Evaluating the ecosystem effects of variation in recruitment and fishing effort in the western rock lobster fishery

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We used a regional, ecosystem mass-balance model (Ecopath with Ecosim) to evaluate the possible changes in flows of mass and energy between the benthic and pelagic components of the marine ecosystem of Jurien Bay in temperate Western Australia (∼30°S, ∼115°E). The effects of varying the biomass of western rock lobster in the system were examined by simulating changes in commercial and recreational fishing mortality as well as recruitment-driven changes in the abundance of lobster puerulus (the first post-larval stage). The model comprised 80 functional groups (more than 200 species). A simulated closure of the commercial lobster fishing was predicted to lead to an increase in lobster biomass of 160% after 20 years, with changes in biomass of up to 20% of the main prey and predators of lobster. Since 2006/2007, the puerulus settlement (recruitment to the benthos) of western rock lobster has declined to the lowest levels recorded in the fishery. The model predicted that under 2005 levels of fishing effort, a simulated depletion of 90% of puerulus biomass resulted in an ∼17% reduction in the biomass of adult lobster biomass after 20 years (i.e. by 2025). In general, the model predicts that the variations in lobster biomass, whether induced from fishing mortality or declining puerulus settlement, have relatively small effects on the biomass of the main predators and prey of lobster in the marine park. The relative biomass of adult rock lobster and their associated predators and prey was more sensitive to fishing than to variations in recruitment.