Coho salmon exhibit a life cycle, transitioning between freshwater and marine environments. This anadromous characteristic influences their dietary needs and feeding strategies. Their diet shifts as they mature, adapting to the diverse prey available in streams, estuaries, and the open ocean.
Freshwater Diet of Young Coho
Young coho salmon begin their lives in freshwater streams and rivers, primarily consuming small, readily available prey. Immediately after emerging from the gravel, they feed on plankton and tiny aquatic invertebrates. As they grow, their diet expands to include a variety of insects, both aquatic and terrestrial.
Juvenile coho are visual feeders, predominantly consuming drifting invertebrates rather than those on the stream bottom. Common aquatic insect larvae, such as midge, caddisfly, mayfly, and stonefly larvae, form a significant portion of their diet. Terrestrial insects, which fall into the water from riparian vegetation, also contribute substantially, often making up 25-50% of their ingested biomass. Small crustaceans can also be a part of their freshwater intake.
Ocean Diet of Adult Coho
Upon migrating to the ocean, coho salmon undergo a significant dietary shift, consuming larger, more energy-rich prey. Their diet in marine waters primarily transitions to various species of small fish. Key forage fish include Pacific herring, sand lance, anchovies, and sardines.
Coho salmon also prey on juvenile salmon of other species, such as pink and chum salmon, and sometimes juvenile sablefish. Beyond fish, their ocean diet incorporates cephalopods like squid and larger crustaceans such as krill and crab larvae. These larger, more mobile prey items support the growth and metabolic demands of adult coho.
Factors Shaping Coho Salmon Diet
Several factors influence the dietary choices of coho salmon throughout their life stages. The availability of prey is a primary determinant, changing with seasons, geographical location, and habitat type. Water temperature can also impact the metabolic rates of coho, affecting their feeding intensity.
Competition with other fish species for food resources can also shape their diet. As coho salmon grow, their metabolic needs increase, prompting them to seek larger and more energetically dense food sources. This natural progression drives the transition from insect-based diets in freshwater to fish and larger invertebrates in the ocean.
The Ecological Role of Coho Feeding
Coho salmon feeding habits play an integral role in the ecosystems they inhabit. As predators, both in freshwater and marine environments, they influence the populations of their prey species. In streams, they help regulate insect and small invertebrate populations.
In the ocean, their consumption of forage fish and invertebrates contributes to marine food web dynamics. When adult coho salmon return to freshwater to spawn and die, their carcasses provide a significant influx of marine-derived nutrients to the riverine ecosystems. This nutrient transfer supports a wide array of organisms, from insects to plants.