Seasonal matching of foraging to anticipated energy requirements in anorexic juvenile salmon

Animals can use fat reserves as a source of energy whenever demand exceeds intake. The size of this 'insurance' is dependent on a trade-off between storage costs and anticipated needs. Previous theoretical and empirical studies in birds have shown that fat levels are tailored to short-term needs. However, in some situations animals must match intake to longer term requirements. Here we develop a model of this situation, and compare it with experimental data from juvenile Atlantic salmon, which become anorexic and gradually deplete their energy reserves over winter. Responses to experimentally-accelerated depletion of reserves matched theoretical predictions: an elevation of appetite to restore lost reserves was dependent not on current energy state, but on projected energy state at the end of the winter. This is the first demonstration of an adaptive regulation of appetite tailored to long-term requirements for an energy insurance rather than short-term needs.