The marine phase of the Atlantic salmon (Salmo salar) life cycle, with comparisons to Pacific salmon

Atlantic salmon (Salmo salar) are distributed over large areas in the north Atlantic Ocean. They usually move very quickly from freshwater to oceanic areas, whereas there is considerable variation among Pacific salmon in earl y marine movements. In some areas, Atlantic salmon of exploitable size are sufficiently abundant that commercial high seas fisheries have developed. S uch areas are off west Greenland, where North American and European fish ar e harvested, and in the Norwegian Sea, north of the Faroe Islands, where ma inly European fish are exploited. Atlantic salmon feed on a wide range of l arge crustaceans, pelagic fish, and squid in the marine environment, suppor ting the hypothesis that Atlantic salmon are opportunistic feeders. In the ocean the salmon grow relatively quickly and the sea age when they become s exually mature depends on both genetics and on growing conditions. Natural marine mortality of salmon is highest during the first few months at sea an d the major mortality factor is probably predation. However, marine mortali ty of Atlantic salmon has increased in recent years, apparently correlated with a decline in sea surface temperatures. Similar relationships between e nvironmental conditions and the growth and survival of Pacific salmon have been reported. Atlantic salmon life histories most closely mimic stream-typ e chinook salmon or steelhead trout among the Pacific species. Finally, Atl antic and Pacific salmon return to their home rivers with high precision an d possible mechanisms controlling the oceanic homing migration are presente d and discussed.