The battle between harvest and natural selection creates small and shy fish

Harvest of fish and wildlife, both commercial and recreational, is a selective force that can induce evolutionary changes to life-history and behaviour. Natural selective forces may create countering selection pressures. Assessing natural fitness represents a considerable challenge in broadcast spawners.

Thus, our understanding about the relative strength of natural and fisheries selection is slim. In the field, we compared the strength and shape of harvest selection to natural selection on body size over four years and behaviour over one year in a natural population of a freshwater top predator, the northern pike (Esox lucius).

Natural selection was approximated by relative reproductive success via parent-offspring genetic assignments over four years. Harvest selection was measured by comparing individuals susceptible to recreational angling with individuals never captured by this gear type. Individual behaviour was measured by high-resolution acoustic telemetry.

Harvest and natural size selection operated with equal strength but opposing directions, and harvest size-selection was consistently negative in all study years. Harvest selection also had a substantial behavioural component independent of body length, while natural behavioural-selection was not documented, suggesting the potential for directional harvest selection favouring inactive, timid fish.

Simulations of the outcomes of different fishing regulations showed that traditional minimum-size based harvest limits are unlikely to counteract harvest-selection without being completely restrictive. Our study suggests harvest selection may be inevitable and recreational fisheries may thus favor small, inactive, shy and difficult-to-capture fish.

Increasing fractions of shy fish in angling-exploited stocks would have consequences for stock assessment and all fisheries operating with hook-and-line.