Pink salmon are anadromous fish that begin their lives in freshwater rivers, migrate to the Pacific Ocean to grow and mature, and then return to their natal streams to spawn. Their diet undergoes substantial changes as they progress through these different environments and life stages.
Early Life Diet
Pink salmon fry generally migrate to saltwater environments very soon after hatching from the gravel. Once they emerge from their nests and begin their migration, their initial diet primarily consists of plankton and various aquatic insects available in freshwater.
As they enter estuaries and nearshore coastal areas, their food sources expand to include small invertebrates. These often consist of zooplankton, copepods, and amphipods. Some research indicates that juvenile pink salmon in estuarine habitats also feed on minute organisms such as gastropods, cladocerans, and bivalves. The availability of these small prey items in their freshwater and estuarine nursery grounds largely dictates their diet during this early life phase, supporting their initial growth before they move into the open ocean.
Oceanic Diet
Upon reaching the open ocean, pink salmon rapidly shift their diet to larger, more energy-dense prey, which fuels their significant growth. They are active predators, consuming a diverse array of plankton, invertebrates, and small fish. Crustaceans are a major component of their marine diet, particularly euphausiids, commonly known as krill, and copepods. The characteristic pink coloration of their flesh is derived from the pigments present in these crustaceans.
Pink salmon also feed on small fish species, including herring, sand lance, capelin, and pilchards. Squid forms another important part of their diet in the ocean. They exhibit pelagic feeding behavior. This intense feeding period, which typically spans about 18 months in the ocean, is crucial for accumulating the necessary energy reserves for their arduous return migration and spawning.
Factors Influencing Food Choices
Beyond their specific life stage, several environmental and biological factors shape what pink salmon consume. The seasonal availability of prey significantly impacts their diet, with certain fish species or crustaceans becoming more abundant at different times of the year. Geographic location also plays a role, as the types of prey organisms vary across the extensive range of pink salmon in the North Pacific and other areas.
Water temperature can influence both the metabolic demands of the salmon and the distribution and abundance of their prey. Competition for food resources with other fish species, including other salmonids like sockeye and steelhead, can affect their diet and growth rates. Studies indicate that high populations of pink salmon, particularly in odd-numbered years, may lead to reduced growth in sockeye salmon due to shared food sources. Pink salmon are also considered opportunistic feeders, consuming whatever suitable prey is readily available in their environment. Their inherent metabolic requirements for rapid growth in the ocean further drive their robust feeding habits.