Marine temperatures experienced by postsmolts and the survival of Atlantic salmon, Salmo salar L., in the North Sea area

The survival of two Atlantic salmon stocks that inhabit rivers confluent with the North Sea was examined in respect to historical distributions of sea surface water temperatures. The rivers Figgjo and North Esk are relatively small salmon rivers in southern Norway and eastern Scotland, respectively. Wild salmon smelts have been tagged in these rivers since 1965. Tag returns were used to evaluate the survival of salmon in the North Sea. Survival rates of one-sea-winter (1SW) and 2SW fish were correlated within stocks, as well as between stocks, Survival rates were compared with the areal extent of thermal habitat in the north-eastern Atlantic Ocean. A positive correlation was found between the area of 8-10 degrees C water in May and the survival of salmon. A reciprocal negative correlation was also found between survival and 5-7 degrees C water in the same month. An analysis of sea surface temperature distributions for periods of good vs. poor salmon survival showed that when cool surface waters dominate the Norwegian coast and North Sea during May, salmon survival has been poor. Conversely, when the 8 degrees C isotherm has extended northward along the Norwegian coast during May, survival has been good. The effect of water temperature distributions on the growth of postsmolts and other survival factors are discussed.