Advancing an Ecosystem Approach in the Gulf of Maine

Thermal Phenological Factors Affecting the Survival of Atlantic Salmon in the Gulf of Maine

Kevin D. Friedland, James P. Manning, and Jason S. Link


Abstract .—The marine survival of Atlantic salmon Salmo salar stocks in the Gulf of Maine area has declined as sea surface temperature in the coastal ocean has increased; in part, this change in recruitment can be attributed to a phenological mismatch between the timing of smolt migrations and initial conditions postsmolts find during their early marine phase. Salmon juvenile migrations to the ocean are released by photoperiod and spring transitional freshwater temperatures, neither of which have changed significantly in recent decades, thus actuating the migrations at nearly the same time each year. However, ocean water temperatures have increased during the spring transition period, suggesting that smolts have entered the ocean under varying physical and biological conditions. The phenological effect observed in the Gulf of Maine is consistent with the relationship observed for the North American stock complex. In light of recent findings related to the growth of postsmolts, the contrast in recruitment for North American salmon, and Gulf of Maine stocks in particular, appears to be the result of mortality that occurs during the first months at sea. This mortality covaries with the thermal changes in the coastal ocean, which we suspect is associated with variation in the predator field.