Salmon are predators, actively hunting and consuming other organisms throughout most of their lives. These fish occupy a dynamic position within aquatic food webs, shifting their dietary needs and roles as they mature and migrate between freshwater and marine environments. Understanding their feeding habits and the animals that prey upon them offers insights into their significant ecological contributions. This dual role underscores their importance in transferring energy across diverse ecosystems.
Salmon as Predators: Their Diet
Salmon exhibit varied predatory behaviors and diets that adapt to their different life stages and habitats. During their early freshwater fry stage, emerging from the gravel, they begin to actively feed on microscopic plankton, aquatic insects, and larvae found in streams and lakes. As they grow into smolts and prepare for their ocean migration, their diet may expand to include small freshwater crustaceans.
Once salmon migrate to the ocean, their diet broadens considerably, reflecting their growth and increased energy demands. They hunt a wider array of marine organisms. Common prey items include smaller fish such as herring, and various crustaceans like krill and amphipods. Sockeye salmon, for instance, primarily consume zooplankton, krill, and small fish, occasionally adding squid to their diet.
Their predatory success in the ocean is supported by physiological adaptations that allow them to efficiently digest a high-protein, high-lipid diet. For example, Atlantic salmon show increased lipid transport abilities from their gut, indicating efficient nutrient uptake. This rich diet, particularly from krill and plankton, contributes to the vibrant coloration of their flesh due to carotenoids.
Salmon continue to feed and grow substantially during their years in the ocean, with some species spending up to six years maturing before returning to freshwater. During this period, their pursuit of prey like small fish, squid, and shrimp allows them to accumulate the energy reserves needed for their arduous spawning migration. Upon returning to freshwater for spawning, adult salmon cease feeding entirely, relying solely on these stored fat reserves to fuel their reproductive journey.
Salmon as Prey: Their Place in the Food Web
While salmon are effective predators, they also serve as a foundational food source for a wide range of other animals across their life cycle. Their eggs, laid in gravel nests, are vulnerable to predation by various fish, including rainbow trout and Dolly Varden, which often target salmon roe. Juvenile salmon, known as fry and smolts, face numerous predators in freshwater environments.
Birds such as gulls, kingfishers, terns, and American dippers hunt juvenile salmon in rivers and streams. Small land mammals like river otters, mink, and ermine also feed on these young fish. Larger fish, including other salmon species and northern pikeminnow, consume smolts in river systems.
Upon entering the ocean, adult salmon become prey for marine mammals and larger fish. Killer whales, dolphins, seals, and sea lions are predators of adult salmon. Salmon sharks also prey on adult salmon. Chinook salmon, for example, constitute a significant portion of the diet for Southern Resident killer whales.
As salmon return to freshwater to spawn, they encounter a new set of predators. Bears, particularly coastal brown bears, can derive a substantial portion of their annual protein from salmon by catching them in streams or scavenging carcasses. Eagles and wolves are also predators. This extensive predation transfers vast amounts of marine-derived nutrients into freshwater and terrestrial ecosystems, enriching the environment even after the salmon die post-spawning.