| Summary: | This thesis informs an ecosystem approach to fisheries management (EAFM) through the development and application of three relevant. novel size-based modelling case studies that demonstrate the importance of considering trophic interactions when managing fisheries. EAFM, which considers ecological interactions between and within fish populations, allows for better fisheries management because these interactions can cause complex changes in ecosystem structure with changing fishing pressure. This size-based modelling approach informs EAFM through the model's ability to accurately represent size-structured fish community dynamics. I refine the model's ecological representation and explore its properties in order to make it more operational for EAFM. I take a test Signal approach to examine the performance of a suite of fish community indicators proposed as being useful for fisheries management. I show that fish community indicators are differentially and nonlinearly sensitive to fishing and that indicators are generally, though not equally, specific to fishing. I examine how fisheries for forage and predator fish populations interact I show that trade-offs occur among yield, spawning stock biomass, and collapse of forage and predator fish populations due to underlying trophic interactions. I examine ecological seal-fishery interactions in southwest Irish waters using a modelling scenario based on fish community survey data, grey and harbour seal diet and population data, and fisheries catch and discard data. I specify seal diet preference for fish prey using a diet correction method. I find that under the modelled circumstances, seals do not have a major direct or indirect impact on the main modelled commercial fish species, but they do have a negative impact on salmonids, which are vulnerable group. These case studies illustrate that the model used in this thesis is a useful tool that shows how trophic interactions cause ecosystem change with changing fishing pressure, which has policy implications for fisheries management.
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