Salmon are active, often voracious predators for most of their lives, and at the same time they rank among the most heavily hunted animals in the ecosystems they move through. What a salmon eats depends on its species, its age, and whether it is in fresh water or the open ocean, but the short answer is that salmon spend most of their lives chasing and consuming other organisms. Their role as prey is equally dramatic: everything from killer whales and seals to wolves, eagles, bears, and even non-native bass targets them at different life stages. This dual identity as both hunter and hunted makes salmon one of the more ecologically influential groups of fish on the planet.
What Salmon Eat in the Ocean
Once salmon leave their home rivers and head to sea, they begin feeding aggressively, and what they eat splits roughly along species lines. A large study of over 6,500 salmon stomachs from the Gulf of Alaska found that Chinook and coho salmon are primarily fish-eaters: adult Chinook stomachs contained about 75% fish by diet weight, with the rest split between small crustaceans called euphausiids and squid. Coho were similarly piscivorous, with fish making up around 55% of their diet. Sockeye, chum, and pink salmon, by contrast, lean heavily toward plankton and small invertebrates rather than fish.1Deep Sea Research Part II: Topical Studies in Oceanography. Feeding ecology of salmon in eastern and central Gulf of Alaska
The specific prey fish that Chinook and coho target vary by region and season. In the Salish Sea, Pacific herring turned up as a key prey item across all areas and throughout the year. Other forage fish like Pacific sand lance, northern anchovy, and lanternfish (myctophids) showed up more in certain places or at certain times of year.2Fisheries Oceanography. Using the Diet Composition of Adult Chinook Salmon to Understand the Regional Structure of Salish Sea Forage Communities Atlantic salmon feeding off the coast of Greenland follow a similar pattern of fish-heavy diets, with capelin dominating in some areas and squid dominating in others. Amphipods and sand lance also showed up as important supplementary prey.3Fisheries Oceanography. Assessing the diet of North American Atlantic salmon (Salmo salar L.) off the West Greenland coast using gut content and stable isotope analyses
This diet flexibility matters. Salmon do not rigidly depend on a single prey species. When conditions change, they switch. During a five-year study in Southeast Alaska that spanned both warm and cool ocean phases, juvenile salmon shifted among zooplankton, euphausiids, and fish prey depending on what was available. In one unusual summer, euphausiids dominated the diet of all four species studied, including coho, which normally eat fish. Researchers concluded that within the range of conditions they observed, juvenile salmon managed to meet their energy needs by switching to alternative prey.4Progress in Oceanography. Trophic responses of juvenile Pacific salmon to warm and cool periods within inside marine waters of Southeast Alaska
What Juvenile Salmon Eat in Fresh Water
Before they ever reach the ocean, young salmon spend weeks to years in rivers and estuaries, and during that phase their diet looks very different. River-dwelling juvenile salmon are drift feeders: they hold position in a current and snap up insects and invertebrates carried past them by the water. Classic behavioral research showed that young Atlantic salmon respond to drifting prey based on size relative to the fish’s own body. They react most strongly to items between roughly 2.5% and 7% of their body length, with the upper and lower limits set by jaw width and gill raker spacing.5Animal Behaviour. Behavioural aspects of predation by juvenile Atlantic salmon (Salmo salar L.) on particulate, drifting prey
The types of invertebrates available to juvenile salmon can be surprisingly diverse. A field study of tidal freshwater wetlands in the Pacific Northwest collected over 63,000 individual invertebrates being transported out through tidal creeks, representing 14 major groups. Insects made up 46% of all individuals, with flies (dipterans) and true bugs (hemipterans) together accounting for the vast majority of the insect haul. Amphipods, spiders, mites, and various small crustaceans rounded out the rest.6PLOS ONE. Export of macroinvertebrate prey from tidal freshwater wetlands provides a significant energy subsidy for outmigrating juvenile salmon These wetland exports represent a significant energy subsidy for young salmon heading out to sea: the prey drifts right to them.
Salmon as Competitors
Salmon are not the only mid-level predators in the ocean. In the sub-Arctic North Pacific, squid and lanternfish occupy similar feeding niches and go after similar prey. Stable isotope analysis revealed substantial diet overlap between sockeye salmon and certain squid species, and chum salmon showed considerable overlap with multiple competitors in different regions. The overall biomass of these non-salmon competitors may actually exceed that of Pacific salmon, which means competition for prey could constrain salmon growth and survival in ways that fisheries managers have not always accounted for.7PubMed Central. The Hunger Games: Stable Isotopes Indicate Winter Inter-Guild Competition for Resources by Marine Meso-Predators in the Sub-Arctic North Pacific
Marine Mammals That Eat Salmon
For all their predatory capabilities, salmon sit squarely in the middle of the food web. The most famous salmon specialists are resident killer whales, whose diets are so dominated by salmon that they are essentially salmon-obligate predators. But pinnipeds (seals and sea lions) consume enormous quantities too. A meta-analysis of marine mammal diets in the Northeast Pacific found that harbor seals and Steller sea lions both had intermediate but meaningful levels of salmon in their diets.8Ecological Informatics. A century of Chinook salmon consumption by marine mammal predators in the Northeast Pacific Ocean
What has changed dramatically in recent decades is the sheer scale of pinniped predation. Estimates from Washington State’s inland waters show that the annual biomass of Chinook salmon consumed by pinnipeds rose from 68 metric tons in 1970 to 625 metric tons by 2015. When researchers converted juvenile Chinook eaten by seals and sea lions into adult equivalents, they found that by 2015, pinnipeds were consuming double what resident killer whales ate and six times more than the combined commercial and recreational fishing catch.9Canadian Journal of Fisheries and Aquatic Sciences. Estimates of Chinook salmon consumption in Washington State inland waters by four marine mammal predators from 1970 to 2015 This shift is partly a conservation success story (pinniped populations have rebounded since receiving legal protection) and partly a conservation problem (more seals eating more of the same salmon that endangered killer whales depend on).
Modeling work covering 1979 to 2020 confirmed this broader pattern: total mortality of Chinook salmon returning to the Salish Sea stayed roughly constant over that period, but the composition changed. Fishing mortality dropped substantially, by 60% to 84% depending on the specific population group. Meanwhile, predation mortality from marine mammals increased to fill the gap.10PLOS ONE. The combined effects of predation, fishing, and ocean productivity on salmon species targeted by marine mammals in the northeast Pacific So the fish that humans stopped catching did not necessarily survive. They were eaten by something else.
Bears, Wolves, and Other Terrestrial Predators
When salmon return to rivers to spawn, they become available to a different set of predators entirely. Brown bears and grizzlies feasting on spawning salmon are one of the most iconic wildlife spectacles in North America, but wolves also take advantage of salmon runs in ways that reshape entire food webs. In coastal British Columbia, researchers found that when salmon became available in autumn, wolves shifted their diets away from deer. Salmon appeared in 40% of wolf fecal samples across the population, and in some groups that figure reached 70%. The correlation was strong: the more salmon wolves ate, the less deer they consumed.11PubMed Central. Spawning salmon disrupt trophic coupling between wolves and ungulate prey in coastal British Columbia
Wolves are surprisingly effective salmon fishers. Detailed observations at one British Columbia river documented wolves capturing salmon at a pooled rate of about 21.5 fish per hour, with a success rate of roughly 39% per attempt. Wolves oriented themselves facing upstream to detect approaching fish and, interestingly, often ate only the heads. Researchers speculated this was either for nutritional reasons (salmon brains are energy-dense) or to avoid parasites concentrated in the body cavity. Salmon offer wolves a spatially concentrated, calorie-rich food source with much lower injury risk than hunting large mammals.12Canadian Journal of Zoology. Foraging behaviour by gray wolves on salmon streams in coastal British Columbia
Bald eagles, river otters, mink, and various bird species also feed heavily on salmon during spawning runs, though the research literature is deepest on bears and wolves because of their outsized ecological effects.
Non-Native Fish Predators
In freshwater habitats, some of the biggest threats to juvenile salmon come from fish that were never supposed to be there. A study of Columbia Basin reservoirs found that non-native warmwater game fish, including bass and crappie, preyed more heavily on Chinook salmon fry in spring than native predators like northern pikeminnow.13Ecosphere. Damming salmon fry: evidence for predation by non‐native warmwater fishes in reservoirs Reservoirs created by dams provide the warm, slow-moving habitat that these introduced species prefer, essentially creating predation traps for young salmon that have to pass through on their way to the ocean.
Non-native brook trout present a more nuanced picture. In an Idaho stream study, differences in feeding behavior and diet suggested minimal direct competition for prey between brook trout and juvenile Chinook. However, individual Chinook fed less actively as encounter rates with other fish increased. Interestingly, despite being the invading species, brook trout sometimes lost aggressive encounters to slightly smaller Chinook. The researchers suggested that frequent same-species interactions among the Chinook themselves may have been more limiting to growth than the presence of brook trout.14Ecology of Freshwater Fish. Effects of non‐native brook trout (Salvelinus fontinalis) on threatened juvenile Chinook salmon (Oncorhynchus tshawytscha) in an Idaho stream
Egg Eating and Cannibalism
Salmon even prey on each other, at least in a limited sense. It was long assumed that Pacific salmon stop feeding entirely once they return to spawn, but research has overturned that belief. Analysis showed that about 13% of spawning salmon stomachs contained eggs, with feeding rates estimated at up to 14 eggs per day. The energy content of that egg consumption was estimated at up to 52 kilojoules daily, a modest but real caloric intake during a phase of life when salmon are supposedly not eating at all.15Canadian Journal of Fisheries and Aquatic Sciences. Egg consumption in mature Pacific salmon (Oncorhynchus spp.)
In aquaculture settings, cannibalism among salmon can be a more significant issue. Cannibalism in cultured fish is influenced by gape size limitations, growth differences within a cohort, and the inability to escape through habitat separation or migration, which fish in the wild can do. The intensity varies by species and life stage, and environmental conditions in the hatchery play a role in how often it occurs.16Aquaculture Research. Dynamics of intracohort cannibalism in cultured fish
Hatchery Salmon and Predator Avoidance
How well salmon handle predators depends partly on whether they grew up in the wild or in a hatchery. This matters because billions of hatchery-raised salmon are released into rivers every year across the Pacific and Atlantic, and their survival after release is consistently lower than that of wild fish. One reason is that hatchery fish are worse at recognizing and escaping predators.
Experiments comparing wild-caught, first-generation hatchery (F1), and second-generation hatchery (F2) Atlantic salmon showed a clear gradient. Wild fish exhibited the strongest antipredator response to alarm cues, F2 hatchery fish showed the weakest response, and F1 fish fell in between. The difference between wild and F1 fish pointed to the role of experience: wild fish had learned from real encounters. The further decline from F1 to F2 suggested that even a single full generation of hatchery rearing can select for maladaptive predator responses.17Canadian Journal of Fisheries and Aquatic Sciences. Differences in antipredator behaviour between wild and hatchery-reared juvenile Atlantic salmon (Salmo salar) under seminatural conditions
The flip side is also true. After release into the wild, captive-reared salmon show lower foraging rates on natural prey and impaired ability to avoid predators, both contributing to lower survival compared with wild-born fish.18Ecology of Freshwater Fish. Wild origin and enriched environment promote foraging rate and learning to forage on natural prey of captive reared Atlantic salmon parr So hatchery salmon are weaker on both sides of the predator-prey equation: they hunt less effectively and they get eaten more often.
How Salmon Feed the Forests
The story of salmon as prey does not end when the predator finishes eating. When bears drag salmon carcasses into the woods, when wolves leave half-eaten heads on the riverbank, and when spawned-out salmon die and decompose in streams, marine-derived nutrients flow into freshwater and terrestrial ecosystems. Salmon spend years accumulating nitrogen, phosphorus, and carbon from the ocean, and they bring those nutrients back in their bodies.
Measurements in an Alaskan stream showed that spawning salmon increased stream nitrogen concentrations by an average of 190% and phosphorus by 390% above pre-salmon values. Algae and other primary producers incorporated some of these nutrients into their tissues, effectively converting ocean productivity into freshwater productivity.19PubMed. Marine-derived nutrients, bioturbation, and ecosystem metabolism: reconsidering the role of salmon in streams The effect extends beyond water chemistry. Experimental work showed that salmon carcasses added to streams caused invertebrate consumers to shift their diet away from dead leaves and toward the higher-nutrient carcass material, temporarily slowing the breakdown of leaf litter.20PubMed Central. Impacts of marine-derived nutrients on stream ecosystem functioning
A synthesis of 31 studies examining relationships between Pacific salmon and surrounding ecosystems found 172 documented ecological relationships. The most commonly studied link was between salmon density and the presence of marine-derived isotopes in nearby organisms (40% of the relationships), essentially tracking how salmon nutrients spread through the food web. Far fewer studies quantified broader ecosystem-level responses, suggesting that the full scope of how salmon shape the ecosystems around them is still being mapped out.21PubMed Central. Relationships between Pacific salmon and aquatic and terrestrial ecosystems: implications for ecosystem-based management
Parasites as a Record of What Salmon Eat
One unexpected way researchers learn about salmon diets is through the parasites salmon carry. Many intestinal parasites are trophically transmitted, meaning the salmon pick them up by eating infected prey. The specific parasites a salmon hosts can therefore serve as a biological record of what it has been eating and where.
A study of Atlantic salmon post-smolts across four Norwegian fjords found highly variable parasite communities that reflected local feeding conditions. In northern fjords, where salmon fed more heavily on amphipods and fish larvae, infection rates with certain parasites were much higher (67–85% for trematodes, for instance) than in a southern fjord where those prey were less common (19% for the same trematodes). Researchers proposed that parasites of both freshwater and marine origin could work as bio-indicators of salmon feeding and migration patterns.22Journal of Fish Biology. Trophically transmitted parasites in wild Atlantic salmon post‐smolts from Norwegian fjords
How Marine Heatwaves Reshape the Menu
Climate change is altering the ocean conditions that salmon hunt in, and researchers have been watching to see whether rising temperatures affect what salmon eat or how well they grow during early marine life. In Southeast Alaska, data spanning years that included marine heatwaves showed that sea surface temperature anomalies were the primary driver of interannual variability in salmon stable isotope signatures, which reflect what they have been eating. Warmer conditions appeared to increase terrestrial nutrient input into nearshore waters. Yet despite measurable ecosystem shifts, fish body condition did not show a significant relationship with temperature anomalies. The complex topography and oceanography of the region may temporarily buffer juvenile salmon from the worst effects of heatwaves before they migrate into the open Gulf of Alaska.23Marine Ecology Progress Series. Juvenile salmon body condition in Southeast Alaska is buffered during marine heatwaves
That buffering capacity has limits, of course. Reduced return rates for some salmon populations in the Gulf of Alaska have been linked to climate variability, and the longer-term effects of repeated heatwaves on prey availability are still being studied. The picture so far suggests that salmon are flexible predators capable of switching prey when conditions shift, but that flexibility may not be enough if the changes to their prey base become too extreme or too sustained.