Ocean climate influences on critical Atlantic salmon (<i>Salmo salar</i>) life history events

Ocean climate and ocean-linked terrestrial climate affect nearly all phases of Atlantic salmon (Salmo salar) life history. Natural mortality in salmon occurs in two main phases: juvenile stages experience high mortality during freshwater residency and pre-adult salmon experience high mortality in estuarine and ocean environments. Freshwater survivorship is well characterized and tends to be less variable than marine mortality. Sources of marine mortality are poorly known due to a lack of basic knowledge about post-smolt distributions and habits. Coherence patterns among regional and continental stock groups suggest broad scale forcing functions play a more important role in defining recruitment than mortality effects associated with individual rivers. The action of mesoscale regional environment is most prominent during the post-smolt year when survival, maturation, and migration trajectories are being defined. During the early weeks at sea, growth mediated survival defines recruitment patterns. A correlation between sea surface water temperature and survival has been observed for salmon stocks in the northeast Atlantic suggesting temperature either directly affects growth or modifies post-smolt behavior. Age at first maturity is controlled by environmental as well as genetic factors. The abundance of two seawinter spawners in North America is directly scaled to the size of overwintering thermal habitat in the northwest Atlantic, which suggests a link between maturation and environment.