Abstract

Fisheries have impacted seabird populations worldwide, either via bycatch mortality or resource depletion. Understanding the overlap between seabird distributions and fisheries is an important element for bycatch risk assessment, though the drivers of variation in seabird-fishery overlap are not well understood for some seabird populations. Here, we quantified the spatial overlap between foraging Cory's shearwaters Calonectris borealis during the breeding season and commercial fisheries operating within the Portuguese Exclusive Economic Zone. In addition, we evaluated whether overlap varied as a function of an individual's boldness, sex or breeding stage. For this, we GPS tracked 361 foraging trips by 72 Cory's shearwaters nesting at Berlenga Island, Portugal, over 5 consecutive breeding seasons (2012-2016). Simultaneously, we used fishing effort data from Global Fishing Watch detailing the distribution of industrial fisheries within the temporal and spatial range of Cory's shearwater tracks. Although fishing vessels were present during 88.1% of foraging trips, Cory's shearwaters spent only on average 13.3 % of the time foraging in the same areas as fisheries. Such low spatial overlap is likely driven by high prey availability near the colony and suggests low direct competition for resources. We also found variation in overlap with fisheries across the breeding period, with Cory's shearwaters spending approximately 11% more time foraging in the same areas as fixed gear and purse seine vessels during pre-laying than during chick-rearing. Surprisingly, no sex or boldness-related differences were found in the overlap with any fishing gear. Our findings have implications for understanding within population variation in the overlap between fisheries and seabirds and, in turn, bycatch risk.