Effects of bycatch mortality on population dynamics in model food webs

Background/Question/Methods

Fisheries impact many more species than the target species alone, necessitating the use of multi-species management to account for non-target anthropogenic effects and ecological interactions.   Harvest effort often decreases the abundance of non-target species through unintended effects such as habitat destruction and mortality from bycatch.  However, extraction of target species can also have a positive effect on non-target species abundance by reducing competition or predation pressure.  Understanding the relative positive and negative effects of harvest in a multi-species system is necessary to accurately predict changes in community dynamics and manage fisheries sustainably.  I use a three-species model food web to determine the conditions under which the impact of harvest with bycatch shifts between net positive and net negative for the non-target species.  The model consists of two prey species with Ricker or Beverton-Holt growth and competition, and one predator following Nicholson-Bailey dynamics.  This allows a comparison between compensatory and over-compensatory growth, and assumes an obligate predator with two prey species.  The three-species results are compared with two-species competition and predator-prey models to examine the effect of bycatch on species interactions in isolation or in combination.  Additionally, I examine the effects of bycatch on community stability.

Results/Conclusions

Unsurprisingly, harvest in a two-competitor system benefits the non-target species through competitive release.  However, a species with a harvested strong competitor is likely to be more susceptible to bycatch due to its ecological similarity and shared habitat.  This additional mortality can counteract competitive release, but the likelihood of capture of the non-target species must almost equal that of the target species for this to happen.  Bycatch in a predator-prey system, with the predator harvested, results in a smaller population decrease than in the two-competitor system.  The bycatch loss is offset by a dampening of predator growth.  The three-species (predator-competitor-competitor) model can have similar dynamics to either two-species model or exhibit unique behavior, depending on the strength of species interactions.  When competition and predation are strong, harvest increases the abundance of the non-target species by lowering the numbers of both competitors and predators.  The combined release from both these interactions is greater than most levels of bycatch.  Dynamics are similar between Ricker and Beverton-Holt models, and bycatch does not greatly affect system stability.  These results suggest that, when species interactions are accounted for, non-target species in highly competitive or predator-mediated systems may increase within a fishery despite high levels of bycatch.