State-dependent shifts between nocturnal and diurnal activity in salmon

Animal species have usually evolved to be active at a specific time of the daily cycle, and so are either diurnal, nocturnal or crepuscular. However, we show here that the daily timing of activity in juvenile Atlantic salmon is related to the life-history strategy that they have adopted (i.e. the age at which they will migrate to the sea) and their current state (body size/relative nutritional state). Salmon can detect food more easily by day than by night, but the risk of predation is greater. Nocturnal foraging should generally be preferred, but the greater the need for growth, the greater should be the shift towards diurnal activity. In line with this prediction, all fish were predominantly nocturnal, but salmon preparing to migrate to the sea, which would experience size-dependent mortality during the forthcoming migration, were more diurnal than fish of the same age and size that were delaying migration for a further year. Moreover, the proportion of activity by day was negatively correlated with body size within the intending migrants. It has previously been shown that overwinter survival in fish delaying migration is maximized not by growth but by minimizing exposure to predators. As predicted, daytime activity in these fish was correlated with the prior rate of weight loss, fish being more diurnal when their risk of starvation was greater. To our knowledge, these are the first quantitative demonstrations of state-dependent variation in the timing of daily activity.