Harvest‐induced evolution and effective population size

Much has been written about fishery‐induced evolution ( FIE ) in exploited species, but relatively little attention has been paid to the consequences for one of the most important parameters in evolutionary biology—effective population size ( N e ). We use a combination of simulations of Atlantic cod populations experiencing harvest, artificial manipulation of cod life tables, and analytical methods to explore how adding harvest to natural mortality affects N e , census size ( N ), and the ratio N e / N . We show that harvest‐mediated reductions in N e are due entirely to reductions in recruitment, because increasing adult mortality actually increases the N e / N ratio. This means that proportional reductions in abundance caused by harvest represent an upper limit to the proportional reductions in N e , and that in some cases N e can even increase with increased harvest. This result is a quite general consequence of increased adult mortality and does not depend on harvest selectivity or FIE , although both of these influence the results in a quantitative way. In scenarios that allowed evolution, N e recovered quickly after harvest ended and remained higher than in the preharvest population for well over a century, which indicates that evolution can help provide a long‐term buffer against loss of genetic variability.