Use of bird-borne radar to examine shearwater interactions with legal and illegal fisheries.

Seabirds interact with fishing vessels to consume their discards and baits, sometimes resulting in incidental capture (bycatch) and the death of the bird, with clear conservation implications. To understand seabird-fishery interactions at large spatiotemporal scales, researchers increasingly use simultaneous seabird and fishing vessel tracking. However, vessel tracking data can contain relevant gaps due to technical issues, illicit manipulation, or the lack of adoption of tracking monitoring systems. Such hidden vessel activity might lead to underestimating the fishing effort and bycatch rates, jeopardizing the effectiveness of marine conservation management. In this work, we deployed bird-borne radar detector tags, capable to record the occurrence of radar vessels, on three shearwaters species foraging in the NW Mediterranean Sea and the Canary Current Large Marine Ecosystem. We modelled radar detections in relation to gridded AIS vessel tracking data to understand the spatiotemporal dynamics of seabird-vessels interactions and identify unreported fishing activity areas. Our models showed a moderate fit to vessel tracking data, indicating a strong association of shearwaters to fishing vessels in major fishing grounds, although in high marine traffic density regions radar detections were also driven by non-fishing vessels. Moreover, radar detector tags registered the presence of potential unregulated and unreported fishing vessels in West African waters. Overall, we showed bird-borne radar detectors can unveil the areas and periods when the association of seabirds to both, reported and unreported fishing vessels, is more intense, which should help to improve the focus of conservation efforts. Article impact statement: Radar detector unveils shearwaters associations with reported and unreported fishing vessels across NW Mediterranean and NE central Atlantic. This article is protected by copyright. All rights reserved.