Anorexia and Defended Energy Levels in Over-Wintering Juvenile Salmon

1. Juvenile Atlantic salmon (Salmo salar L.) that do not migrate to sea within the next year spend the winter under stones in the stream bed. In a laboratory study, we show that this coincides with a loss of appetite even in the presence of excess food: an index of appetite dropped by an average of 96% between early September and late December. 2. This period of anorexia leads to the virtual cessation of growth under environmental conditions that would otherwise allow growth to occur. 3. Juvenile salmon normally exhibit a gradual loss of fat reserves during the winter. This depletion was experimentally accelerated by depriving fish of food for 3 weeks. The fish responded by increasing their appetite; appetite was negatively correlated with the estimated energy reserves of individual fish. 4. Those fish previously deprived of food regained their appetite and made up their lost fat within 4 weeks of resuming feeding; their appetite then decreased to the level of control fish. 5. The parallel changes in habitat choice and appetite in autumn may be a consequence of the decreased profitability and increased predation risk when feeding in winter. The degree and duration of anorexia in overwintering salmon is not preprogrammed but is regulated by energy reserves, the salmon effectively having a 'defended energy level' below which appetite is increased until lost energy reserves have been restored.