Using trophic flows and ecosystem structure to model the effects of fishing in the Jurien Bay Marine Park, temperate Western Australia

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Understanding the impacts of fishing on the trophic structure of systems has become increasingly important because of the introduction of Ecosystem Based Fisheries Management and the legislative requirements of fisheries to demonstrate that they are not having a negative impact on other species. A biomass-based dynamic model of Jurien Bay Marine Park (∼30°S) was constructed using Ecopath to investigate the ecosystem impacts of fishing (mainly commercial rock lobster, Panulirus cygnus) in the park, as an example of the potential responses of temperate marine ecosystems in Western Australia to commercial fishing. A simulated 50% reduction in fishing mortality for commercial finfish predicted that after 20 years, the biomass of important fished species (i.e. Pagrus auratus and Choerodon rubescens) would increase by up to 30%. A simulated total fishing closure resulted in much larger (2.5–8 fold) increases in targeted populations, but did not result in any predicted cascading effects on grazing invertebrates and benthic primary producers. The simulations suggest that the structure of this ecosystem is characterised more by bottom-up than top-down processes; i.e. benthic primary production is a major limiting factor. The present study identified trophic linkages and ecosystem processes such as the role of both low and high trophic-level groups and the impact of fishing mortality in the marine park, an essential step towards distinguishing the impacts of fishing from those attributable to natural or other human-induced changes.