Behavioural plasticity in circadian foraging patterns increases resistance of brown trout populations to environmental change

Stream-dwelling salmonids in the low-latitude and -altitude margins of their range are particularly threatened by climate change. However, they possess a variety of evolutionary, plastic, and behavioural mechanisms that provide resistance against rapid changes in their environment. Behavioural plasticity can be important under rapid environmental change because it is relatively fast and flexible. In particular, salmonids can exhibit flexible diel activity patterns in response to new environmental conditions, but the consequences of this capability for long-term population persistence in the face of climate change remain unclear. We used an individual-based model to simulate the trajectory of a brown trout population at the warmest edge of its range under three environmental-change scenarios of increasing warming and streamflow reduction. We assessed (1) how simulated trout responded behaviourally to climate change by modifying their circadian foraging patterns, and (2) how much this behavioural plasticity buffered the population-level consequences of environmental change. Our simulations showed that under current conditions trout of different age classes segregated foraging both temporally and spatially. The most consistent response to environmental change was more diurnal feeding in all age classes and under all scenarios, with the strength of this response increasing with the severity of change. In addition, total daily foraging activity increased in all age classes. A second experiment indicated that virtual populations of individuals capable of flexible circadian feeding were more resistant to environmental change than populations restricted to fixed feeding patterns. Thus, our computational experiment suggests that the ability of fish to adaptively select when as well as where to feed, well-documented at the individual level in the empirical literature, could potentially buffer the demographic impacts of long-term environmental change.