vital to freshwater ecosystems. Carcasses of adult salmon significantly boost productivity in nutrient-poor lakes and streams. Their biomass supports a wide range of organisms, including bears, birds, salmonids, insects, and bacteria, extending benefits to trees and other plants. Reduced adult salmon returns, notably from commercial fishing, diminish ecosystem productivity and biodiversity.

Carrying Capacity

Carrying capacity is the maximum population an environment can sustain, considering resources like food and habitat. 

In freshwater, salmonid carrying capacity fluctuates due to natural variation and is influenced by factors like spawning success and juvenile residence time. Streams and lakes vary in biomass support, not always correlating to surface area, highlighting complex dynamics. 

The ocean carrying capacity for salmonids is also crucial to understand. It’s impacted by hatchery production and ocean-ranching, affecting food resource competition, size, and survival of wild and hatchery salmon. This intricate balance underscores the need for cautious management within natural limits.

Caudal Fin

The caudal fin, or tail fin, is crucial for propulsion and steering. Female salmonids also use their caudal fin to dig redds to deposit their eggs. 

Caudal Peduncle

The caudal peduncle is the tapered area connecting the body to the caudal fin. The muscle in the peduncle powers the tail (caudal) fin. The caudal peduncle’s depth varies among species, with narrow peduncles in fast swimmers like tunas and broader ones in short-burst ambush predators.

Char

Char is the common name for fish of the genus Salvelinus. They are salmonids found in cold, fresh waters of the northern hemisphere. 

Char are distinguishable by light spots over dark bodies and small scales. Popular as gamefish, char species like the lake trout and arctic char are also commercially fished. Other well-known char species include bull trout, Dolly Varden, brook trout, and white-spotted char. 

The genus Salvelinus has a complex taxonomy and includes subgenera Baione, Cristovomer, and Salvelinus sensu stricto, covering a diverse array of species. 

Cherry Salmon

Cherry salmon is a common name for masu salmon, particularly anadromous variants. This is because they return to freshwater in the spring when cherry blossoms bloom. 

Chinook Salmon

Chinook salmon (Oncorhynchus tshawytscha), also known as king salmon, are the largest Pacific salmon species.  

Chinook have a semelparous life cycle: they’re born in streams, migrate to the ocean, and return to spawn once, dying post-reproduction. They are the least abundant Pacific salmon. The native range of king salmon spans from Alaska to California and across the Pacific to Asia, with notable populations in Kamchatka. They’ve been introduced successfully around the world, including the Great Lakes, Argentina, Chile, and New Zealand. 

Chinook salmon transition from olive-brown juveniles with parr marks to silvery ocean dwellers. As they return to freshwater to spawn, adults turn darker reds and browns. 

Adult chinook salmon average 10 to 50 pounds (4.5 to 22.7 kg), and the largest ever caught weighed 126 pounds (57 kg)! Siberian taimen are the only salmonid that potentially grow larger.

Learn more about chinook salmon!

Circuli

Circuli are concentric growth rings on fish scales, similar to tree rings. They are used to determine age and growth rates in salmonids. They form as fish grow, with wider spacing indicating faster growth. Analyzing circuli patterns provides insights into a fish’s life history and environmental conditions encountered​​.

Chum Salmon

Chum salmon (Oncorhynchus keta) inhabit North Pacific and Arctic Ocean areas from Asia to North America. Their common names include keta salmon, dog salmon, and calico salmon. Like most Pacific salmon, chums are semelperous. 

Chum grow large, averaging 6.5 to 11 pounds (3 to 5 kg), and grow to 35 pounds (15.9 kg) or more. 

These salmon migrate to the ocean as soon as they’re born and have a silvery appearance. Upon returning to freshwater to spawn, they transition to olive green with purple bars. Male chums have deep bodies and often exaggeratedly large jaws. Chum salmon typically spawn in the lower reaches of rivers, but some populations make notably long upstream migrations of over 1200 miles (1930 km). 

Learn more about chum salmon!

Coastal Cutthroat Trout

Coastal cutthroat trout (Oncorhynchus clarkii clarkii) are a subspecies of cutthroat trout that range from Alaska to Northern California. Common names include speckled trout for resident forms and harvest trout, SRC, salter, and blueback trout for anadromous forms. They display various life forms like resident, lacustrine, and sea-run (SRC). Coastal cutthroat are also iteroparous, breeding multiple times. 

Recognizable by a red slash under their jaws and heavy spotting, they vary in color and size. Juvenile coastal cutthroat resemble rainbow trout with numerous parr marks and an olive-yellow color. As they mature, they often become lighter and more silvery in appearance. Stream residents are typically small, while sea-run cutthroat and fluvial forms can reach 5 pounds (2.3 kg). Lake residents reach the largest sizes of up to 17 pounds (7.7 kg). 

Coastal cutthroat inhabit diverse habitats, feeding on insects and small fish. They are relatively secure across most of their range. 

Coastal Rainbow Trout

Coastal rainbow trout (Oncorhynchus mykiss irideus) are a subspecies of rainbow trout that range from Alaska’s Kuskokwim River to Baja California. Found up to elevations of 8,000 feet on the Sierra Nevada’s west slope, these trout exhibit resident, fluvial, adfluvial, and anadromous (steelhead) life forms. They are also iteroparous. 

Coastal rainbow trout in streams typically measure 6-8 inches (15.2-20.3 cm) and 12-16 inches (30.5-40.6 cm) in larger rivers and lakes. The steelhead variant can reach up to 43 inches (109 cm) and 42 pounds (19 kg). Their diet varies based on habitat and includes aquatic insects, invertebrates, salmon eggs, fish, and squid.

Anadromous coastal rainbows face challenges from dams, land use changes, and a warming climate. Efforts to conserve steelhead focus on hatchery propagation and habitat restoration but are often hindered by dams blocking access to historic spawning grounds. 

Coaster Brook Trout

Coaster brook trout (Salvelinus fontinalis) are a lake-run (adfluvial) form of brook trout native to Lake Superior and its tributaries. Typical of char, they are iteroparous and capable of multiple spawning runs. 

Coasters typically measure less than 12 inches (30.5 cm) but can grow to over 24 inches (61 cm). They have dark bodies with light spots and colors ranging from bronze to olive. They’re often bright when in the lake. And while spawning, they may have vivid orange bellies and spots. 

Historically abundant, their numbers have declined due to overfishing, habitat loss, and competition from introduced salmonids. Efforts to restore their populations focus on habitat enhancement and protective regulations, with a minimum catch size to ensure sustainability. 

Coho Salmon

Coho salmon (Oncorhynchus kisutch), or silver salmon, are native to the North Pacific. They’re anadromous, migrating from ocean to freshwater to spawn. Like most Pacific salmon, they’re also semelparous. Coho range from Alaska to California and parts of Asia.

Adult coho average 28 inches (71 cm) and weigh 7 to 11 pounds (3.2 to 5.0 kg), potentially reaching up to 36 pounds (16.3 kg). In freshwater, juveniles show vibrant orange fins and distinct markings, while adults in the ocean phase display bright silver sides and limited spots.

Coho salmon rely on diverse habitats, from freshwater streams for spawning to the ocean for adulthood. Their diet shifts from aquatic insects in freshwater to fish in the ocean. Threats to coho include habitat loss, pollution, and overfishing.

Learn more about coho salmon!

Columbia River Redband Trout

Columbia River redband trout (Oncorhynchus mykiss gairdneri) are a subspecies of rainbow trout native to the Columbia River Basin of Idaho, Oregon, Washington, and Montana. They are also present in the upper Fraser River in Canada. They exhibit various life history forms like resident, fluvial, adfluvial, and anadromous (steelhead). Like other rainbow trout, Columbia redbands are iteroparous.

Adults typically measure 18 to 24 inches (45 to 61 cm) and weigh 2 to 5 pounds (0.9 to 2.3 kg), with some reaching up to 20 pounds (9.1 kg). Columbia River redbands have a reddish hue along their lateral line and larger spots than coastal varieties.

Columbia River redbands prefer cold, clear streams with complex habitats. They feed on insects, small fish, and crustaceans. Threats include habitat degradation, including dams, hybridization, and competition from non-native species.

Copepods

Copepods are a group of small crustaceans found in most freshwater and saltwater habitats. Cold-water copepod species play a crucial role in the marine food chain. These copepods hibernate in winter, storing fats essential for the food web. Predatory fish like salmon benefit from feeding on these nutrient-dense copepods, enhancing their survival rates. 

Not all copepods are beneficial to salmonids. Sea lice parasitize salmon, reducing fitness, spreading disease, and occasionally killing them. 

Cormorants

Cormorants are an aquatic bird and skilled predators feeding primarily on fish, including salmonids. In North America and Europe, their predation on salmon, trout, and other marine species has prompted drastic human interventions. Strategies included scaring, egg oiling, and adult shooting to reduce the colony size. Notably, explosives were used to destroy a colony of double-crested cormorants on the Columbia River to protect endangered salmon populations. 

Despite these actions, the impact of cormorant predation on salmon recovery remains debated. Some studies suggest cormorants may preferentially consume unhealthy salmon and have less impact on overall salmon populations than previously thought.

Courtship

In salmon and trout spawning, males fiercely compete for dominance and access to females. Courtship includes vivid displays where males quiver and perform crossover movements over the female’s back. Males also occasionally dig as a courtship display. Males intensify these behaviors as spawning approaches. Dominant males may fertilize several nests, while subordinate males, or “satellite males,” adopt sneaking strategies to participate in fertilization.

Crescenti Cutthroat Trout 

Lake Crescent cutthroat trout are a unique form of the coastal cutthroat that resides solely in Washington’s Lake Crescent. They were historically isolated by a landslide that formed the lake. This isolation led to their divergence from other coastal cutthroat and also led to the localized divergence of coastal rainbow trout into Beardslee trout. 

Crescenti cutthroat have highest gill raker and vertebrae counts among coastal cutthroats. Despite hybridization challenges, a genetically pure population thrives in the Lyre River.

Crustaceans

Most salmonids are opportunistic feeders. Their diets often include a variety of crustaceans. In freshwater, this may include crayfish, shrimp, daphnia, and scuds. They continue to feed on shrimp in the ocean, along with krill, various crustacean larvae, and copepods. 

Sea lice are a parasitic crustacean that significantly impacts anadromous salmonids, posing a challenge to some populations.

Culverts

Road Crossings in fish-bearing watersheds, particularly culverts, are significant obstacles to salmonid migration and habitat access. Culverts are often poorly designed or maintained and hinder the movement of adult and juvenile salmon and trout. 

Issues with culverts affecting fish passage include excessive water velocity, hydraulic jumps, vertical barriers, and insufficient hydraulic roughness. These factors are influenced by engineering decisions like culvert diameter, slope, and additional features like baffles and weirs. 

Solutions focus on making culverts transparent to watershed processes, thereby maintaining natural stream functions. This involves ensuring fish passage at all life stages, appropriate hydrology, sediment transport, large woody debris handling, habitat connectivity, tidal influence, and floodplain processes. The goal is to preserve ecological functions like habitat connectivity, sediment movement, and natural flow conditions. Careful design and management can mitigate these impacts, maintaining the ecological balance of watersheds.

Cutthroat Trout

Cutthroat trout (Oncorhynchus clarkii) are a Pacific trout that inhabits western North America. They have several life history forms, including resident, fluvial, adfluvial, and anadromous. Like other species of trout, they are iteroparous. 

Cutthroat trout vary in color from yellow to olive, with rosy red gill plates and possibly red or crimson bellies. They feature a distinct red or orange slash under their mandibles and numerous small dark spots above the lateral line.

Small streams cutthroat rarely exceed 12 inches (30.5 cm) in length. Fluvial and sea-run cutthroat grow larger, with a maximum of around 6 pounds (2.7 kg). Lake dwellers can grow up to 30 inches (76.2 cm) or more, with Lahontan cutthroat reaching 41 pounds (19 kg).

Cutthroat trout utilize diverse habitats, from rivers to lakes, and their diet varies from insects to fish. Major threats include habitat loss, hybridization with introduced species, and climate change.

Learn more about cutthroat trout!

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Dams

Dams significantly impact salmon, trout, and other migratory fish species, disrupting their natural life cycles and habitats. Those that completely block passage prevent these fish from reaching critical spawning and rearing areas. For example, over 40% of the historic habitat in the Columbia River Basin is now inaccessible due to dams. 

The creation of reservoirs further alters habitats by inundating spawning grounds and slowing water flow. This slower flow leads to increased water temperatures, which can be lethal to salmon and trout. Dams also change river food webs and sediment transport, affecting downstream ecosystems. Spawning gravel and large woody debris cannot pass most dams, which significantly reduces the quality of the habitat below. 

Water level fluctuations in reservoirs have historically stranded salmon nests and juveniles. Although many agreements have been made to regulate dam outflows for fish protection, these changes come after considerable impact and population declines. 

Early dams like splash dams and tributary dams began impacting salmon populations well before the era of large hydroelectric projects. These smaller dams devastated many spawning streams, many of which haven’t recovered. 

Dams not only obstruct adult fish migration but also hinder the downstream passage of juveniles. Without bypass systems or spillways, juveniles must navigate through turbines, often resulting in high mortality rates. 

Fish ladders have been installed at some dams but often don’t fully mitigate the impacts on fish populations. Many only provide partial passage. And many others were installed retroactively, well after populations crashed. 

Most salmon and migratory trout populations have faced significant declines, with dams being a key contributing factor. 

Danube Salmon

See huchen

Daphnia

Daphnia are small planktonic crustaceans and are critical prey for juvenile salmonids in some freshwater environments. This is especially true in lakes, where juvenile sockeye salmon, kokanee, rainbow trout, and cutthroat trout often exhibit strong preferences for Daphnia. 

Diurnal Rhythms

Salmonids exhibit diel shifts (day-night) influenced by season, temperature, and habitat. In freshwater during the summer, salmonids are active day and night. During the winter, nocturnal activity is much more common, especially in colder interior environments. 

During spawning migrations, salmonids often favor nocturnal migration in clear or small streams. In the ocean, anadromous salmonids typically prefer deeper waters during the daytime, particularly in winter.

Dog Salmon

See chum salmon

Dolly Varden

Dolly Varden (Salvelinus malma) are a species of char inhabiting cold-water tributaries of the Pacific Ocean in Asia and North America. They’re adaptable fish with resident, fluvial, and semi-anadromous life history forms. Similar to other char, they’re also iteroparous. 

These colorful fish got their name from the Charles Dickens character of the same name known for her colorful dress. Interestingly, the specimens responsible for the common name were likely the closely related bull trout. Dolly Varden have olive green or gray backs with pale yellow or pinkish-yellow spots and small red spots on the lower sides. Spawning males have a vibrant turquoise color with orange bellies and spots. 

Dolly Varden have a wide range of sizes depending on their habitat and life history form. In small streams, they rarely exceed a few inches in length. Most Dolly Varden average from 12 to 18 inches (30.5-45.7 cm), with a record of 19.25 pounds (8.7 kg). 

Found in coastal waters, lakes, and rivers, Dolly Varden feeds on insects, fish, and salmon eggs. They face threats from habitat loss and overfishing. 

Dorsal Fin

The dorsal fin is situated atop a fish. It has two primary functions in salmonids. One is to stabilize the fish, maintaining an upright position in the water. The other is to help direct the fish during turns and stops.

Drift

Besides eggs, drifting aquatic insects are the primary food source in streams for most juvenile salmonids and some adults. It is the downstream journey of larval stages of aquatic insects. 

This phenomenon manifests in three distinct forms: constant drift, which persists at minimal rates; catastrophic drift, when benthic insects are forcefully dislodging into the water column; and behavioral drift, paramount for salmonids’ feeding strategies. Behavioral drift is undertaken to seek food, evade predators, or due to overcrowding. It predominantly occurs at night to minimize predation risks. Salmonids most actively feed at dusk and dawn so they can see the drifting insects. 

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Eelgrass

Eelgrass (Zostera marina) is vital for juvenile anadromous salmonids, offering critical foraging grounds during their early marine phase. Its dense beds support abundant prey like harpacticoid copepods, sand lance, and sand shrimp. 

Embryos

Salmon and trout embryos begin as single cells with ample yolk, providing early developmental energy. Following fertilization, cells differentiate, creating a fragile embryo that matures once its rudimentary mouth (blastopore) closes. Survival hinges on water temperature, oxygen levels, and nest protection from disturbances.

Emergence

Salmonids begin life as eggs under gravel. After a number of weeks or months, they emerge from their eggs as alevin with yolk sacs attached to their underdeveloped bodies. Sometimes called sac fry, alevin remain hidden under the safety of their gravel nest while they develop. 

Once most or all of their yolk has been absorbed, the young salmonids will eventually emerge from the gravel as fry. How quickly salmonids emerge from their eggs, as well as from the gravel, depends on the species, water temperature, and levels of dissolved oxygen.  

Endangered Species Act – ESA

The Endangered Species Act was enacted in 1973 and aims to conserve endangered and threatened species and their ecosystems. It has listed numerous salmon and trout populations as threatened or endangered, including the Upper Willamette River Chinook, Snake River Sockeye, Central California Coast Coho, and Lahontan cutthroat trout. 

Under the ESA, the US government implements habitat restoration, enforces conservation laws, establishes protected areas, and manages recovery plans for listed species. It also monitors populations, regulates harmful activities, and promotes sustainable practices to protect endangered and threatened species.

Escapement

Escapement in salmon management refers to the number of salmon that successfully avoid capture by fishing activities and reach their freshwater spawning grounds. It’s important for understanding the health and sustainability of salmon populations. 

Escapement is assessed through various methods. This includes fish counts at barriers like weirs or dams and carcass surveys on spawning grounds. These data help managers estimate population sizes and set fishing quotas for future seasons.

Escapement goals vary: Biological Escapement Goals (BEGs) aim for maximal harvest while ensuring enough spawning to sustain future populations. Sustainable Escapement Goals (SEGs), used when catch data is limited, ensure yield sustainability over a decade. Optimal Escapement Goals (OEGs) balance biological needs and fishing allocations, setting a sustainable range to guide fishing practices. These goals are essential for managing salmon stocks effectively, ensuring both conservation and continued availability for commercial and recreational fishing.

Estuary

Estuaries serve as crucial transition zones for anadromous salmon and trout, easing their adaptation from freshwater to saltwater. In general, species like pink salmon, chum, and ocean-type chinook, which enter saltwater within a year of freshwater life, rely heavily on estuary habitats. Yearling and older smolts like sockeye, steelhead, and stream-type chinook use estuary habitat less. Estuaries provide abundant food, enabling rapid growth before the fish venture into the ocean.

ESUs

An Evolutionarily Significant Unit (ESU) is a distinct population crucial for conservation, encompassing species, subspecies, or populations. The term guides conservation actions under the US Endangered Species Act, especially for vertebrates. ESUs for salmonids are determined by distinct genetic characteristics and significant ecological variation, focusing on unique adaptation and reproductive isolation.

Eutrophic Lakes

Eutrophic lakes are nutrient-rich, fostering large populations of aquatic plants, algae, and fish. They have soft, mucky substrates. Plant and algae growth can reach nuisance levels. 

Fish species in eutrophic lakes are typically warm-water and low-oxygen tolerant, like carp, bullheads, and bluegills. Deep lakes can have oxygen-depleted hypolimnions by mid-summer, releasing phosphorus and triggering algae blooms.

Eyed Eggs

Eyed eggs are fertilized salmonid eggs with visible eyes and early nervous systems through the translucent shell. Gyrating movements are also observable, indicating significant development.

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Fall Chinook

Fall chinook are one of the four seasonal runs of king salmon. In general, fall kings migrate from the ocean to freshwater spawning grounds beginning in late July, peaking in September, and concluding in December. Spawning takes place from October through December. Run timing of fall chinook varies in some locations like the Columbia River basin. There, they’re often categorized as brights or tules. Bright chinook arrive less mature, brighter, and spawn later. Tule chinook enter freshwater more mature, darker, and spawn soon after. 

Fall Chum

Fall chums are one of three life history patterns of chum salmon, along with summer and winter chums. They spawn in fall or early winter, often within cold upwelling areas. This strategy allows fall chums to adapt to seasonal temperature shifts and optimize egg incubation conditions. 

Fecundity

Fecundity is the number of eggs a salmonid lays and varies by fish species, size, and age. It’s a measure of reproductive capacity influenced by environmental conditions. Larger salmonids generally have higher fecundity, indicating a direct relationship between body size and reproductive potential. 

Ferox Trout

Ferox trout are a variety of brown trout found in oligotrophic lakes of Ireland, England, Scotland, and Wales. Once considered a distinct species (Salmo ferox) due to their reproductive isolation from other brown trout, ferox don’t appear to share a common origin. 

Ferox trout are piscivorous and feed largely on Arctic char. They can reach 31.5 inches (80 cm) in length.  

Fingerling

Fingerling is a general term for juvenile fish. Fingerlings typically resemble the size of a human finger, hence their name. The size range for fingerlings can vary by species, but they are generally between 10 and 15 centimeters long, as defined by the Food and Agriculture Organization (FAO). Fingerlings are generally up to one year or even two years of age.

The fingerling stage marks the transition to a more developed and robust state. Fingerlings are particularly significant in aquaculture and fish stocking programs, where the term is often used. Due to their enhanced survivability, fish stocking programs often release fish at the fingerling stage to improve the success rate of population establishment or replenishment efforts. 

Fisheries

Fisheries involve raising or harvesting fish and aquatic life. They are crucial for economic dependence worldwide but are not without ecological downsides. Fisheries management, influenced by legal frameworks and environmental challenges like overfishing and climate change, aims for sustainable practices. Salmon, trout, and char fisheries require careful management to ensure survival and ecological balance.

Fish Ladder

Fish ladders, or fishways, enable migrating fish like salmon and eel to bypass obstructions like dams. They feature ascending pools where fish leap through water streams, rest, and continue upstream. These structures are crucial for maintaining fish migrations.

Fluvial

Fluvial salmonids exhibit a lifecycle pattern where adults inhabit larger river systems and migrate to smaller tributaries for spawning. They rear in their natal tributary streams after hatching. Following this juvenile period, they migrate to the main river system, where they grow to maturity. 

The fluvial life history exists in many trout and char species like brown, rainbow, cutthroat, and bull trout. Masu salmon are the only Pacific salmon that exhibits this life history. 

This life history strategy involves movement within freshwater ecosystems without migrating to marine environments. Fluvial salmonids adapt to varying conditions across both the larger river habitats where they mature and the smaller tributaries where they spawn and begin their life cycle.

Formosa Salmon

Formosan salmon (Oncorhynchus masou formosanus) are a subspecies of masu salmon endemic to Taiwan’s mountain streams. They’re the southernmost masu salmon subspecies. Other common names for this rare fish include Taiwanese trout and slamaw trout.

Adults are deep-bodied with dark green bodies and silver bellies. They also have distinctive dark and black spots. Formosan salmon reach around 12 inches (30.5 cm) in length. 

Fry

Fry are an infant stage of salmon, trout, and char. They emerge after the alevin stage as tiny, vulnerable fish.  

Fry rise to the water’s surface to fill their swim bladders with air to aid in buoyancy. This is done typically at night to avoid predators. At this stage, most fry measure just about 1 inch (2.5 cm) in length. 

Emergent fry are often too weak to swim upstream. Instead, they drift or move laterally to calmer pools to establish feeding territories. Their diet consists of plankton and other organic material at first. This expands to terrestrial insects, insect nymphs, and larvae when they’re large enough. 

Predation is a major threat to trout and salmon fry. Birds and larger fish are common predators. Fry develop camouflage markings known as parr marks, which help them blend into the stream or lakebed. 

Fry undergo imprinting, memorizing their home stream’s unique scent. This is essential for their return as adults. Imprinting is sensitive to environmental changes, which can disrupt their homing instinct. 

Fry rely on fresh, oxygen-rich, shaded water with hiding spots like boulders or fallen logs to survive. 

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Gila Trout

Gila trout (Oncorhynchus gilae gilae) are native to the Gila River tributaries in Arizona and New Mexico. They live in high-elevation streams and only have resident life history forms. 

Gila trout are yellow with small dark spots. Like the closely related Apache trout, Gila trout have faint or no parr marks. 

Due to the irregular habitats utilized by Gila trout and the limited food availability, they are small trout. Most are less than 10 inches (25.4 cm). The largest Gila trout are introduced into lakes and reservoirs. The biggest recorded trout was 5 pounds, 2 ounces (2.3 kg) and 23 inches (58.4 cm) long. 

Threats include competition with introduced species, habitat loss from environmental changes, and human impacts. Conservation efforts have expanded the range of Gila trout, improving their status. It was down-listed from endangered to threatened in 2006, enabling limited sport fishing. 

Gilbert-Rich System

The Gilbert-Rich system is a method used to show the age of anadromous salmonids. It features the number of years the salmonid was alive, from egg to death. A subscript after the primary number indicates the number of full years spent at sea. For example, a chinook salmon that lived five years with three spent at sea would be 5₃.

Gillaroo Trout

Gillaroo are a type of brown trout found in Ireland, and they’re sometimes classified as seperate species (Salmo stomachicus). Gillaroo feed primarily on snails and other benthic organisms. They can be distinguished by bright golden flanks and large crimson spots. They also have a specialized gizzard for digesting hard-shelled prey.

Gill Nets

Gill nets are a fishing gear type consisting of vertical panels of netting suspended in water. They’re typically made of monofilament or multifilament nylon and are invisible to fish. 

Gill nets are designed so fish can get their head through but not their body, trapping them by the gills as they try to retreat. Mesh size varies based on the target species and fishing area. 

There are two main types of gill nets: set gillnets, anchored to the substrate, and drift gillnets, kept afloat with weights and buoys. 

Gillnetting poses significant risks to non-target species like sea turtles and marine mammals, which can become entangled and drown or suffer injuries. Bycatch in gill nets is also a significant risk to species of salmonids. Measures like acoustic pingers, weak links in nets, and fishing area closures are sometimes employed to reduce bycatch. 

Gillnetting is highly regulated to minimize bycatch and ensure size selectivity, particularly in salmon fisheries. They are highly efficient at capturing fish. Gill net fisheries are controversial and closely monitored.

Gills

Salmon breathe by extracting oxygen from water using eight gills arranged in four pairs on either side of the head. Each gill features a bony arch and filaments rich in blood vessels. This structure enables efficient gas exchange as water flows over them.

Gill Rakers

Gill rakers are comb-like projections on the gill arches. They strain food and prevent it from entering the gills, guiding it to the throat. Juvenile salmon can often be identified by their gill rakers. Plankton feeders like pink and sockeye salmon have many long, thin rakers. Fish eaters like chinook and coho possess fewer, stouter, and sharper rakers.

Golden Trout

Golden trout originate in the Kern River basin, connected to the San Joaquin River system of southern California. They encompass three subspecies of ancient rainbow trout. These include the South Fork Kern and Golden Trout Creek golden trout (Oncorhynchus mykiss aguabonita), Little Kern River golden trout (Oncorhynchus mykiss whitei), and Kern River rainbow trout (Oncorhynchus mykiss gilberti). 

Renowned for their stunning golden hue and rosy lateral lines, aguabonita is the most brilliant with large purple parr marks. Gilberti looks the most like rainbow trout, and whitei showcases an intermediate appearance.

Stocking of coastal rainbow trout and brown trout has seriously compromised the three golden trout subspecies through hybridization and competition. The genetic integrity of Kern River rainbow trout has been particularly compromised. 

Gradient

Stream gradient, measured in feet per mile (ft/mi), indicates a stream’s slope by the elevation drop over a horizontal distance. Gradient can also be expressed as a percentage, indicating the elevation change as a proportion of the horizontal distance traveled. 

Stream gradient influences water flow speed and erosion capability and varies along a stream’s course. High gradients mean steep, rapid streams, while low gradients indicate level beds and slow water. This factor shapes valleys, affecting sediment transport and stream shape.

Greenback Cutthroat Trout

Greenback Cutthroat Trout (O. c. stomias) are a cutthroat subspecies native to Colorado’s Arkansas and South Platte River basins and select Wyoming tributaries. They’re closely related to the Colorado River cutthroat subspecies. Greenback cutthroat were once endangered but are now threatened due to successful conservation efforts. 

Grilse

Atlantic salmon grilse are a unique life stage that spend one winter at sea before returning to freshwater rivers for spawning. Typically arriving in rivers during early summer, grilse weights range from 2 to 7 pounds. 

Grilse provide exciting fishing opportunities due to their vigorous fighting nature. They’re also critical contributors to the genetic diversity and sustainability of Atlantic salmon populations.

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Habitat

Half-Pounder

Half-pounder steelhead are a unique life history variant of anadromous coastal rainbow trout primarily found in northern Californiaia and southern Oregon rivers. Half-pounders return to freshwater within the year of ocean entry to overwinter in freshwater. They feed actively, rarely spawn, and return to the sea in spring. This life history strategy balances food availability, potential reduced predation risk, and capitalization on growth opportunities. 

Harbor Seals

Harvest

Hatcheries

Herring

Holomictic Lake

Holomictic lakes are the most prevalent type globally. They undergo uniform mixing of water layers at least once a year, contrasting with the continuous stratification seen in rarer meromictic lakes. This mixing distributes oxygen and nutrients, supporting diverse fish and aquatic life. 

Holomictic lakes are classified into four categories based on their mixing frequency: oligomictic with irregular mixing, monomictic with annual mixing common in polar areas, dimictic with biannual mixing typical in temperate zones, and polymictic with frequent mixing found in tropical regions. Dimictic lakes specifically experience turnover during spring and fall, with distinct stratification phases in summer and winter.

Homing

Homing in salmonids involves returning to their natal site for spawning, a trait seen in 95% or more of both anadromous and nonanadromous adults. This process includes learning natal stream odors before seaward migration, retaining these memories, and being stimulated to swim upstream upon detecting these odors at maturity. When salmonids return to a non-natal watershed to spawn, it’s called straying. 

Hook Nose

Huchen

Huchen (Hucho hucho), or Danube salmon, are native to the Danube basin in Central and Eastern Europe. They exhibit primarily resident and fluvial life history forms and are iteroparous. 

Huchen are large salmonids, with adults reaching up to 4 ft 11 in (1.5 m) and over 110 lb (50kg). They generally have reddish-brown backs with dark patches and large heads. 

Smaller huchen fish feed on aquatic insect larvae or small fish. Eventually, they become primarily piscivorous. Huchen spawn in shallow streams, laying eggs covered by sand. They are threatened by damming, habitat loss, and overfishing. Hatchery efforts aim at reintroduction with some success. 

Hucho

See taimen

Humboldt Cutthroat Trout

Humboldt cutthroat trout (O. c. humboldtensis) are a cutthroat trout subspecies that’s closely related to Lahontan cutthroat. They are found in Nevada’s Humboldt River drainage and have a fluvial evolutionary history, as opposed to the lacustrine Lahontan cutthroat. Almost all Humboldt cutthroat populations are limited to isolated headwater streams in their extremely arid environments. 

Humpy

See pink salmon

Hydro

Hybridization

Hyper-eutrophic Lakes

Hyper-eutrophic lakes are extremely nutrient-rich and frequently experience significant algae blooms, including harmful blue-green algae. 

Dominated by species like carp, which tolerate warm temperatures and low oxygen, these lakes often result from urban or agricultural watershed runoff. Hyper-eutrophic lakes are often small impoundments. They can suffer from severe ecological imbalances due to their high nutrient content.

Hyporheic Flow

Hyporheic flow refers to water movement into spaces within riverbed gravels, banks, and floodplains that originate from a stream and returns to the channel. It’s a mix of stream water, local groundwater, and regional groundwater. 

Hyporheic zones connect mainstem and off-channel habitats and reduce water temperatures in the summer. This process also facilitates water’s chemical alteration through contact with sediments, affecting nutrient balance, dissolved gases, and minerals. Hyporheic zones are important habitat for microbes, macroinvertebrates, and fish like salmonids and sculpins. 

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Imprinting

Inconnu

Inconnu (Stenodus nelma), or sheefish, are large, anadromous freshwater whitefish native to Arctic rivers from the Kola Peninsula across Siberia to North America’s Yukon and Mackenzie Rivers. Reaching lengths of up to 60 inches (152 cm) and weights of 60 pounds (27 kg). 

Inconnu have large mouths, protruding lower jaws, high dorsal fins, and silver colorations with variably colored backs. Their diet transitions from plankton to smaller fish.

Iteroparity

Iteroparity is a reproductive strategy where organisms undergo multiple reproductive cycles throughout their lives. This contrasts with semelparity, where only one reproductive event occurs before death. 

In the context of salmon and trout, iteroparity means these fish can spawn more than once before they die. Iteroparity isn’t as common in salmon species but does occur. For instance, Atlantic salmon and many trout species are iteroparous.

This strategy offers multiple opportunities to contribute genetically across several seasons. Iteroparous salmonids still face risks and energy costs with each spawning cycle, but unlike their semelparous counterparts, they do not invest all their life energy into a single reproductive event. The ability to spawn repeatedly potentially allows these fish to produce more offspring over their lifespan. However, each spawning event might produce fewer offspring compared to a semelparous event. 

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Jack Salmon

Jack salmon are precocious male salmon that mature rapidly, spending less time in the ocean compared to other salmon. 

Jacks are typically smaller and represent a unique life history strategy. Some populations have up to 50% jacks. They’re known for their ‘sneak-spawn’ tactic during reproduction, contributing genetically despite their size. Rapid growth in hatcheries can increase jacking rates, influenced by both nutrition and genetics. Governed by the same biological cycle as larger males, jacks die post-spawning, contributing nutrients back to the stream.

The smaller size of jack salmon makes them less commercially valuable and harder to monitor. While not large trophy fish, their presence can indicate future adult salmon returns, making them ecologically significant. 

Jellyfish

Juvenile Salmonids

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Kamchatkan Rainbow Trout

Kamchatkan rainbow trout (O. m. mykiss), also known as mikizha, are native to the Russian Far East, primarily the Kamchatka Peninsula. This subspecies includes anadromous steelhead forms that can reach up to 48 inches (122 cm) in length and 26 pounds (12 kg), as well as resident forms. 

Kelp

Kelt 

Kelts are iteroparous, anadromous salmonids like steelhead and Atlantic salmon that survived spawning. They journey back to the marine environment to prepare for potential future spawning events.

Research highlights the significant reproductive advantage of this strategy. Steelhead (anadromous rainbow trout) that undergo repeat spawning demonstrate over twice the lifetime reproductive success compared to their counterparts that spawn only once. The journey back to the ocean and then to freshwater spawning grounds again is fraught with risks like predation and physical barriers.

Only a small fraction of steelhead (about 2.4%) are repeat spawners. Kelts that manage to survive and return for subsequent spawning contribute significantly to the genetic diversity and resilience of their populations.

Fisheries biologists are actively working on kelt reconditioning projects to promote this valuable iteroparous trait in steelhead populations. In these projects, post-spawn adults are nurtured back to health, ensuring they are robust and ready for their subsequent spawning journey.

King Salmon

See chinook salmon

Kin Recognition

Kokanee Salmon

Kokanee salmon (Oncorhynchus nerka) are the non-anadromous form of the sockeye. They inhabit freshwater lakes across western North America, including Alaska, Washington, Oregon, California, Idaho, and Canada, plus Japan and Russia. 

Recent divergence from its sea-going relative around 15,000 years ago led to speciation, marked by geographic isolation and lack of interbreeding. Kokanee average 9–12 inches (23–30 cm) in length and 1 pound (0.45 kg), with variations up to 20 inches (51 cm) and 3–5 pounds (1.4–2.3 kg). They exhibit morphological differences from sockeye salmon like higher gill raker counts.

Kokanee feed primarily on zooplankton.

Korean Taimen

Korean taimen (Hucho ishikawae) are a large salmonid inhabiting rivers in the China-North Korea border region, including the Am-nok. Access restrictions hinder research, rendering its conservation status as data deficient by the IUCN. 

Krill

Kype

A kype is a pronounced hook-like growth on the lower jaw of some male salmonids, appearing before the spawning season. It’s a secondary sex characteristic crucial for mating rituals. Forming in the lower jaw’s distal tip, the kype develops alongside a matching depression in the upper jaw. This ensures the mouth closes properly despite the kype’s presence.

Kypes rapidly emerge from bony growths in the jaw and are composed of mesh-like bone tissues. This unique structure allows for quick development with minimal material use. While primarily made of bone, the kype also includes cartilage and chondrocytes. Its rapid growth results in dense networks of osteoblasts and proteoglycans along its structure.

The kype’s size and presence vary. It’s most prominent in anadromous males like Dolly Varden and coho salmon. Nonanadromous species like Arctic char, cutthroat, and lake trout often lack a visible kype. In some salmonid species, the upper jaw elongates more than the lower, forming a snout instead of a kype.

Kype development usually coincides with other physical changes during spawning. This includes skull morphogenesis, breeding teeth emergence, and scale resorption, especially in males. Some species even develop a prominent dorsal hump.

The kype’s function is believed to be a display of dominance and sexual fitness. Larger kypes often indicate higher status among males. It’s also thought to influence mate selection, with females possibly preferring males with larger kypes. After spawning, iteroparous salmonids may partially reabsorb their kypes, with some species retaining a permanent, growing kype.

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Lacustrine

The lacustrine life history form in salmonids involves living and maturing in lakes. Lacustrine salmonids may spawn in tributary streams, where their young rear before migrating to a lake. This variation is often called adfluvial. Other species, like lake trout, may spawn in the lake, spending their entire lives there. 

Lacustrine salmonids benefit from the lake’s rich food resources. This life history form is adapted to large, stable lacustrine ecosystems.

Lahontan Cutthroat Trout

Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) are the largest cutthroat subspecies. They’re native to the Great Basin’s Pyramid Lake and tributaries like the Truckee, Humboldt, Carson, Walker, and Quinn Rivers. Historically populating ancient Lake Lahontan’s vast waters, these cutthroat evolved into formidable predators, capable of reaching up to 39 inches (1 m) and 41 pounds (18.6 kg). 

There are both lacustrine and resident forms of Lahontan cutthroat. Once close to extinction due to overharvesting, habitat loss, and competition, conservation efforts have brought them back to threatened status. 

Lake Trout

Lake trout (Salvelinus namaycush), also known as mackinaw or lake char, are a freshwater char found in northern North America’s lakes. Their native distribution includes large parts of Canada, plus Alaska and parts of the northeastern United States. 

Lakers are notable for their size, with records of fish weighing almost 102 pounds (46 kg) and 50 inches (130 cm) long. Lake trout inhabit cold, oxygen-rich waters and are slow-growing, late-maturing fish.

They’re vulnerable to overfishing and acidification. Lake trout have three subspecies: the common, siscowet, and rush lake trout, with variations in feeding habits and habitat depth affecting their size and abundance. Lake trout hybrids, known as “splake,” result from natural crossbreeding with brook trout.

Lake Whitefish

Lake whitefish (Coregonus clupeaformis) are found throughout Canada and the northern United States, including Alaska and all Great Lakes.

Lake whitefish have earned the nickname humpbacks due to their small heads relative to body length. They’re typically between 24–36 inches (61–91 cm) long and weigh an average of 4 pounds (1.8 kg). The current world record is 14 pounds 6 ounces (6.5 kg). 

Lake whitefish feed on plankton, benthic organisms, and other aquatic invertebrates. They need cool water habitats and spawn on rubble and gravel shoals.

Lamprey

Large Woody Debris

Large woody debris (LWD), including fallen trees and branches, plays a critical role in aquatic ecosystems. 

Despite negative perceptions linking it to flooding and bank erosion, LWD has numerous benefits. It connects stream channels to floodplains, acting as a barrier during high flows and aiding floodplain water storage. LWD slows water flow, reducing erosion and sediment transport. This creates diverse habitats, forming pools and shelter for aquatic life like fish and macroinvertebrates. It can also serve as a basking spot for reptiles and birds. 

Gravel often accumulates upstream of large woody debris. These gravel deposits form spawning habitat for many species of fish. LWD also helps feed the aquatic food chain from the bottom up.

Recognizing these advantages, restoration projects often incorporate LWD to enhance stream health. LWD forms naturally through windthrow, bank erosion, and landslides. Its formation is disrupted by human activities like shoreline armoring, vegetation removal, and timber harvesting. 

Despite its ecological importance, LWD is sometimes removed for safety in recreational areas. 

Lateral Line 

The lateral line in fish, including salmonids, is a key sensory system that detects water movements and vibrations. This line runs along each side of the fish’s body. It’s crucial for orientation, navigation, and detecting prey or predators. 

The lateral line contains neuromasts, sensory cells that respond to water displacement. These cells translate vibrations into electrical signals sent to the brain. Trout and salmon rely on this system for survival, especially in murky waters or during migration. The lateral line helps these fish maintain balance, avoid obstacles, and efficiently school with others.

Life Cycle

Life History

Limnetic Zone

The limnetic zone is the open, sunlit surface layer of a lake away from the shore. This zone extends to the depth where sunlight penetration is sufficient for photosynthesis. It contrasts with the littoral zone, which is near the shore and can support rooted plants. The limnetic zone is too deep for sunlight to reach the bottom, preventing plant growth there. It’s rich in oxygen from photosynthesis and surface mixing, supporting abundant plankton and fish. 

The photic zone (aka: sunlight zone) is comprised of the limnetic and littoral zones. This is opposed to the aphotic zone with little or no sunlight. 

Littoral Zone

The littoral zone is the shallow, nearshore area of lakes, rivers, and seas. Light penetrates to the bottom of the littoral zone, supporting aquatic plant growth. 

It’s distinguished from the deeper and open-water limnetic zone. The littoral zone is characterized by abundant and diverse plant and animal life due to the ample sunlight and shallower waters. It’s crucial for ecological processes, including nutrient cycling and providing habitat for various species. 

Longevity

Longline Fishing

Loons

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Mackerel

Mackinaw

See lake trout

Magnetic Fields

Maiden Fish

Marble Trout

Marble trout (Salmo marmoratus) are a rare salmonid from the Adriatic Basin’s cold rivers. Their range stretches from Italy to Montenegro. 

Notable for their marbled pattern and large size, marble trout have been recorded up to 55 inches (140 cm) and over 53 pounds (24 kg). There have even been reports of much bigger marble trout. 

The diet of marble trout transitions from invertebrates to smaller fish as it matures. Reproduction occurs from November to January, with females laying thousands of eggs in gravel beds of swift rivers. Threats include hybridization with brown trout, habitat loss, and pollution. 

Masu Salmon

Masu Salmon (Oncorhynchus masou), or Cherry Trout, inhabit the North Pacific from Russia to Taiwan. They are iteroparous, unlike the other species of Pacific salmon. 

Adult masu generally weigh 4.4 to 5.5 pounds (2 to 2.5 kg) and measure 20 inches (50 cm) long, with some reaching 28 inches (71 cm) and 20 pounds (9 kg). 

Masu salmon have diverse life history forms, including resident, adfluvial, and anadromous. They feed on aquatic insects, small fish, and crustaceans. Masu are a commercially significant species caught wild, farmed in aquaculture, and pursued as a game fish.

Mature Male Parr

Mature male parr are found in certain anadromous salmonids and represent a unique life history strategy. Instead of migrating to the ocean, these males mature and remain in freshwater for their entire life cycle. They are typically the fastest-growing males in a population. 

Mature male parr occur in chinook salmon, masu salmon, rainbow trout, cutthroat trout, Dolly Varden, and Atlantic salmon. They are absent or rare in chum, pink, and sockeye salmon. 

Despite their small size, mature male parr can compete against larger, more dominant males. They do so indirectly by hiding on the periphery of the female and dominant male. When the female releases her eggs, the mature male sneaks in to release milt. This sneaking behavior is also seen in jacks. 

Their small size makes them challenging to sample. Mature male parr significantly contribute to the genetic diversity and effective population size in salmon populations. 

Maximum Sustainable Yield 

Maximum Sustainable Yield (MSY) is a core concept in fisheries management. It represents the largest average catch that can be continuously taken from stocks without compromising its future productivity. This yield is calculated at the point where a population grows most rapidly, typically at around half its carrying capacity. 

MSY is determined using models that require data on catch, effort, abundance, and various biological factors. However, MSY has been criticized for its oversimplified assumptions and inability to account for ecosystem complexities, species interactions, and environmental conditions. Despite these criticisms and the uncertainties of more sophisticated models, MSY remains a primary management tool in global fisheries. It has evolved from a direct management target to a limit on fishing mortality and biomass depletion, guiding sustainable practices. 

The concept often struggles in practical application due to data limitations and challenges in estimating population dynamics. As a result, MSY is viewed as a starting point for fisheries management, albeit one that requires careful consideration and adjustment to address real-world complexities and ensure the long-term health of fish stocks.

Mayflies

Meromictic Lake

Meromictic lakes are unique water bodies where different layers of water do not mix. Unlike holomictic lakes, which undergo regular mixing, meromictic lakes maintain stable stratification for extended periods, sometimes lasting years, decades, or even centuries. 

Meromictic lakes form in deep, steep basins or due to density differences caused by salinity variations. They are important in studying ecological dynamics and past climatic conditions.

These lakes typically have three distinct layers: the oxygen-depleted, often saltier monimolimnion at the bottom; the mixolimnion at the top, behaving like a regular lake; and the chemocline in between. The bottom layer’s lack of oxygen and high nutrient levels create specific ecological niches and are often inhabited by specialized bacteria like purple sulfur bacteria. 

Sediments in meromictic lakes are undisturbed and useful for climate research. However, any disturbance in stratification can have drastic effects on resident organisms, especially those in the oxygen-rich upper layer. 

Mesotrophic Lakes

Mesotrophic lakes possess moderate nutrient levels and support diverse aquatic life, including plants, algae, and fish. They may experience occasional algae blooms. 

In deeper lakes, the hypolimnion can become oxygen-depleted by summer’s end, potentially releasing phosphorus from sediments. These lakes strike a balance between nutrient richness and aquatic life diversity.

Mexican Golden Trout

Mexican golden trout (Oncorhynchus chrysogaster) are restricted to three high-elevation watersheds that drain into the Gulf of California: the Rio Fuerte, Rio Sinaloa, and Rio Culiacuan drainages. These rare fish are threatened by overfishing, habitat degradation, and hybridization with introduced rainbow trout. 

Midges

Milt

Milt is the seminal fluid containing sperm in fish and certain aquatic animals. It’s used for reproduction by spraying onto roe.

Mini Jack

Mountain Pine Beetle

Mountain Whitefish

Mountain whitefish (Prosopium williamsoni) are widespread across western North America. They thrive in cold, clear mountain streams and lakes. 

These slender salmonids are nearly cylindrical. They feature a dusky olive-green back, silver body, and a small mouth under the snout. Adults can reach up to 28 inches (70 cm) and weigh 6.4 pounds (2.9 kg). 

Spawning in gravel or cobbles at shallow depths, mountain whitefish lay eggs that hatch in early spring. Juveniles feed primarily on insect larvae. Known for demersal feeding habits, they stir the substrate to expose prey while contributing significantly to their ecosystem as a prey species themselves.

Morphology

Morphology is the study of the form and structure of organisms, encompassing their physical shape, size, and structural features.

The morphology of salmonids is characterized by a streamlined shape, small scales, and spineless fins. These fish possess a single soft dorsal fin, and their pelvic fins are located abdominally. Key characteristics distinguishing the salmonid family include the presence of an adipose fin and a pelvic axillary process at the base of the pelvic fin.

Mortality

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National Marine Fisheries Service (NMFS)

The National Marine Fisheries Service (NMFS), informally known as NOAA Fisheries, is a US federal agency within the National Oceanic and Atmospheric Administration (NOAA). It is responsible for the stewardship of US national marine resources. 

Founded in 1871, NMFS is the oldest federal conservation and environmental research agency in the country. Its primary role is to conserve and manage fisheries, promoting sustainability and preventing economic losses due to overfishing, species decline, and habitat degradation. NMFS operates under the Department of Commerce and regulates commercial and recreational marine fishing in the US, focusing on sustainable management and conservation.

Northern Anchovy

Northern Anchovy (Engraulis mordax) are prevalent in the Pacific from Mexico to British Columbia. They are a crucial diet component for Pacific salmonids like chinook salmon, coho, and steelhead. Historically canned as sardine populations fell, their harvest peaked at 43,000 tons in 1953. Today they are often used in animal feed and as bait.

Northern Pikeminnow

Northern pikeminnow (Ptychocheilus oregonensis) are large predatory freshwater fish native to northwestern North America. They are a major predator of juvenile salmonids in the impounded Columbia and Snake Rivers. They can reach up to 35 inches (89 cm) and 15 pounds (6.8 kg). A bounty program exists to manage their predation on salmon.

Nutrient Cycling

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Ocean-Type Chinook 

Chinook salmon are often divided into two categories: ocean-type and stream-type. Ocean-type chinook head to the ocean within the first year of their lives, utilizing coastal waters and estuaries significantly more than their stream-type counterparts. Their shorter freshwater residency contrasts with stream-types that migrate as smolts after their first year in freshwater. 

Off-Channel Habitat

Off-channel habitats (OCH) are water bodies adjacent to a river’s main channel, connected at summer discharge levels. They include side channels with upstream and downstream connections and alcoves connected downstream but not upstream during low flow. 

OCHs vary in surface and groundwater mix. They provide essential refugia, feeding grounds, and cooler temperatures and are crucial for young and migrating fish. OCH loss has impacted habitat quality and abundance of fish.   

Ohrid Trout

Ohrid trout (Salmo letnica) are a type of brown trout endemic to Lake Ohrid and the Black Drin river in North Macedonia and Albania. This adfluvial trout can grow to 30 inches (76 cm) and 14.3 pounds (6.5 kg). Ohrid trout face threats from overfishing and habitat destruction. 

Oligotrophic Lake

Oligotrophic lakes are characterized by their clear, deep, and cold waters. They typically have a firm, sandy substrate and low nutrient levels. This results in limited aquatic plant, animal, and algae populations. Fish are rarely abundant but tend to be large. 

Oligotrophic lakes often stratify, forming a cold, oxygen-rich hypolimnion. This makes them suitable for species like lake trout and cisco.

Operculum

The operculum, or gill cover, safeguards salmonids’ delicate gill filaments. It functions similarly to a human’s rib cage by protecting vital organs. It plays a key role in respiration, forcing oxygen-rich water over the gills for gas exchange.

Orcas

Osprey

Otoliths

Otters

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Pacific Herring

Pacific herring (Clupea pallasii) range from Baja California to Alaska. They are a vital food source for many predators like Pacific salmon. Spawning early in the year on vegetation, they do not die after, allowing multiple breeding seasons. Despite a collapse in 1993, conservation efforts are aiding their slow recovery. 

Pacific Salmon

Pacific salmon belong to the genus Oncorhynchus and are native to the coldwater tributaries of the North Pacific basin. They include six distinct species: chinook, coho, chum, pink, sockeye, and masu salmon. 

With the exception of masu, Pacific salmon are semelparous, meaning they die after spawning. Most species are anadromous, as well. They play a crucial ecological role, supporting diverse marine and freshwater food webs. 

Learn more about species of Pacific salmon and trout!

Pacific Trout

Pacific trout are closely related to Pacific salmon. They also fall within the genus Oncorhynchus. There are several species of Pacific trout, including rainbow, cutthroat, Gila, Apache, and the recently recognized Mexican golden trout. These species inhabit diverse freshwater environments across the North Pacific basin, extending from Alaska through the western United States and into Mexico. 

Pacific trout species are more adaptable to staying within freshwater ecosystems throughout their life cycles than most Pacific salmon. Most species of trout have numerous life history strategies, sometimes including anadromy. Another key distinction between Pacific salmon and trout is iteroparity. Trout are able to survive spawning attempts in some cases, whereas salmon always perish. 

Learn more about species of Pacific trout and salmon!

Paiute Cutthroat Trout

Paiute cutthroat trout (Oncorhynchus clarkii seleniris) are native to Silver King Creek in California’s Sierra Nevada Mountains. They’re notable for their nearly spotless bodies and unique purple hue. 

Closely related to Lahontan cutthroat trout, this rare subspecies is named after the Northern Paiute peoples. 

Palomino Trout

Palomino trout are a type of rainbow trout developed from a hatchery experiment in West Virginia. They were derived from a crossing of “golden rainbow trout” and a regular rainbow trout. Golden rainbow trout, not to be confused with golden trout, originate from a single hatchery rainbow trout with an unusual golden color. This mutant trout was bred with a standard rainbow trout, resulting in the lighter-colored Palomino trout.

Palomino trout are sometimes referred to as lightning trout or banana trout. They’re capable of rapid growth and can reach sizes up to 30 inches (70 cm) and 13 pounds (5.9 kg). They’re also known for strong fighting abilities.

Parr

Parr are juvenile salmon. It’s a general term and can refer to all smaller juveniles, or those that have developed past the fry stage. Parr have protective scales, stronger fins, and a carnivorous diet. They exhibit distinctive spots and vertical bars for camouflage, which are called parr marks. 

Parr feed on small invertebrates and can remain in this stage for several years or more, depending on the species. Some mature male parr can even fertilize eggs.

Parr Marks

Parr marks are distinct dark bands on the sides of some species of young salmonids, leading to the term parr. These marks  camouflage fry and young trout or salmon. While prominent in juveniles, parr marks fade as salmonids grow, especially when they mature or migrate to the sea or a lake. The pattern of these marks varies by species, aiding in identification during biologists’ stream surveys.

Passive Integrated Transponder Tag – PIT Tag

PCBs

Peak Flows

Pectoral Fin

Pectoral fins are located on a fish’s sides and are crucial for steering, vertical movement, and lateral motion. Alongside pelvic fins, they ensure stability, assist in stopping, and help maintain balance, allowing precise control over direction.

Pelvic Fin

Pelvic fins are paired and found on a salmonid’s ventral side in front of the anus. They aid in stability, deceleration, and vertical movement. Paired with pectoral fins, they prevent rolling and maintain balance, sometimes allowing fish to rest by sitting on them.

Pesticides

Phytoplankton

Pink Salmon

Pink salmon (Oncorhynchus gorbuscha) are the smallest and most abundant salmon species. They range from California to the Mackenzie River and across Asia. Mature males develop pronounced humps, earning them the nickname “humpies.

Pinks are notable for their unique 2-year life cycle with distinct odd and even-year populations. They’re semelparous, so they spawn once and die. Their fry immediately migrate to the sea, unlike most salmon. Recently, pink salmon have expanded their range into the Arctic and are also invasive in the North Atlantic. 

Learn more about pink salmon!

Pinniped

Pollution

Pool

Pools are deeper areas in streams with slower-moving water. They often form after riffles or when water flows over large obstacles like logs or boulders. 

Pools accumulate organic debris, offering a habitat for animals like mollusks and worms. Pools also provide a stable environment for trout and other species, especially during lower stream levels. The slow flow allows for a concentration of nutrients and shelter, making pools critical for various aquatic organisms.

A sequence of riffles, pools, and then runs is common in streams. 

Precocious Parr

Prespawn Mortality

Prespawn mortality in salmonids occurs when adult fish reach their spawning grounds but die before reproducing. This phenomenon varies widely in occurrence, with rates ranging from 0% to over 90% across different years and locations. 

Factors influencing prespawn mortality include environmental conditions like stream temperature, fish traits, and origin (hatchery or natural). Climate change is expected to increase prespawn mortality rates, especially in low elevation streams and for larger, hatchery-origin fish.

Purse Seine

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Quinnat Salmon

See chinook salmon

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Rainbow Trout

Rainbow trout (Oncorhynchus mykiss) are native to Pacific basin rivers, extending from Alaska to Mexico and west to Kamchatka. They display diverse life history forms, including resident, fluvial, adfluvial, and anadromous, which are called steelhead. Rainbow trout are iteroparous, meaning they’re capable of multiple reproductive cycles. 

Rainbow trout exhibit a greenish back with a distinct reddish stripe and black spots. They average 10-22 inches (25-56 cm) and 0.5-5 pounds (0.2-2.3 kg), with some reaching up to 30 pounds (13.6 kg) and 40 inches (101.6 cm). The record is a 48-pound (21.8 kg) fish, likely a triploid, from Lake Diefenbaker.

Rainbows thrive in varied freshwater environments, consuming a wide-ranging diet from zooplankton to smaller fish. Despite their resilience, they face threats from habitat loss, a warming climate, and overfishing. Steelhead are particularly challenged by these issues. 

Learn more about rainbow trout and steelhead!

Rain Gardens

Rawner 

Rawner is a term for mature male Atlantic salmon that did not spawn. Kipper is another word for these fish. 

Redband Rainbow Trout

Redband trout are an interior form of rainbow trout (as opposed to coastal) inhabiting drainages east of the Cascade Mountains, Klamath Mountains, and California Coast Ranges. There are several forms of redbands. 

The Columbia River redband trout (Oncorhynchus mykiss gairdneri) which has resident, fluvial, and adfluvial forms. There are also steelhead (anadromous) Columbia redbands. They are distributed primarily in the upper Columbia River and upper Fraser River systems. The lake-dwelling (adfluvial) Columbia redbands of the Fraser are sometimes called Kamloops trout. 

Northern Great Basin redbands (Onchorhynchus mykiss newberri) are another rainbow trout subspecies. They are primarily distributed in southeast Oregon, plus parts of northeast California and northwest Nevada. There is great diversity among this group, with the Upper Klamath Lake redbands as the most well-known. 

Finally, the Northern Sacramento redbands are a diverse group of California redband trout. They include the McCloud River redband (Oncorhynchus mykiss stonei), Sheepheaven Creek Redband, and Eagle Lake rainbow trout (Oncorhynchus mykiss aquilarum). 

Redd

Red Salmon

See sockeye salmon

Repeat Spawners

See kelt

Resident

The resident life history of some species of salmonids is characterized by spending their entire lifecycle within their natal tributary or small headwater stream. Residency is a common life history pattern for trout and char like rainbow, brown, cutthroat, bull, and brook trout. It’s much less common in Pacific salmon, with only the masu having a resident form. There are also a few known river-resident populations of Atlantic salmon. 

Unlike their migratory counterparts, these fish do not travel to larger rivers, lakes, or the ocean. This life history variant is often a result of geographical barriers or ecological preferences. Resident salmonids are typically adapted to the specific conditions of their native stream, including its food resources, water temperature, and flow characteristics.

Riffle

Riffles are shallow areas in a stream where water flows rapidly over rocks, creating fast, turbulent conditions. This environment is ideal for species that can firmly attach themselves, like net-winged midges, caddisflies, and stoneflies. 

Riffles are oxygen-rich due to the agitated water and support limited plant life, primarily diatoms and small algae. They provide essential habitat for certain aquatic invertebrates and feeding grounds for fish.

A sequence of riffles, pools, and then runs is common in streams. 

Rio Grande Cutthroat Trout

Rio Grande cutthroat trout (Oncorhynchus clarkii virginalis) are the southernmost cutthroat trout form. They inhabit the Rio Grande Basin of New Mexico and Colorado. Sometimes called red-bellied trout, they were first encountered by Europeans during Francisco de Coronado’s gold-seeking expedition in 1541. 

Rio Grande cutthroat live in mountain headwater streams and exhibit both resident and fluvial life strategies. They are usually small, achieving a maximum length of about 15 inches (38 cm) and a weight of up to 1 pound (0.45 kg).

Riparian

Roe

Run

Runs are areas in a stream characterized by deep, fast-flowing water with little to no turbulence. These sections often follow pools and precede riffles. 

Runs typically have a uniform flow that supports a diverse range of aquatic life. Fish often inhabit runs due to the availability of food and shelter. Runs play a vital role in the stream ecosystem, connecting different habitats.

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Saber-Toothed Salmon

Saber-toothed salmon (Oncorhynchus rastrosus) are an ancient species of salmon of the North Pacific in North America and Japan. They were alive from the late Miocene to the Early Pliocene. They reached lengths up to 7 ft 10 in (2.4 m) and weights of 440 lb (200 kg). 

The saber-tooth name comes from their small fangs for territory defense and nest marking during breeding. Scientists believe these giants were anadromous and planktivorous. 

Sakhalin Taimen

Sakhalin taimen (Parahucho perryi) is a critically endangered salmonid found in Northeast Asia, including parts of Russia and Hokkaido, Japan. It’s also commonly called Japanese huchen and stringfish. Though once included in the genus Hucho, it’s now often placed in its own monotypic genus. 

This rare salmonid primarily feeds on aquatic insects when young, switching to a piscivorous diet as an adult. It commonly reaches 11 pounds (5 kg), though unverified records exist of individuals up to 6.9 feet (2.1 m).

The Sakhalin taimen are in severe decline due to environmental degradation, commercial bycatch, poaching, and recreational fishing pressures. The IUCN lists it as critically endangered, with populations below 5% of historical levels.

Salmonflies

Salmonid

Solmonids are fish belonging to the family Salmonidae. They include Pacific and Atlantic salmon, trout, char, grayling, whitefish, lenok, and taimen. These coldwater fish are native to the subarctic and cool temperate waters of the Northern Hemisphere.

Salmonids feature pelvic fins set far back on their bodies, an adipose fin near the end of their backs, slender bodies, and rounded scales.

Salmon Sharks

Salmon sharks (Lamna ditropis) are a top predator in the northern Pacific, notably preying on salmon. They reach up to 8.6 feet (260 cm) and 485 pounds (220 kg). 

Sand Lance 

Sand lances belong to the family Ammodytidae. They are slender, pointed-snout fish known for burrowing into sand. Found globally, especially in the North Pacific and North Atlantic, sand lances lack pelvic fins and swim bladders, favoring a bottom-dwelling lifestyle. Both adults and larvae serve as crucial prey for significant species like cod, salmon, and birds. Interestingly, they exhibit chameleon-like eye movements and a ballistic tongue for feeding. 

Satellite Males

Schooling Behavior

Scour

Sculpins

Sculpins encompassing nearly 400 species within the superfamily Cottoidea. They inhabit diverse environments, from rivers to submarine canyons and kelp forests. Scuplins are benthic fish with pectoral fins designed for gripping substrates. Predatory towards salmonid eggs and young, sculpins also serve as prey for larger fish like bull trout.

Sea Lice

Sea lice, specifically the salmon louse (Lepeophtheirus salmonis), are parasitic copepods thriving on salmon species in the Pacific and Atlantic Oceans. They feed on fish mucus, skin, and blood. They impact salmon farms and wild populations by causing physical damage and potentially spreading diseases. Their lifecycle includes several planktonic and parasitic stages, allowing them to drift and attach to new hosts.

Sea Lions

Sea-Run Cutthroat Trout

Sea-run cutthroat, often called SRC or harvest trout, are the anadromous form of coastal cutthroat trout. They range from northern California to southeast Alaska. 

Despite improved feeding opportunities in the ocean, sea-run cutthroat don’t grow very large. They rarely exceed 20 inches (50.8 cm) or 3 pounds (1.4 kg), and their maximum is around 6 pounds (2.7 kg). 

Unlike their anadromous rainbow trout (steelhead) cousins, SRC don’t spend as much time or travel as far in saltwater. They tend to favor estuaries and coastal waters near their natal streams. Sea-run cutthroat tend to return to freshwater in the fall before spawning the next spring. 

Sea Trout

See brown trout

Sedimentation

Sediment Transport

Semelparity

Semelparity is a reproductive strategy where organisms experience a single reproductive event before death. This contrasts with iteroparity, which involves multiple reproductive cycles. In the context of salmon and trout, semelparity implies these fish spawn once in their lifetime, which is common among Pacific salmon species.

This strategy involves the allocation of all available energy and resources into one reproductive effort. Semelparous salmon migrate from the ocean to freshwater for a single, often exhaustive spawning event. Afterwards, they die. Semelparity is characterized by the production of numerous offspring in this one reproductive phase, maximizing the chance of successful reproduction at the expense of future life and additional reproductive opportunities.

In environments where the chances of survival and subsequent reproduction are uncertain, semelparity can be an effective strategy. It ensures an organism has at least one opportunity to pass on its genes. However, it also means that the organism does not survive to contribute to future generations beyond this single event.

Siberian Taimen

The Siberian taimen (Hucho taimen) are probably the largest species of salmonid. They’re native to Siberia’s rivers, extending from the Volga to the Amur River basins across Russia, Kazakhstan, Mongolia, and China. 

Noted for their substantial home range, some individuals roam up to 93 kilometers. Taimen can reach lengths of 83 inches (210 cm) and weights of 231 pounds (105 kg), out-sizing even king salmon. Predominantly piscivores, they also consume terrestrial prey like ducks and mice. 

Siberian taimen mating is unique, occurring as a solitary pair. They often pair days before the actual spawning event. Male taimen do not allow satellite males near the nesting female, unleashing vicious attacks on any competitors that get close. Unlike other salmonids, female taimen do not immediately cover their eggs post-spawning, instead resting over the redd before undertaking the task.

Despite their significance as game fish, Siberian taimen face vulnerability due to habitat change, pollution, overfishing, and climate impacts. 

Sichuan Taimen

Sichuan taimen (Hucho bleekeri) are a large piscivorous salmonid endemic to China’s Yangtze basin. They inhabit high-elevation mountain streams and rivers and require high oxygen levels and cold waters to survive. 

Adults Sichuan taimen have dark gray backs and silvery undersides. Their maximum size is unclear but have been recorded up to 6.5 feet (200 cm) and 110 pounds (50kg). 

Sichuan taimen are critically endangered and face threats from dams, other forms of habitat degradation, and illegal fishing. 

Silver Salmon

See coho salmon

Slow Water Habitats

Smelt

Smelt are small fish belonging to the family Osmeridae. They’re found in the North Atlantic, North Pacific, and inland waters of Europe, North America, and Northeast Asia. They serve as crucial forage for many species of salmon, trout, and char. 

Many species of smelt are anadromous, migrating from the ocean to freshwater to spawn. Other smelt species spend their entire lives at sea. 

Smolts

Smoltification

Sockeye Salmon

Sockeye salmon (Oncorhynchus nerka) are a Pacific salmon found in North America and Asia. They generally require lakes for the juvenile stage of their life cycle, though a few river-type populations exist. Sockeye are primarily anadromous. However, they have a freshwater lacustrine variant called kokanee. Both kokanee and anadromous sockeye salmon die after spawning. 

Sockeye are recognized for their red spawning color, green heads, and lack of spots. This contrasts with their silvery-blue appearance in the ocean. Sockeye salmon average 4.4 to 8.8 pounds (2-4 kg), reaching up to 15 pounds 3 ounces (6.9 kg). 

Like most salmon, sockeye are opportunistic feeders but are primarily planktivorous. Their unique lifecycle contributes significantly to nutrient cycling in aquatic ecosystems.

Learn more about sockeye salmon!

Softmouth Trout

See Adriatic trout

Sonaghen Trout

Sonaghen trout (Salmo nigripinnis) are endemic to Lough Melvin of Ireland and the United Kingdom. They favor the lake’s deep waters and migrate to tributary streams to spawn in November and December. Sonaghen range from light brown to silver with large black spots and dark fins.

Spawning Gravel

Spring Chinook

Spring chinook, or springers, are one of the four population types of king salmon. They migrate from the ocean to freshwater spawning grounds from around April to July. Unique for their early arrival and extended freshwater residency, they spawn from August through early November. Spring chinook are noted for their size and the long distances they travel upstream. Springers are highly valuable for their quality taste. 

Sneaker Males

Hierarchies form at salmon spawning sites with more males than females. Dominant males pair with females, surrounded by satellite males. Smaller, non-hierarchical sneaker males, such as jacks and mature parr, stay hidden on the periphery. These sneakers opportunistically rush in to fertilize eggs the moment females release them, bypassing direct competition.

Sneaker males are more common in the southern part of the range of salmon. Some species, like chum and pink salmon, don’t exhibit sneaking strategies at all. 

Spawning Sites

Salmonids tend to select spawning sites near where they were born. In general, salmon select spawning sites in coarse gravel with high sediment mobility. These areas mostly occur in tailouts, or the pool-riffle transition, with high hyporheic flows. 

Squid 

Anadromous trout and salmon are typically opportunistic feeders in freshwater and the ocean. Various squid species are important prey for all species of Pacific salmon, Atlantic salmon, and steelhead. Squid make up a higher percentage of their diet when foraging at greater depths. 

Humboldt squid, another opportunistic predator, occasionally prey on salmon in California. 

Status (Conservation) 

The conservation status of organisms reflects their existence and extinction risk, considering population trends, breeding success, and threats. The IUCN and CITES provide international frameworks, classifying species into categories like Extinct, Endangered, and Vulnerable. 

NOAA Fisheries lists various salmon and steelhead populations in the US as threatened or endangered. This includes Atlantic salmon and various populations and species of Pacific salmon and trout. Conservation efforts include habitat restoration, hatchery programs, and fish passage improvements to support recovery. 

Steelhead

Steelhead are the anadromous form of coastal rainbow trout and Columbia River redband trout. They’re native to the Pacific basin’s cold-water tributaries. Unlike their Pacific salmon cousins, steelhead are iteroparous, capable of surviving spawning. However, survival rates post-spawning are low. 

There are two distinct runs of steehead: summer steelhead and winter steelhead. Summer steelhead, or summer-runs, spend more time in freshwater as juveniles. And they return to freshwater immature. They often migrate further upriver to spawn. Winter-run steelhead spend less time in freshwater as juveniles and return to freshwater sexually mature. Their freshwater spawning migrations are shorter than summer steelhead. 

Steelhead face threats from habitat blockage by dams, other forms of habitat degradation, overfishing, poor ocean conditions, and more. 

Sticklebacks

Sticklebacks (Gasterosteidae) are ray-finned and primarily marine fishes. The threespine stickleback (Gasterosteus aculeatus) is notable for its ecological overlap with salmonids. They are carnivorous, feeding on aquatic insects, crustaceans, fish larvae, and zooplankton, sometimes preying on salmonid eggs and juveniles. They, in turn, serve as prey for trout, salmon, and char species.

Stocking

Stocking salmon and trout supports fisheries but has ecological downsides. Hatcheries produce salmon and trout to boost commercial and recreational fisheries. But they often lack local genetic adaptation. They compete with natural-original fish, reducing the biomass of available food and survivability. The limited genetic diversity of stocked fish can dilute native gene pools when they interbreed, threatening natural populations. 

Stoneflies

Stoneflies belong to the order Plecoptera, comprising about 3,500 known species globally. Along with other aquatic insects, stoneflies are an important food source for steam salmonids. Their presence also indicates excellent water quality. The general anatomy of stoneflies is characterized by long antennae, robust legs, and membranous wings, which are often not strong in flight. Nymphs and adults feed primarily on vegetation but also on salmon carcasses. Adult stoneflies’ lifespans are short, living close to their aquatic birthplaces.

Straying 

Straying is when salmonids utilize a non-natal watershed for spawning. The propensity to stray varies by species, life-history type, and watershed. Straying is partly a failure to home and partly a response to environmental conditions. Essential for salmon persistence and distribution through climatic changes, straying’s mechanisms and reasons are complex. They involve environmental cues and possibly genetic factors, yet are critical for adaptation and species survival.

Stream-Type Chinook

See ocean-type chinook

Subadult Salmon

Salmon in the ocean are generally called subadults or immature salmon. They become mature or adult during their spawning migration. 

Substrate

Stream substrate is the material at a stream’s bottom and includes mud, sand, granules, pebbles, cobbles, and boulders. These materials are classified by size. This substrate influences stream health and clarity, affecting the habitat’s life. 

Summer Chinook

Summer chinook, also known as summer kings, are one of the four seasonal runs of king salmon. They initiate their migration from the ocean to freshwater spawning grounds between June and August, with spawning occurring from late August to November. Summer chinook generally spawn soon after entering freshwater. 

Summer Chum

Summer chums are one of three life history patterns of chum salmon, along with fall and winter chums. They arrive in freshwater in the summer, much earlier than other types of chum salmon. Summer chums often spawn in the lower to middle river sections. 

Summer Kill 

Summer kills in lakes result from warm temperatures and low dissolved oxygen (DO) levels. Fish are forced from cooler, oxygen-depleted depths to warmer, oxygen-rich surface waters, creating stress. Factors like fertilizers, livestock waste, septic fields, and decaying plant matter contribute to these dangerously low DO levels, making environments lethal for many fish species that require cool water.

Summer Steelhead

Summer steelhead are one of two types of steelhead life history patterns and are often called summer-runs. Winter steelhead are the other type. They both spawn during late winter or spring, but summer steelhead return to freshwater streams in an immature state and much earlier. They may spend up to a year maturing in freshwater before spawning. Summer steelhead enter their natal streams from spring until late summer. 

This strategy allows summer steelhead to undertake arduous migrations to distant spawning grounds. Sometimes, summer steelhead spawn above seasonal barriers that are impassible to winter steelhead during high winter flows. 

Sunapee Trout

Sunapee trout (Salvelinus alpinus oquassa) are a subspecies of Arctic char. They’re found in northeastern New England and Canada, including Québec and New Brunswick. Depending on their location, these char are also called blueback trout, Quebec red trout, and Sunapee golden trout. Most are found in deep, cold lakes, though anadromy occurs in some populations connected to the ocean.

Sustainable Fisheries

Sustainable fisheries, particularly for salmonids and forage fish like smelt and herring, focus on maintaining fish populations and ecosystems over time. This involves harvesting at rates that do not cause long-term decline, considering fluctuating natural populations and environmental conditions. 

Controversy arises over the definition of sustainability. Some view it as minimal fishing for historical population recovery. Others see it as maximizing catch without further depletion. Heavy fishing pressure, ecosystem and stock structure changes, and trophic balance shifts are key concerns. 

Sustainable practices include managing fishing quotas, protecting habitats, and addressing overfishing. Aquaculture, like salmon farming, often relies on wild forage fish for feed, raising sustainability issues. Environmental impacts, such as habitat destruction and pollution, are also significant factors. 

Ultimately, sustainable fisheries aim to balance ecological health with economic and social needs, a complex and sometimes contentious goal.

Sympatry

Sympatric salmonids coexist in the same geographic area where they frequently encounter each other. In such environments, multiple salmonid species often occupy distinct ecological niches. This minimizes competition and allows for the coexistence of closely related species within the same habitat.

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Taimen

Hucho is a genus within the Salmonidae family, featuring large, piscivorous salmonids known as taimens. This includes the Sichuan taimen in China’s Yangtze basin, the huchen endemic to Europe’s Danube basin, the Korean taimen in North Korea and China, and the widespread Siberian taimen across Siberia. 

Territorial Behavior 

Salmon, trout, and char often exhibit territorial behavior in freshwater during foraging and spawning. Juvenile salmonids establish feeding territories to capture drifting aquatic organisms, with dominant individuals securing prime feeding stations. Adults fight over mates and optimal spawning gravel. Males battle for access to females, displaying territorial aggression to deter rivals, while females compete for and defend the best spawning sites.

Tiger Trout

Tiger trout are a sterile hybrid between brown trout and brook trout with distinctive vermiculations resembling tiger stripes. They’re the result of hatchery breeding and stocking. 

Tree Canopy

Tree canopies are vital for freshwater ecosystems, especially for salmon and trout habitats. They offer shade, cooling waters, and enhance habitat by contributing organic material that benefits invertebrates. Canopies help stabilize streambanks, support late summer flows, and add complexity with fallen trees, improving reproductive and refuge spaces for aquatic life. Their removal can significantly elevate stream temperatures and reduce habitat quality. 

Trout

Trout is a generic name for various species within the Oncorhynchus and Salmo genera of the Salmoninae subfamily. Though referred to as char, many species within the Salvelinus genus are also commonly called trout. 

Trout are generally iteroparous, capable of multiple spawning attempts. While mainly potamodromous, many species are capable of anadromy, like steelhead, sea-run cutthroat, and sea trout. 

Essential as both predators and prey, trout significantly influence aquatic and terrestrial food webs. They are carnivorous freshwater fishes and mostly opportunistic feeders.

Tule

See fall chinook

Turbidity

Turbidity measures water’s relative clarity, quantified in Nephelometric Turbidity Units (NTU). High turbidity is caused by things like silt, clay, algae, plankton, and other organic and inorganic materials. Turbid water diminishes light penetration, affecting water quality, habitat, and potentially human health by sheltering pathogens.

Triploid

Triploid trout possess three sets of chromosomes, rendering them infertile. This contrasts with diploid trout and most other vertebrates, which are fertile and have two chromosome sets. Although visually similar to diploids, triploids may grow larger. They’re created in hatcheries through pressure treatment or heat shock, which retain an extra chromosome during fertilization. Triploids are not considered genetically modified organisms.

Tyee

Tyee is Chinook Jargon for large chinook salmon, typically over 30 pounds (13.6 kg). 

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Upriver Brights

See fall chinook

Upriver Migration

Anadromous salmon, trout, and char embark on upriver migrations throughout the year, with each species and population following a distinct pattern to match their home stream. These migrations start as adult fish leave their ocean feeding grounds, aiming for their birth streams to spawn. 

The journey’s timing, driven by genetic cues rather than current river conditions, ensures they reach spawning sites energetically prepared for reproduction. Temperature and water flow determine the exact timing of migration, as well as the optimal time to spawn for the maximum survival of their offspring. 

Upwelling

Upwellings are when deep, cold water is pulled up towards the shore. They’re driven by winds pushing warmer surface water offshore, which pulls the nutrient-rich, deep water to the surface to fill the space. This nutrient-rich water fuels phytoplankton growth and supports vast marine food webs. Similar phenomena can occur in large, deep freshwater lakes. 

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Vertical Slot Fishway

Vertical slot fishways are fish passage structures designed to help migratory fish species bypass barriers like dams. They consist of a series of pools separated by vertical slots that allow fish to rest and gradually move upstream. 

These fish passage structures do have some downsides, as well. Vertical slot fishways can be costly, species-specific, and environmentally disruptive. Their effectiveness varies with water conditions, and they may attract predators, requiring regular maintenance to prevent blockages.

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Westslope Cutthroat Trout

Westslope cutthroat trout (Oncorhynchus clarkii lewisi) are a unique subspecies of cutthroat trout. Named after Meriwether Lewis, they were one of the cutthroat variants encountered by Lewis and Clark. 

Westslope cutthroat trout were historically the most abundant and widely distributed cutthroat subspecies. They thrived east and west of the Continental Divide in cold, clear streams, rivers, and lakes in the US and Canada. However, their range has significantly diminished, primarily due to habitat loss and hybridization with non-native cutthroat trout subspecies.

Whirling Disease 

Whirling disease, caused by the parasite Myxobolus cerebralis, severely affects juvenile salmonids and leads to skeletal and neurological damage. Discovered in 1893, it now impacts salmon and trout worldwide. It necessitates a tubificid worm for its lifecycle. 

This parasite induces abnormal swimming patterns, reduces survival rates significantly, and can devastate fish populations economically. Whirling disease leads to high mortality rates among infected fingerlings. M. cerebralis demands stringent control and prevention measures to mitigate its spread and impact on aquaculture and wild fish stocks.

White Spotted Char

Whitespotted char (Salvelinus leucomaenis) are found in East Asia known as iwana in Japanese and kundzha in Russian. Their range includes Japan, northeastern Korea, Sakhalin, the Kuril Islands, and Kamchatka. They exhibit both landlocked and ocean-run forms. Landlocked individuals prefer streams below 59°F (15°C) and grow up to 14 inches (35 cm). Their anadromous counterparts have been recorded up to 47 inches (120 cm) and 33 pounds (15 kg). Whitespotted char feed on plankton, insects, and small fish. Anadromous juveniles enter the sea at two years and return to spawn at four.

Whitefish

Freshwater whitefish (Coregoninae) are a type of salmonid and include whitefishes, ciscoes, and inconnu, spanning over 60 species. They are one of three subfamilies within the Salmonidae. Whitefish inhabit cool northern waters across the Northern Hemisphere. While primarily freshwater, some Arctic species are anadromous, venturing into the ocean to forage. 

Winter Chinook

Winter chinook are one of the four seasonal runs of king salmon. They are primarily located in Caliornia’s Sacramento River basin. This run evolved to deal with the warm and low water conditions found in summer and fall. Winter chinook migrate to freshwater spawning grounds from December through May, peaking in March. Spawning occurs from mid-April to early August. Winter kings historically have been less common than other seasonal types. They spend 1 to 3 years maturing in the ocean before returning to freshwater.

Winter Chum

Winter chums are one of three life history patterns of chum salmon, along with summer and fall chum salmon. Winter chum salmon are a rare race. They time their spawning to align with late-season stream flows, allowing them to avoid winter floods. This timing ensures faster egg incubation due to warmer water temperatures, enhancing survival rates. 

Winter Kill

Winter kills occur when dissolved oxygen depletes in frozen shallow lakes or ponds. Ice prevents oxygen exchange from the air, and when heavy snow blocks sunlight into the water, aquatic plants cannot photosynthesize and produce oxygen. Decaying plants and organic matter accelerate oxygen loss. These conditions can lead to fish stress or death from low oxygen levels unless they can access more oxygenated areas like inlet streams. 

Winter Steelhead

Winter steelhead, also known as winter-runs, are one of two primary life history patterns of steelhead. Unlike their summer steelhead counterparts, winter-runs return to freshwater fully mature and ready to spawn. They typically enter their natal streams from late fall through winter, timing their arrival closer to the spawning season in late winter or spring. 

This strategy minimizes their time in freshwater, allowing them to conserve energy for reproduction. Winter steelhead also don’t migrate as far from the ocean as summer steelhead. 

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Yellowstone Cutthroat Trout

Yellowstone cutthroat trout (Oncorhynchus clarkii bouvierii), sometimes called black-spotted trout, are native to the Yellowstone Lake and River drainage. They are the most widely recognized cutthroat subspecies. 

Large individuals have been recorded up to 24 inches (61 cm) and 6 pounds (2.7 kg). Their populations have declined due to predation by introduced lake trout in Yellowstone Lake. Additionally, introduced brook and rainbow trout have caused issues elsewhere. 

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Zooplankton

Zooplankton are microscopic to macroscopic drifting organisms that are pivotal in aquatic ecosystems. They bridge primary producers, like phytoplankton, to higher trophic levels, like salmonids. In freshwater and marine habitats, zooplankton consume phytoplankton, thus controlling their populations and transferring energy up the food web. Salmon, trout, and other salmonids rely on zooplankton as a direct food source, alongside preying on organisms that zooplankton themselves consume, making these drifters essential for the nutrition and survival of many fish species.

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