Salmon are powerful, migratory fish known for their incredible journeys between freshwater rivers and the saltwater ocean. These carnivores are impressive predators that feed across vast aquatic environments. Understanding their diet provides insight into their complex lifecycle and the ecosystems they inhabit. Salmon do consume worms, but the answer depends on the specific type of worm and the fish’s life stage.
Worms as a Food Source
Salmon do eat worms, but consumption varies based on the type of worm and the context. In the ocean, adult salmon consume polychaete worms, which are marine segmented worms, as a small part of their overall diet. Wild salmon often consume worms opportunistically in freshwater, especially when aquatic invertebrates are abundant.
A distinction exists between aquatic worms, which are a natural food item, and terrestrial earthworms. Earthworms are not typically found in the aquatic environment but are washed into rivers and streams by rain, where salmon readily consume them. Juvenile coho salmon, for example, have been observed feeding on earthworms in Alaskan streams, which provides a significant and energy-dense food source. One earthworm can provide the energy equivalent of hundreds of smaller aquatic insect larvae.
The presence of worms in salmon is also related to parasitic nematodes, often called roundworms or anisakids, which use salmon as an intermediate host. These parasites are acquired when salmon eat infected prey, such as small crustaceans or fish, and are found in the muscle tissue of the salmon. While not a food source, the high prevalence of these worms shows a direct link to a worm-related organism in the salmon’s biology.
Diet Variation Across Life Stages
The likelihood of a salmon eating a worm is influenced by the fish’s life stage and habitat. Juvenile salmon, including fry and parr, spend their early years in freshwater rivers and streams where worms are a regular component of their diet. They feed predominantly on zooplankton and aquatic invertebrates, including aquatic worms and insect larvae.
As young fish mature and migrate to the ocean, their diet shifts to suit the saltwater environment. For large, ocean-dwelling adults, worms become a negligible part of their food intake, replaced by larger, more mobile prey. When adults return to their natal rivers to spawn, they typically cease feeding entirely, relying on stored fat reserves for the journey. However, adult salmon may still feed opportunistically during their upriver migration, making them susceptible to consuming bait used by anglers.
Broader Natural Diet Composition
While worms are part of the diet, especially for younger salmon, the bulk of their natural food intake consists of other organisms. In freshwater streams, juvenile salmon primarily consume aquatic insects, including the larvae of mayflies, caddisflies, and stoneflies, alongside small crustaceans. They also consume terrestrial insects that fall into the water, such as grasshoppers.
Once they reach the ocean, the diet of adult salmon expands significantly to include energy-dense prey necessary for rapid growth. This marine diet is largely composed of smaller forage fish like herring, sand lances, and mackerel, which provide high caloric intake. They also consume vast quantities of crustaceans, such as amphipods, shrimp, and krill, which are a major food source in the open ocean. This diverse diet supports the salmon’s growth before they return to freshwater.