Penalising Yield: MSY and bycatch

Non-target species such as marine mammals and elasmobranchs are by-caught in commercial fisheries (despite technical measures designed to alleviate such risks) and this may result in population declines for these species. We investigate the relationship between fishing effort and bycatch, taking account of relevant covariates, to enable the prediction of bycatch resulting from different levels of fishing. Using data on life-history parameters for bycaught species (e.g. fecundity, natural mortality rates) we explore methods to evaluate the impact of additional bycatch mortality on their populations. The MSY-based management of fisheries relies on optimizing an objective function such as net catch of target species or net profit. We explore some methods whereby such a function can be modified to take account of bycatch. The simplest approach, given perfect knowledge of the system e.g. of target and non-target populations, would be to set absolute limits to the bycatch (and fishing effort) permissible. However, neither our knowledge nor our model-fitting is perfect, and in the light of the resulting uncertainty, more subtle approaches to penalizing the objective function (e.g. a penalty that is a monotonically increasing function of effort) may be more useful