Abstract

We examined two long-term tagging studies with wild salmon stocks in the North Sea area. The salmon stocks, the Figgjo in southern Norway and the North Esk in eastern Scotland, reside in relatively un-impacted rivers that continue to sustain healthy runs of salmon. The return rates for one seawinter fish (1SW), the predominant age at maturity for both stocks, were highly correlated. An analysis of sea surface temperature distributions for periods of high versus low return rate showed that when low sea surface temperatures dominate the North Sea and southern coast of Norway during May, salmon survival has been poor. Conversely, when high sea surface temperatures extend northward along the Norwegian coast during May, survival has been good. Ocean conditions can be further related to the recruitment process through growth studies for the North Esk stock. Post-smolt growth increments for returning 1SW fish showed that enhanced growth was associated with years during which temperature conditions were favorable, which in turn resulted in higher survival rates. The implicit linkage between growth and survival suggests that growth-mediated predation is the dominant source of recruitment variability. Mechanisms by which ocean climate may affect post-smolt growth are discussed.