Salmon are primarily considered carnivores, meaning the bulk of their natural diet consists of other animals. A salmon’s nutrition depends heavily on its age and location, shifting from small invertebrates in rivers to larger fish in the open ocean during its anadromous life cycle. This natural feeding behavior stands in sharp contrast to the manufactured diets of commercially farmed salmon.
Understanding Biological Dietary Classifications
Animals are categorized into dietary groups based on their primary source of nutrition, which provides the necessary context for analyzing the salmon’s diet. A carnivore is an animal that obtains its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue. These animals typically possess specialized digestive systems adapted to efficiently process protein and fat from meat.
An omnivore, by contrast, consumes both animal and plant matter, exhibiting a flexible diet that allows them to take advantage of various food sources. Herbivores complete the main classification by feeding solely on plant material. While some carnivores are considered “obligate,” relying entirely on animal flesh, others, like the salmon, can be “facultative,” meaning they may consume non-animal matter under certain circumstances.
The Natural Diet of Wild Salmon Across Life Stages
The natural diet of wild salmon evolves significantly as the fish transitions from a freshwater juvenile to a mature ocean dweller. This anadromous life cycle creates a dietary profile dependent on available prey in two entirely different ecosystems.
In the freshwater phase, young salmon, known as fry and smolt, feed on a variety of small organisms. Their diet in rivers and streams is composed mainly of aquatic insects, insect larvae, zooplankton, and small crustaceans like amphipods. They may incidentally consume some plant material, such as algae, which provides a slightly more omnivorous slant to their early diet.
Once salmon migrate to the ocean, their feeding habits solidify their classification as carnivores. Adult salmon become active predators, consuming a diet rich in high-fat and high-protein marine life. Their oceanic prey includes smaller schooling fish, such as herring, sand eels, and capelin, along with squid and larger crustaceans.
The distinctive pink or red color of wild salmon flesh is a direct result of this marine diet. It comes from the consumption of crustaceans, which contain high levels of the natural pigment astaxanthin, a carotenoid stored in the muscle tissue.
Diet in Commercial Aquaculture
The diet of commercially farmed salmon differs substantially from their wild counterparts, relying on manufactured pellets designed for rapid growth. Historically, these pellets were predominantly composed of fishmeal and fish oil, derived from small, wild-caught forage fish. This composition closely mimicked the natural, marine-based carnivorous diet of adult salmon.
However, over the last few decades, the composition of farmed salmon feed has shifted dramatically due to sustainability concerns and rising costs of wild fish stocks. Modern feed contains an increasingly large proportion of plant-based ingredients, such as soy protein concentrates, wheat, and corn gluten. In 2020, for example, the feed for Norwegian farmed salmon consisted of approximately 73% plant-based ingredients, with only 22% marine components.
This shift toward terrestrial ingredients has created nutritional challenges, as a salmon’s digestive system is not naturally adapted to process large amounts of plant matter. Untreated soy, for instance, must be processed into a protein concentrate to prevent intestinal inflammation in the fish. Furthermore, the reliance on plant oils, rather than marine oils, has been linked to a decline in the beneficial Omega-3 fatty acid content in the final farmed product.
To ensure farmed salmon have the expected red color, synthetic or naturally derived carotenoids are added to the feed. This ensures the flesh color meets consumer expectations, providing a visual example of how the artificial diet influences the physical characteristics of the commercially raised fish.