An efficient tool to find multispecies MSY for interacting fish stocks

Natural ecological communities exhibit complex mixtures of interspecific biological interactions, which makes finding optimal yet sustainable exploitation rates challenging. Most fisheries management advice is at present based on applying the Maximum Sustainable Yield (MSY) target to each species in a community by modelling it as if it was a monoculture. Such application of single-species MSY policies to strongly interacting populations can result in tragic overexploitation. However, the idea of ‘maximising the yield from each species separately’ can be extended to take into account species interactions. This leads to a form of Nash Equilibrium, where the yields of each species are simultaneously maximised. Here we present ‘nash’, an R package that streamlines the computation of Nash equilibrium reference points for fisheries and other systems represented by a user-defined multispecies or ecosystem model. We present two real-world fisheries management applications alongside performance benchmarks. Satisfactory search results are shown across models with an approximate factor 7 increase in performance when compared to the expensive round-robbing sequential optimisation algorithms used by other authors in the literature. We believe that the nash package can play an instrumental role in fully implementing ecosystem-based management policies worldwide.