Size-selective mortality induces evolutionary changes in group risk-taking behavior and the circadian system in a fish

Abstract

Aim: Size-selective mortality is a major evolutionary driver of life-history and behavioral traits in human-dominated marine ecosystems. These changes might in turn alter the circadian system and therefore affect daily physiological processes and behavioral outputs. Methods: The proximate mechanisms governing evolution are difficult to disentangle in the wild. We overcome this limitation by studying the evolutionary impact of size-selective harvesting on group risk-taking behavior and the circadian system in a model fish species. We exposed zebrafish (Danio rerio) to either large or small size-selective harvesting relative to a control over five generations, followed by eight generations during which harvesting halted. Results: Size-selective mortality affected fine-scale timing of behaviours. In particular, small size-selective mortality, typical of gape-limited predators targeting smaller size classes, increased group risk-taking behavior. Moreover, small size-selective mortality increased early peaks of daily activity as well as extended self-feeding daily activity to the photophase compared to controls. By contrast large size-selective mortality, typical of most wild capture fisheries, only showed an almost significant effect of decreasing group risk-taking behavior and no clear changes in fine-scale timing of daily behavioral rhythms compared to controls. We also found changes in the molecular circadian core clockwork in response to both size selective mortality treatments. These changes disappeared in the clock output pathway because both size-selected lines showed similar transcription profiles suggesting the presence of a molecular switch. Main conclusion: To conclude, our experimental harvest left an asymmetrical evolutionary legacy in group risk-taking behavior and in fine-scale daily behavioral rhythms. Yet, the overall timing of activity showed evolutionary resistance probably maintained by a molecular switch. Our experimental findings suggest that human-induced changes in size selective mortality can have consequences for behavior and physiological processes in marine ecosystems