Foraging ecology and fisheries overlap drive bycatch risk in a declining New Zealand shag population

Summary

Set net fisheries are a global threat to diving seabirds, yet fine-scale data on interactions with fishing gear remain limited, particularly in small-scale inshore fisheries. The kawau tikitiki (Spotted Shag Phalacrocorax punctatus ) is a coastal endemic of Aotearoa New Zealand that has undergone regional declines, with the Hauraki Gulf population now endangered. We combined a decade of population monitoring (2015–2025) with bird-borne GPS and dive logging, commercial set-net location data and coastal recreational set-net surveys to investigate population trends, foraging ecology, habitat use, and fisheries impacts. Nest counts declined by 7% per year over the study, halving the breeding population. Tracking of 14 adult birds (2023–2024) showed that foraging was restricted to inshore habitats within the Firth of Thames, with small core areas (50% utilisation distribution: 47.6 km²) along the southeastern coastline of the Firth and adjacent mussel farms. Birds performed frequent, shallow dives (mean depth 4.1 m; ~150–200 dives per day) with seasonal shifts in habitat use, including increased use of mussel farms in late summer and autumn. Spatial and temporal overlap with commercial set-net fisheries was pronounced, particularly during November–December when birds concentrated in shallow coastal habitats used by fishers. Over 30% of set-net fishing deployments occurred within the overall foraging range and analysis identified repeated foraging dives within 50 - 300 m of active nets. Surveys along the same coastline revealed widespread deployment of recreational set nets within core foraging areas during this high-risk period. Two tracked birds drowned during the study, providing direct evidence of bycatch mortality. Findings indicate that set-net fisheries represent a significant threat to the kawau tikitiki population in the Hauraki Gulf. Limiting fisheries mortality and protecting key roosting and breeding sites will be critical to recovering the population.