Overwintering strategy of sandeel ecotypes from an energy/predation trade-off perspective

The overwintering strategy is widespread among planktivorous fish from temperate to sub-polar regions, and is particularly pronounced in sandeel ecotypes. This adaptation is presumably a behavioural adaptation to strong seasonal fluctuation in prey availability, day length and temperature. Yet so far, insight into the evolutionary and ecological mechanisms involved has remained elusive.

In the present study, the overwintering behaviour is considered to be a strategy requiring 2 annual decisions: When to end overwintering and when to start overwintering, which combined comprise the timing and duration of the foraging window. We present a model framework for sandeel that demonstrates how the optimal timing of the foraging window involves a trade-off between energy gain and survival probability.

Physiological components of the model are based on laboratory experiments with sandeel, some of which are presented as part of this study. The major findings were that optimisation of individual fitness is strictly dependent on the temporal match between the foraging window and the peak period of the spring burst of zooplankton.

In contrast, fitness is only marginally reduced in individuals that fail to optimise the foraging window in relation to the level of mortality and prey availability experienced during the zooplankton spring burst. Lastly, overwintering remained the optimal strategy, even in scenarios where winter prey abundances approached abundances near that of the peak abundance in spring.