Salmon are migratory fish whose diet adapts significantly as they transition between freshwater and saltwater environments. This dietary shift influences their growth, development, and overall survival. Understanding what salmon consume at each stage provides insight into their ecological role and the intricate connections within aquatic ecosystems.
Freshwater Foraging: The Diet of Young Salmon
During their early life stages, young salmon, known as fry and parr, inhabit freshwater rivers, streams, and lakes. These juvenile fish primarily forage on aquatic invertebrates found in their natal waters. Their diet includes insect larvae and nymphs, such as mayflies, caddisflies, stoneflies, blackflies, and midges. Small crustaceans like amphipods, scuds, and cladocerans also form a significant part of their diet.
Young salmon are opportunistic feeders, consuming what is readily available in their immediate environment. They also eat zooplankton and terrestrial insects that fall into the water. As they grow larger, their diet can expand to include small fish and occasionally salmon eggs. This diverse foraging behavior supports their rapid growth before their migration to the ocean.
Oceanic Feasting: The Diet of Adult Salmon
Upon migrating to the ocean, adult salmon undergo a substantial dietary shift, becoming predators of larger prey. Their oceanic diet consists primarily of smaller fish species, abundant in their feeding grounds.
- Capelin
- Atlantic herring
- Sand lance
- Barracudina
- Lanternfish
- Smelt
- Alewives
- Small mackerel
Crustaceans, particularly euphausiids (krill), and various types of shrimp and amphipods, also constitute a significant portion of their diet. Some salmon species also consume squid and polychaete worms. The specific diet can vary by salmon species and prey availability.
Beyond Sustenance: How Diet Shapes Salmon
Salmon’s diet influences their physical characteristics and biological processes. The distinctive pink or red coloration of salmon flesh comes from carotenoids, particularly astaxanthin, found in the crustaceans they consume. Salmon cannot produce these pigments, so they accumulate them directly from their prey. The intensity of their flesh color can vary based on the proportion of carotenoid-rich crustaceans in their diet, with sockeye salmon often displaying a deeper red due to their heavy consumption of zooplankton.
Abundant food sources in the ocean allow salmon to accumulate significant fat reserves, which are important for their return migration to freshwater spawning grounds. Adult salmon typically cease feeding upon entering freshwater for spawning, relying on these stored energy reserves to complete their reproductive journey. This stored energy, influenced by their oceanic diet, impacts their overall health, growth rate, reproductive success, and nutritional value for other species.