Uncovering the links between foraging and breeding regions in a highly mobile mammal

Summary
1. The annual cycle of many animals is characterized by the need to satisfy different life history priorities, often requiring seasonal movements. For such species, investigating carryover effects (such as the year-long drivers of breeding success) and managing protected areas effectively, relies on quantifying these movements. Here, we model the seasonal movements of
the UK population of grey seals Halichoerus grypus and show how insights from the model can improve its management.
2. We fit a hidden process model to two types of information – regional population redistribution and individual movements – to estimate the seasonal transition probabilities of breeding female grey seals among four regions around the UK.
3. We found that between 21% and 58% of females used different regions for breeding and foraging.
4. For our study period, we detected an increase in the breeding performance of animals that foraged in the Hebrides and South-East Coast.
5. Grey seal Special Areas of Conservation (SACs) were designed to encompass a significant proportion of the UK breeding population: ~ 40% of the breeding females in our study area. Of the females breeding on SACs, only 15% breed in Northern Scotland, but up to 50% forage there. Our results indicate that, by only considering the breeding distribution of females that breed in SACs, the impact of anthropogenic activities on nearby SACs may be overestimated,
whereas impacts on remote SACs may be underestimated. 6. Synthesis and applications. By quantifying the link between the foraging and breeding distributions of grey seals, management of breeding populations can be focused on the foraging regions where the resources necessary for reproduction are acquired. The construction of marine developments is dependent on demonstrating that they will not have an adverse effect on
the integrity of Special Areas of Conservation (SACs), and we have shown that this requires consideration of the seasonal transition probabilities estimated in this study. Our specific results provide support for management strategies that jointly consider SACs and Marine Protected Areas (MPAs). More generally, we prescribe combinations of data on population size, breeding performance and individual movement that can enable our framework to be applied to seasonally migrating species.