Spatial distribution of seabird biomass removal and overlap with fisheries in a large marine ecosystem

Globally, seabirds remove a prey biomass equivalent to commercial fishery landings, suggesting likely competition for resources that will vary spatially and seasonally. Using extensive aerial surveys combined with species-specific energetic requirements, we calculated and mapped at-sea removal of prey biomass by seabirds during the breeding and non-breeding seasons and compared this to the seasonal distribution of fishing effort. The distribution of prey biomass removal by seabirds was concentrated in coastal areas, with up to 6.39 kg/km2/day of biomass consumed in the breeding season and up to 4.3 kg/km2/day in the non-breeding season. Offshore, average biomass consumption ranged from 0.28–1.53 to 0.75–1.84 kg/km2/day in the breeding and non-breeding seasons, respectively. Total prey biomass removal by seabirds across the study area (the Irish Exclusive Economic Zone covering an area of 341 183 km2) was estimated at around 441.2 tonnes/day in the breeding season and 478 tonnes/day in the non-breeding season, highlighting the significant role that seabirds play in marine ecosystems. The fine-scale spatial distribution of prey biomass removal by seabirds was significantly correlated with the distribution of fishing effort in both inshore and offshore areas, highlighting potential competition for resources and/or attraction of seabirds to vessels for discards.