Diet selection and predation mortality in the North Sea fish assemblage - improving multi species stock assessment

Abstract

This Ph.D. thesis focused on the processes determining the dynamic of predator-prey interactions inside the North Sea food web. As further focus the predation impact on the recruitment of North Sea fish stocks was evaluated. By applying the obtained process understanding on predation and recruitment currently used multi species assessment models could be enhanced. This thesis is structured in five chapters that correspond to manuscripts for publications. An introduction and a chapter on conclusions and perspectives complete the thesis. In the course of this thesis the performance of currently used diet selection models in multi species fish stock assessment was investigated (Manuscript 1 and 2). The structural uncertainties in multi species stock assessment due to critical assumptions on involved processes were quantified. Especially the so far ignored role of changes in spatial predator-prey overlap for the diet selection behavior of fish predator populations was highlighted (Manuscript 3). This gave new insights in the mechanisms of predator-prey interactions on population level and guided the way for further improvements of currently used diet selection models in multi species stock assessment. Outside of the multi species model context, the predation impact on the recruitment of North Sea fish stocks was analysed in a survey-based study on the suitability of hydrographic fronts as nursery areas from a top-down perspective (Manuscript 4). Finally, the obtained knowledge on predation processes and the diet selection of North Sea fish predators was utilized to create a survey-based predation index (Manuscript 5). The index was used to demonstrate the role of predation in determining recruitment success of selected North Sea fish stocks.