Abstract
Reproductive behaviour and mating system complexity may influence fisheries-induced evolution. Mate choice and intrasexual competition might favour late-, large-maturing genotypes in contrast to the selection imposed by many fisheries. Here, we simulate changes to the mean and variance in body size of Atlantic cod (Gadus morhua) concomitant with increased fishing intensity. Comparing selection differentials (S) for length under the assumptions that size does and does not affect reproductive success, we find that the strength of selection for smaller body size associated with increased fishing pressure depends on: (i) the initial variance in body size; (ii) changes to the variance in size with increasing fishing intensity; and (iii) the influence of size on reproductive success. If the initial variability in length is sufficiently high and its coefficient of variation (CV) increases with fishing intensity, the predicted evolutionary shift towards smaller size generated by fishing is less than that expected under the assumption that reproductive success is independent of size. However, if size influences reproduction and if the CV in body size declines as fishing pressure increases, a trend that may be characteristic of many intensively exploited populations, the strength of selection for smaller size is predicted to be comparatively rapid. We conclude that fisheries-induced evolution can be influenced by changes to the mean and variance of traits under sexual selection, and that the benefits of maintaining broad phenotypic variability in traits such as body size may be greater than previously thought.
Highlights
The potential for fishing to cause evolutionary change is not appreciably different from other forms of predatorinduced mortality, given its propensity for generating differential survival among genotypes
We considered three means by which the coefficient of variation (CV) in body size might change with changes to fishing intensity
If the initial variation in size is sufficiently high, and if the CV in size increases with fishing intensity, the strength of fisheries-induced selection for smaller body size is predicted to be less than that under the assumption that size has no effect on reproductive success
Summary
The potential for fishing to cause evolutionary change is not appreciably different from other forms of predatorinduced mortality, given its propensity for generating differential survival among genotypes. The probability of death by fisheries-induced predation increases with the age, size and growth rate of the prey (Law 1991; Stokes et al 1993). Given that these traits are heritable (Roff 2002) and that fishing mortality often exceeds that effected by the predator in most natural predator-prey relationships, fishing has almost certainly generated evolutionary changes in at least some harvested populations (Stokes and Law 2000; Law 2007). Reductions in age and size at maturity in exploited populations
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