Salmon are generally considered to be omnivorous creatures that will eat just about anything. They mostly feast upon plants and/or plant material but have also been known to eat smaller fish from time to time and in some cases may even be found eating their own eggs. What a salmon eats also depends on their age, species and location.
When salmon are young and still residing in freshwater they eat tiny zooplankton and adult invertebrates. However, this too varies upon the species. For instance, young Coho salmon typically feed during the day and have been found to prefer aquatic insects often found at the surfaces of a stream such as mayflies, caddis flies and stone flies. While the young Chinook salmon often prefers plankton it locates off of the river floor as well as terrestrial insects and small crustaceans. Another food source that is not uncommon for young salmon is found on overhanging riparian plants. Larvae and insects feeding upon this vegetation often fall into the stream allowing the salmon to add this to, to their diet.
As a salmon matures and eventually leaves the freshwater to venture off into the ocean, their diet may begin to change. While Chum and Sockeye salmon prefer to continue eating zooplankton and occasionally other small adult fish, other species have been known to begin eating larger fish and aquatic insects. This often includes shrimp, surf smelt, sand lance, crab, herring, amphipods, and krill. When salmon return to freshwater to spawn, feeding efforts virtually stop as a means to conserve more energy for the journey upstream to produce eggs and dig a nest.
While salmon are hungry little creatures that can be found feasting upon plankton and the like, there are plenty of things that are ready to feast upon the salmon. This fish is a hearty staple for humans and also a favorite treat for birds, bears, sea lions, dolphins and other large predatory fish.
Salmon kept on farms are often fed things such as mixtures that contain ground up fish, and/or other organisms from the ocean, as well as food sources with a soy base. Something that should be considered about salmon who are kept on farms is that they may come with a couple of hazards. Salmon that were farmed and fed on vegetable rather than animal proteins may be lacking in Omega-3, which is one of the main reasons salmon are so healthy for a human diet. Farmed salmon are also usually highly susceptible to sea lice, a relentless parasite that has been known to spread throughout the salmon farm in the blink of an eye and that can also be spread into the wild if the farmed salmon have been kept in connecting waters.