Wild salmon are opportunistic predators whose diets range from tiny copepods to squid and smaller fish, depending on the species and life stage. Farmed salmon, by contrast, eat manufactured pellets whose recipe has changed dramatically over the past few decades, shifting from almost entirely marine-derived ingredients to a mix dominated by plant proteins and oils. The gap between a wild salmon’s natural menu and what an aquaculture operation delivers has consequences for the fish’s health, the nutritional profile of the fillet on your plate, and the environmental footprint of the industry.
What Wild Salmon Eat at Sea
Not all salmon eat the same things. Among Pacific species, Chinook and coho salmon are the most carnivorous, feeding primarily on fish. Adult Chinook in the Gulf of Alaska derive roughly three-quarters of their diet from other fish, with krill and squid making up about ten percent each. Coho are similar, getting over half their calories from fish prey. Sockeye, chum, and pink salmon take a different approach: they are planktivores, filling up on tiny crustaceans like copepods and krill rather than hunting other fish.1Deep Sea Research Part II: Topical Studies in Oceanography. Feeding ecology of salmon in eastern and central Gulf of Alaska Historical stomach-content surveys from the Strait of Georgia confirmed this split, finding that calanoid copepods made up between 42 and 63 percent of what sockeye, chum, and pink salmon ate, while Chinook and coho stomachs were dominated by insects, small fish, and decapods.2Oxford Academic. Historical Diets of Forage Fish and Juvenile Pacific Salmon in the Strait of Georgia, 1966–1968
Atlantic salmon have their own dietary preferences. Off the west coast of Greenland, where many North American Atlantic salmon spend their feeding years, capelin and sand lance are major prey items, along with boreoatlantic armhook squid and hyperiid amphipods. The exact balance depends on location: capelin dominated stomachs at some sampling sites, while squid dominated at others.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 Young Atlantic salmon entering the ocean as post-smolts depend heavily on fish larvae and krill, and those that managed to find these energy-rich prey items grew significantly faster than post-smolts feeding on less nutritious alternatives.4Fisheries Research. Diet analysis of Atlantic salmon (Salmo salar) post-smolts after the ecological regime shift in the Northeast Atlantic
How Diet Changes with Life Stage
A salmon’s menu transforms as it grows. Juvenile salmon living in freshwater streams eat aquatic and terrestrial insects, zooplankton, and tiny invertebrates. Once they enter the ocean, the youngest fish eat whatever is small enough for them to capture: copepods, amphipods, and larval fish. As they grow, the piscivorous species like Chinook graduate to bigger prey. Even juvenile Chinook in the ocean already get about half their diet from fish, but they rely more on squid than adults do.1Deep Sea Research Part II: Topical Studies in Oceanography. Feeding ecology of salmon in eastern and central Gulf of Alaska
One particularly interesting difference shows up in river deltas, where wild-born and hatchery-raised juveniles can be compared side by side. In the Sacramento–San Joaquin River Delta, wild juvenile Chinook derived 24 to 31 percent of their diet from terrestrially sourced prey such as land-dwelling insects that fall into the water. Hatchery-raised juveniles released into the same habitat only got 2 to 8 percent of their diet from terrestrial sources, apparently because they had spent their early life eating formulated feed and were less adept at exploiting the full range of natural food available.5Transactions of the American Fisheries Society. Integrated Diet Analyses Reveal Contrasting Trophic Niches for Wild and Hatchery Juvenile Chinook Salmon in a Large River Delta That gap matters because terrestrial insects are a calorie-rich food source in river habitats, and fish that ignore them may grow more slowly.
Why Salmon Stop Eating Before They Spawn
One of the more dramatic chapters of a salmon’s feeding life is its end. Pacific salmon famously stop eating entirely as they begin their spawning migration, burning through fat and muscle to fuel the journey upstream. Atlantic salmon do the same, though they can theoretically survive to spawn again. Sockeye salmon tracked during their 2006 migration stopped feeding after passing the Queen Charlotte Islands, still roughly 850 kilometers from the mouth of the Fraser River. Their bodies shifted into starvation mode almost immediately, breaking down proteins and dialing back genes related to muscle contraction.6PubMed. Salmon spawning migration: metabolic shifts and environmental triggers Everything they need to complete the journey, fight for mates, and produce eggs comes from the energy they stored during their ocean feeding years. This is why the quality of ocean prey matters so much: a salmon that fed well at sea arrives at the spawning grounds with more reserves.
What Farmed Salmon Eat
Farmed Atlantic salmon eat extruded pellets, and the composition of those pellets has been one of the biggest changes in aquaculture over the last thirty years. In 1990, about 90 percent of Norwegian salmon feed came from marine ingredients, mainly fishmeal and fish oil ground from wild-caught forage fish like anchovies, sardines, and herring. By 2013, marine ingredients made up less than 30 percent of the feed.7Aquaculture. Utilisation of feed resources in production of Atlantic salmon (Salmo salar) in Norway The rest is now largely plant-based: soy protein concentrate, rapeseed oil, wheat gluten, corn gluten, and fava bean meal, among others.
This shift was driven by economics and sustainability concerns. Harvesting millions of tonnes of wild forage fish to feed farmed fish raised an obvious ecological contradiction. A useful metric for tracking this is the “fish in, fish out” ratio, which estimates how many kilograms of wild fish go into producing one kilogram of farmed fish. For salmonids, that ratio dropped from about 3.8 in 1995 to roughly 1 by 2020, meaning the industry now uses about as much wild fish as it produces farmed fish, at least on a weight basis.8Aquaculture. Fish as feed: Using economic allocation to quantify the Fish In : Fish Out ratio of major fed aquaculture species
The Trouble with Plant-Based Feed
Replacing marine ingredients with plant proteins is not as simple as swapping one powder for another. Soybean meal, one of the cheapest and most abundant plant protein sources, causes intestinal inflammation in Atlantic salmon and Chinook salmon. Within a week of being fed a soybean-meal-based diet, both species developed thickening of the intestinal wall and infiltration of immune cells, a condition researchers call soybean-meal-induced enteritis. The inflammation worsened over time. Interestingly, pink salmon seemed immune to the problem, suggesting the sensitivity is species-specific.9Aquaculture. Soybean meal-induced enteritis in Atlantic salmon (Salmo salar) and Chinook salmon (Oncorhynchus tshawytscha) but not in pink salmon (O. gorbuscha)
The enteritis involves the loss of normal cell structure in the distal intestine, shrinkage of the intestinal folds, and an influx of inflammatory cells. One research group found that feeding salmon a bacterial protein grown on natural gas instead of soybean meal prevented the inflammation entirely, suggesting the problem lies with specific plant compounds rather than with non-marine proteins in general.10The Journal of Nutrition. Bacteria Grown on Natural Gas Prevent Soybean Meal-Induced Enteritis in Atlantic Salmon The industry has largely worked around the issue by using soy protein concentrate, which has the problematic compounds removed, rather than raw soybean meal. But the episode illustrates why feed reformulation requires careful biological testing, not just nutritional bookkeeping.
Reducing fishmeal also affects growth if the replacement is not formulated well. One trial found that cutting fishmeal from 35 percent to 15 percent of the diet using plant proteins slowed growth in young Atlantic salmon. However, partially replacing that fishmeal with fish protein hydrolysate, a more digestible form of fish-derived protein, restored normal growth rates even when fishmeal was reduced to just 6 percent of the dietary protein.11PubMed Central. Replacing fishmeal with plant protein in Atlantic salmon (Salmo salar) diets by supplementation with fish protein hydrolysate
Novel Ingredients on the Horizon
The search for sustainable protein and oil sources has pushed researchers well beyond conventional crops. Some of the more promising alternatives include black soldier fly larvae, tunicates (sea squirts), and microalgae. A trial that tested all of these found that each one supported high growth rates and good feed efficiency in Atlantic salmon, and a combined diet that removed fishmeal and fish oil entirely still produced healthy fish with normal gut function.12PubMed Central. Atlantic Salmon (Salmo salar) Performance Fed Low Trophic Ingredients in a Fish Meal and Fish Oil Free Diet
Fish oil replacement is arguably the harder problem, because it is the main source of the omega-3 fatty acids EPA and DHA that make salmon nutritionally valuable to humans. Algae-derived oil is emerging as a viable substitute. A trial that fully replaced fish oil with oil from the microalga Schizochytrium found that the resulting fillets had higher DHA levels than fillets from salmon fed conventional fish oil, partly because the algae oil had a naturally high ratio of DHA to EPA.13Aquaculture. Full replacement of fish oil with algae oil in farmed Atlantic salmon (Salmo salar) – Debottlenecking omega 3 If these ingredients can be produced at industrial scale and competitive cost, future farmed salmon could deliver omega-3 levels as high or higher than today’s fish without any wild fish in the supply chain.
How Diet Shapes the Fillet
The most visible way diet affects salmon is flesh color. Wild salmon get their pink-to-red hue from astaxanthin, a carotenoid pigment they accumulate by eating krill, shrimp, and other crustaceans. Farmed salmon do not encounter these prey naturally, so their feed includes either synthetic astaxanthin or astaxanthin from natural sources like the alga Haematococcus pluvialis, the yeast Phaffia rhodozyma, or the bacterium Paracoccus carotinifaciens. Trials have shown that all three natural sources produce flesh color and antioxidant status comparable to synthetic astaxanthin.14Aquaculture. Effect of natural carotenoids obtained from Haematococcus pluvialis, Paracoccus carotinifaciens, and Phaffia rhodozyma on flesh pigmentation and related biochemical mechanisms in Atlantic salmon (Salmo salar L.) Without any added astaxanthin, farmed salmon flesh would be a pale gray, a fact that surprises many consumers.
Wild salmon flesh tends to contain substantially more astaxanthin than farmed salmon, roughly ten times more in one analysis. Cooking reduces astaxanthin content in both, but the starting gap is large enough that wild salmon retains a significant advantage in this department even after being cooked.15PubMed Central. Bioaccessibility and intestinal cell uptake of astaxanthin from salmon and commercial supplements
Fat content and omega-3 levels are also diet-dependent. Farmed salmon generally end up fattier than wild salmon because they eat energy-dense pellets and swim less. The composition of that fat reflects whatever oils are in the feed. As fish oil was replaced with rapeseed and soy oil, the ratio of omega-3 to omega-6 fatty acids in farmed salmon shifted. Feed composition and feeding strategy can readily manipulate the levels of fat and protein in the fillet, though sensory traits like flavor and texture are less responsive to dietary changes.16Aquaculture Research. Quality of farmed salmonids with emphasis on proximate composition, yield and sensory characteristics
Contaminants in Wild Versus Farmed Salmon
A common assumption is that wild salmon must be “cleaner” than farmed. The reality is more nuanced, and in some respects the opposite is true. Dioxins, dioxin-like PCBs, mercury, and arsenic were all found at roughly three times higher concentrations in wild Atlantic salmon compared to farmed Atlantic salmon in a Norwegian study, although every sample from both groups fell well below European Union safety limits.17PubMed Central. An Update on the Content of Fatty Acids, Dioxins, PCBs and Heavy Metals in Farmed, Escaped and Wild Atlantic Salmon (Salmo salar L.) in Norway A separate analysis confirmed the pattern: dioxins, PCBs, organochlorine pesticides, and mercury were all higher in wild salmon, as were the essential minerals selenium, copper, zinc, and iron.18PubMed. Lower levels of Persistent Organic Pollutants, metals and the marine omega 3-fatty acid DHA in farmed compared to wild Atlantic salmon (Salmo salar)
The explanation is straightforward: wild salmon eat prey from the open ocean, where persistent organic pollutants have accumulated in the food chain for decades. Farmed salmon eat manufactured feed whose ingredients are screened and regulated. The trade-off, however, is that wild salmon also had higher levels of DHA, the omega-3 fatty acid most associated with cardiovascular and brain health benefits. EPA levels were comparable between the two groups. So the “which is healthier” question does not have a clean answer. Wild salmon give you more DHA and more astaxanthin but also somewhat more persistent pollutants. Farmed salmon give you more total fat, comparable EPA, lower contaminant loads, and a DHA level that depends heavily on what the farm uses for oil in the feed.
Functional Feeds and Disease Management on Farms
Feed on salmon farms does more than just deliver calories and nutrients. Increasingly, it is used as a tool to manage disease and parasites. Sea lice are one of the most economically damaging parasites in Atlantic salmon aquaculture, and “functional feeds” supplemented with plant-derived immunostimulants or essential oils are being tested as part of integrated pest management. One trial found that a diet containing a mix of plant-based extracts reduced sea lice counts by about 20 percent compared to a standard diet.19Aquaculture Nutrition. Reducing sea lice (Lepeophtheirus salmonis) infestation of farmed Atlantic salmon (Salmo salar L.) through functional feeds That is not enough to replace other lice treatments, but it adds another layer of defense.
Research into the molecular mechanisms behind these functional diets has shown that plant-derived feed additives can activate immune-related genes in salmon, potentially priming the fish’s own defenses against parasites.20PubMed. Transcriptome immunomodulation of in-feed additives in Atlantic salmon Salmo salar infested with sea lice Caligus rogercresseyi The concept of “medicinal food” for fish is still early-stage, but it reflects how far salmon feed has moved beyond simple nutrition. Feed is now a delivery mechanism for pigments, immune boosters, and potentially even antiparasitic compounds, all engineered into a single pellet.
Climate Change and What Wild Salmon Will Eat in the Future
Wild salmon diets are not static. They shift with ocean conditions, and climate change is reshaping the prey base in ways that could ripple up through salmon populations. One consequence that researchers have flagged is a decline in the availability of long-chain omega-3 fatty acids in aquatic food webs. These fatty acids are produced primarily by cold-water algae and concentrate up the food chain through zooplankton and small fish. As ocean temperatures rise and algal communities shift, the total supply of these essential fats in the ocean may drop.21Functional Ecology. Climate change‐induced deprivation of dietary essential fatty acids can reduce growth and mitochondrial efficiency of wild juvenile salmon
For juvenile salmon, this is not a minor nutritional detail. Omega-3 fatty acids are critical for cell membrane function and energy metabolism. When wild juvenile salmon are deprived of these fats in their diet, their growth slows and their mitochondria, the cellular machinery that generates energy, become less efficient. If the base of the food web produces fewer of these fats, young salmon entering the ocean may grow more slowly and have less energy for the journey ahead. Combined with other stressors like warming rivers, habitat loss, and changing prey distributions, a less nutritious ocean could put additional pressure on wild populations that are already struggling in many regions.