Size-selective mortality fosters ontogenetic changes in collective risk-taking behaviour in zebrafish, Danio rerio

Size-selective mortality is common in fish populations and can operate either in a positive size-selective fashion or be negatively size-selective. Through various mechanisms (like genetic correlations among behaviour and life-history traits or direct selection on behaviour co-varying with growth rate or size-at-maturation), both positive- and negative size-selection can result in evolutionary changes in behavioural traits. Theory suggests that size-selection alone favours boldness, but little experimental evidence exists about whether and to what extent size-selection can trigger its evolution. Here we investigated the impact of size-selective mortality on boldness across ontogeny using three experimental lines of zebrafish (Danio rerio) generated through positive (large-harvested), negative (small-harvested) and random (control line) size-selective mortality for five generations. We measured risk-taking during feeding (boldness) under simulated aerial predation threat and in presence of a live cichlid. We found that boldness decreased with ontogenetic age under aerial predation threat, and the small-harvested line was consistently bolder than controls. Collective personality emerged post larval stages among the selection lines. In presence of a cichlid, the large-harvested line was bolder at the highest risk of predation. The large-harvested line showed higher variability and plasticity in boldness across life stages and predation risks. Collectively, our results demonstrate that size-selective harvesting may evolutionarily alter risk-taking tendency. Size-selection alone favours boldness when selection acts on small fish. Selection typical of fisheries operating on large fish favours boldness at the highest risk of predation and increases behavioural variability and plasticity. There was no evidence for positive size-selection favouring evolution of shyness.