Seasonal variation in catch-up growth reveals state-dependent somatic allocations in salmon

The trade-off in the allocation of resources between skeletal growth and the storage of reserves has received little attention, despite relevance to all growing organisms. We explored this trade-off by manipulating food availability for juvenile Atlantic salmon, Salmo salar, so as to create the same reduction in growth and loss of energy reserves at different times of the year. The fish showed seasonal differences in their responses to the nutritional deficit when food was restored. In winter, fish restored lipid reserves, but their growth in length over the recovery period was negligible. In summer, fish allocated resources to growth in length as well as the restoration of lipid reserves; moreover, this skeletal growth was significantly faster than that of control fish that had received food ad libitum throughout. We demonstrate that current physiological and energetic models of animal growth cannot account for such seasonal variation in compensatory growth and allocation patterns, and the regulation of growth and energy reserves is a dynamic and state-dependent process. We then predict – on the basis of expected effects on fitness – how somatic allocation and catch-up growth should vary over time and in contrasting environments.