Assessing bycatch risk from gillnet fisheries for three species of diving seabird in the UK

Incidental mortality (bycatch) in fisheries represents a threat to marine vertebrates. Research has predominantly focussed on bycatch in longline fisheries, but bycatch from gillnet fisheries is of increasing concern. To address this concern, we combined comprehensive biologging data sets and multiple sources of fishing effort data to assess the spatial overlap of 3 diving seabird species during the breeding season (common guillemot Uria aalge, razorbill Alca torda and European shag Phalacrocorax aristotelis) with UK gillnet fisheries. Species distribution models based on birds' diving behaviour identified areas of elevated bycatch risk where high levels of diving activity and fishing effort coincided. In addition, we identified times of day and water depths where diving activity, and hence bycatch risk, may be concentrated. Bycatch risk hotspots for all 3 species were identified along the north-east coast of England. Risk hotspots were also identified along the Pembrokeshire coast for both auk species and along the Cornish coast for shag. Lack of fishing effort data for smaller vessels made it difficult to assess seabird-fishery overlap in Scottish waters. Across species, diving activity was lower at night. For razorbill and guillemot, dive depth tended to increase at sunrise and decrease after sunset. For shag, dive depth showed no diel pattern but was associated with water depth. Our findings should assist in targeting spatio-temporal measures and designing deterrent devices to reduce bycatch. However, scarcity of data on the behaviour of gillnet fishers at comparable spatio-temporal resolution as seabird movement data remains a constraint to fully understanding seabird-fisheries interactions.