Selection on the timing of migration and breeding: A neglected aspect of fishing-induced evolution and trait change

Fishing can drive changes in important phenotypic traits through plastic and evolutionary pathways. Size-selective harvest is a primary driver of such trait change, has received much attention in the literature and is now commonly considered in fisheries management. The potential for selection on behavioural traits has received less study, but mounting evidence suggests that aggression, foraging behaviour and linked traits can also be affected by fishing. An important phenomenon that has received much less attention is selection on reproductive phenology (i.e., the timing of breeding). The potential for this type of temporal selection is widespread because there is often substantial variability in reproductive phenology within fish populations, and fisheries management strategies or fishermen's behaviours can cause fishing effort to vary greatly over time. For example, seasonal closures may expose only early or late breeding individuals to harvest as observed in a range of marine and freshwater fisheries. Such selection may induce evolutionary responses in phenological traits, but can also have demographic impacts such as shortened breeding seasons and reduced phenotypic diversity. These changes can in turn influence productivity, reduce the efficacy of management, exacerbate ongoing climate-driven changes in phenology and reduce resilience to environmental change. In this essay, we describe how fisheries management can cause temporal variability in harvest, and describe the types of selection on temporal traits that can result. We then summarize the likely biological consequences of temporally selective fishing on populations and population complexes and conclude by identifying areas for future research.