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 early marine movements. In some areas, Atlantic salmon of exploitable size are sufficiently abundant that commercial high seas fisheries have developed. Such areas are off west Greenland, where North American and European fish are harvested, and in the Norwegian Sea, north of the Faroe Islands, where mainly European fish are exploited. Atlantic salmon feed on a wide range of large crustaceans, pelagic fish, and squid in the marine environment, supporting the hypothesis that Atlantic salmon are opportunistic feeders. In the ocean the salmon grow relatively quickly and the sea age when they become sexually mature depends on both genetics and on growing conditions. Natural marine mortality of salmon is highest during the first few months at sea and the major mortality factor is probably predation. However, marine mortality of Atlantic salmon has increased in recent years, apparently correlated with a decline in sea surface temperatures. Similar relationships between environmental conditions and the growth and survival of Pacific salmon have been reported. Atlantic salmon life histories most closely mimic stream-type chinook salmon or steelhead trout among the Pacific species. Finally, Atlantic and Pacific salmon return to their home rivers with high precision and possible mechanisms controlling the oceanic homing migration are presented and discussed.