The spatial and trophic ecology of seabirds in the western North Sea: developing the evidence base for ecologically sustainable fisheries and marine spatial planning

Seabirds along the Eastern coast of Britain have been declining over the past three decades, alongside their main breeding season prey sandeel (Ammodytes marinus). Seabird productivity is a critical aspect of a healthy population, and this is highly sensitive to prey availability near to seabird colonies. To understand drivers, outcomes and potential conservation actions of these seabird declines, it is necessary to study the spatial, temporal and trophic ecology of seabirds and their main prey fish. In this thesis I contribute to this understanding by first modelling the fine-scale spatio-temporal distribution of sandeel together with another key prey species, sprat (Sprattus sprattus) across the North Sea over the past two decades. Due to the lack of single comprehensive surveys for these two species, I take a novel approach by combining different available survey datasets with different strengths and weaknesses to extend the coverage and statistical power of the model. These maps allow for the identification of hotspots of biomass, population trends, and important environmental drivers for both prey species. Next, I modelled the specific predator-prey interactions of parent guillemots (Uria aalge) while provisioning their chicks on the Isle of May with their main breeding season prey, sprat and sandeel. I parameterised multispecies functional response models, which show that that guillemot chick provisioning was far more sensitive to changes in sprat availability than sandeel. My results are broadly in agreement with the widely cited ‘one third for the birds’ proposal - i.e., that prey fish should be maintained at least at one third of historical maximum biomass in order to avoid negative impacts on seabirds. However, my flexible multi-species modelling approach showed that high levels of sandeel biomass have the potential to buffer low years of sprat biomass. Finally, I performed habitat risk mapping to identify hotspots where multiple anthropogenic pressures overlap with key seabird foraging areas. I found that two changes in how we use the ocean over the next decade, the cessation of sandeel fishing in the UK EEZ and the large expansion of offshore wind developments, will likely decrease the overall impact risk but increase the spatial overlap of key seabird foraging habitat and anthropogenic stressors. Overall seabirds and their prey are experiencing significant changes and here I lay the groundwork of quantitative evidence with a comprehensive cover of many aspects of seabird ecology that will aid and improve manager’s decisions in relation to commercial fishing, marine spatial planning and seabird conservation.