Effect of rapid evolution on multi-species interactions in fisheries under size-selective harvest

Background/Question/Methods

 Fishery harvest affects target species through mechanisms besides simple population loss.  Heavy mortality can exert strong evolutionary pressure as well through its effect on species demographics.  Size-specific harvest has been shown to produce rapid changes in growth rate and size at maturity in both models and observations.   The effect of species interactions on this evolutionary response is less well understood.  The presence of other species may constrain the evolutionary trajectory through predation or competition while, reciprocally, the strength of those interactions may be altered by the ongoing trait change in the target species.  Marine predators are often gape-limited, and a smaller prey adult size would be expected to increase the effects of predation by increasing the proportion of the population vulnerable to attack.  I use two- and three-species quantitative genetics models to examine how optimal size under harvest changes with the addition of a gape-limited predator, and in the presence of size-sensitive competition.  I compare results for a generalist predator with those for an obligate Nicholson-Bailey predator to determine which forms of predation exert the greatest selective pressure.  Additionally, I analyze the effects of a smaller evolving or non-evolving competitor on the evolutionary trajectory of the harvested species.

Results/Conclusions

The model indicates that the evolutionary effects of fishing can intensify the negative effects of gape-limited predation and size-specific competition and lead to extinction under lighter harvest levels than single-species models would indicate.  Although the target species may be capable of evolving in response to strong directional selection from harvest, the niche space it is evolving toward must be able to support it.  The presence of an evolving competitor further narrows the likelihood of a sustainable evolutionary response to harvest.  However, because an obligate predator responds numerically to the lowered abundance of the target prey, selection from predation diminishes and coexistence is possible under higher harvest levels than with a generalist predator.  This effect is mitigated if an alternate prey is present.  The effects of trait-driven species interactions on the evolutionary trajectory of the target species must be considered when determining evolutionarily sustainable harvest levels.