Gannets wintering in the Canary Current and Iberian Coast feed primarily on pelagic fishes and co-occur with trawl and fixed gear fisheries

Information from throughout the annual cycle is necessary to understand the ecological consequences of global change on migratory marine vertebrates. Northern gannets Morus bassanus are partially migratory seabirds that feed on pelagic fish and fishery discards during breeding periods, but their non-breeding foraging ecology is less well known. Here, we used light-level geolocation tracking of gannets breeding in the UK (Grassholm, Wales) to identify key over-winter locations, combined with stable isotope analysis of winter-grown primary feather keratin and potential fish prey from wintering sites to estimate non-breeding diets using stable isotope mixing models. We tracked gannets to their wintering areas in coastal waters of the Canary Current (n = 15; 75%), Iberian Current (n = 2; 10%) and southwest England/northwest France (n = 3; 15%). Mixing models from the Canary Current and Iberian Coast suggest non-breeding gannets ate mostly pelagic fish, although this varied among individuals (22.3 to 78.2% small pelagics; 17.4 to 70.6% large pelagics), while demersal and/or benthopelagic fish composed 4.4 to 18.4% of the diet. Additionally, we used automatic identification system-derived fishing effort to estimate spatial co-occurrence of wintering gannets with fishing activity. Gannet areas of active use co-occurred with trawler (all types) and fixed gear (anchored longlines, gillnets, pots, and traps) fleets. Direct association between fishing vessels and gannets could not be derived from our tracking data, but the coincident spatial use and dietary preference for heavily targeted fish may indicate a risk of bycatch and potential competition with fisheries. We advocate for increased year-round research, oversight, and monitoring of gannet-vessel interactions and bycatch rates in areas of co-occurrence to understand and mitigate risks and aid in conservation decision making.