Factors affecting marine production of Atlantic salmon (Salmo salar)

This paper reviews recent advances in our understanding of factors influencing the marine production of wild Atlantic salmon (Salmo salar). Population abundance has declined during the last 30 years because of decreased marine growth rate, survival-rate, and production of multi-sea-winter fish. Mortality appears density-independent, indicating that the marine abundance is beneath the carrying capacity for the species. Correlations between the North Atlantic Oscillation winter index for the post-smolt year and production variables indicate that unfavourable climatic conditions are partly responsible for the decline. Low sea temperature may be the ultimate reason for the poor salmon production, whereas predation is one proximate mortality factor, which is probably both size and temperature dependent. Low growth rate during cold years was associated with low sea age at maturity and small salmon, contrary to the common observation that fast growth leads to young age at maturity. It is suggested that low water temperature may stimulate lipid storage relative to protein production and that the energy density needed to attain sexual maturity is lower in small than in large salmon. Future research should focus on the relationships between smolt age and (or) size and adult age and (or) size, and the association among water temperature, growth rate, growth efficiency, and age at sexual maturity.