Differential response of continental stock complexes of Atlantic salmon (Salmo salar) to the Atlantic Multidecadal Oscillation

Atlantic salmon Salmo salar in the North Atlantic are promulgated as a set of population complexes distributed in both North America and Europe. In recent years, the complexes have suffered reduced at sea survival putting individual populations within a complex at risk, in particular, those populations in the southern end of the range. The freshwater portion of the life history is used for spawning and the rearing of juveniles and has tended to stabilize recruitment, whereas the first year at sea, termed the post-smolt year, has been vulnerable to more variable rates of mortality. Strongly developed tele-connections exist between North American and European complexes, which is reflected in a shared response to the thermal variation associated with the Atlantic Multi-decadal Oscillation (AMO) more so than to measures of atmospheric variation like the North Atlantic Oscillation (NAO). The action of the AMO has been to change thermal conditions affecting the post-smolt phase, however, the mechanistic linkage appears to be fundamentally different for continental groups. Whereas ocean climate variability during the first months at sea appears to be important to North American stocks, summer climate variation appears to be central to the recruitment of European stocks. The mechanism behind this difference is related to the varying role of predation pressure and size-related mortality controlled by post-smolt growth.