Salmon are predators, actively hunting and consuming other organisms to fuel their growth and expansive ocean migrations. A predator is any organism that obtains its energy by consuming other living organisms, a definition that applies to salmon across nearly all stages of their life cycle. Their role as a hunter shapes their life stages and their significant place within both freshwater and marine food webs. Understanding this predatory nature requires looking closely at what they target in the ocean and how their diet changes from their freshwater origins to their final return migration.
What Salmon Consume
Adult salmon in the open ocean are capable hunters that consume a wide variety of prey as they accumulate the energy reserves necessary for their return migration. Their diet is primarily carnivorous, focusing on energy-dense marine organisms that allow for rapid growth over their years at sea. Prey often includes smaller forage fish such as capelin, herring, sand lance, and lanternfish, which they pursue in the water column.
Salmon also rely heavily on invertebrates, including crustaceans and cephalopods. This intake includes krill, which are a major food source, as well as squid and polychaete worms. For some species, like Atlantic salmon, crustaceans may represent up to 95% of the food items consumed by number. The bright pink or red color of salmon flesh is a direct result of this diet, caused by the carotenoid pigments abundant in the crustaceans and krill they consume.
Dietary Shifts Across the Life Cycle
The predatory behavior of salmon undergoes a transformation as they transition from freshwater habitats to the open ocean. As newly emerged fry and then as parr, juvenile salmon in streams and rivers are primarily insectivores. Their diet at this stage consists of small aquatic insects, such as the larvae of mayflies, stoneflies, and caddisflies, which they actively hunt.
As they grow into smolts and migrate toward the sea, their physiology changes to adapt to saltwater, and their diet shifts from small invertebrates to increasingly larger prey. Once in the marine environment, the salmon begins its oceanic feeding phase, where the goal is to maximize growth by consuming large quantities of energy-rich fish and crustaceans. This is a move from a diet of tiny stream insects to one of schooling forage fish, reflecting the significant increase in the salmon’s body size and its metabolic needs.
The Salmon’s Role as Prey
While salmon are predators, they simultaneously occupy a position as a major prey item within the larger food web, especially when they return to freshwater to spawn. In the ocean, they are hunted by large marine mammals, including seals, sea lions, and specialized predators like killer whales. Salmon sharks are also significant oceanic predators of adult salmon.
Once adult salmon enter the rivers for their spawning migration, they become a crucial food source that transfers marine-derived nutrients to the terrestrial ecosystem. Land-based predators like black bears and grizzly bears congregate along the riverbanks, catching the migrating fish to build up fat reserves. Avian predators, such as bald eagles, also target the salmon, often scavenging on carcasses or preying on weakened fish. The decomposition of unconsumed salmon carcasses further nourishes the surrounding forest and stream environment.