How does stochasticity affect evolutionary regime shifts in age and size at maturation?

Abstract

Fish in many exploited stocks grow faster and mature earlier at either larger or smaller sizes in comparison to pre-exploitation periods. These changes can be driven by both genetic and phenotypic responses. We have shown recently that their interplay can lead to irreversible evolutionary regime shifts in individual life histories and stock properties. Our results were based on a model which assumed annual spawning and size- and density-dependent individual growth in a deterministic environment. We now extend the analysis to cover stochasticity in recruitment, survival after recruitment and harvesting pressure, including the possibility of bycatch and illegal fishing after fishing moratoria or reduced harvest rates are imposed. We show that under low and moderate stochasticity, early maturation at small sizes and late maturation at large sizes can still persist as alternative, evolutionary and ecologically stable states under otherwise identical environmental conditions. Typically, maturation sizes of the late-maturing phenotypes decrease with increasing stochasticity, while those of the early-maturing phenotypes remain nearly constant. Consequently, we confirm that even in stochastic environments, exploitation of late-maturing populations can induce rapid evolution to smaller maturation sizes associated with stepwise decreases in mean age at first reproduction. These changes can be reversed by fishing moratoria; more stochastic environments and/or harvesting pressure require faster closure of the fishery. Unless stochasticity is too strong, incomplete closure of the fishery may also lead to the counterintuitive, accelerated evolution towards smaller sizes at maturation which we reported for the deterministic system.