Methods and policy pathways to optimise production of multiple interacting fish stocks

Abstract

In a time when food security and sustainable development are prominent societal and international concerns, the literature underlines the importance of ecosystem or multispecies fisheries management to perpetually safeguard transgenerational equity of use of such a resource. The transition from traditional stock-by-stock management to more holistic paradigms necessitates the contextualisation of the fishery within a larger social-ecological setting, achieved through the application of more complex models. However, despite the theoretical and computational advancements in the field, current management practices apply the Maximum Sustainable Yield (MSY) objective to each species in a fishery as if they existed in vacuo; that is, with explicit disregard to indirect effects of fishing through biological interactions and intrinsic dependencies on other abiotic ecosystem components. Furthermore, when operating within a multispecies management framework, the MSY target becomes ambiguous and lacks any formal definition. This thesis aims to reconcile these pressing concerns by performing novel in silico experiments with two EcoPath with EcoSim (EwE) models of the Baltic and North Sea ecosystems that have been ported to the R programming language. Firstly, the MSY concept is formally defined as a form of Nash Equilibrium with respect to fishing mortality rates, conferring MSY with a new meaning in real-world multispecies fisheries. Moreover, an efficient computational tool is developed to streamline its computation for any ecosystem model. Secondly, a practical management exercise is conducted accounting for input parameter uncertainty through Approximate Bayesian Computation. In addition, a novel visualisation is devised so as to facilitate the identification of key components within the managed stock-complex that require prioritised multispecies management. Thirdly, and after UK’s withdrawal from the EU, a thorough framing of Nash Equilibrium MSY within the current legal environment is provided. Results from this thesis have direct management application for the sustainable joint exploitation of communities of wild species.