Spatio-temporal variation in Atlantic bluefin tuna (Thunnus thynnus) catch rates and sizes in a scientific catch-and-release fishery

Effective fisheries management requires high quality data on how species’ distributions vary through space and time. Atlantic bluefin tuna (ABFT; Thunnus thynnus ) are among the largest and most valuable commercial fish species globally and although managed internationally, regional-scale data are required to support evidence-based management. Recreational catch-and-release fisheries offer a cost-effective method for studying fish populations while also providing socio-economic benefits. In this study, we present data from an angler based scientific catch-and-release fishery for ABFT, operating during the summer-autumn months in Ireland since 2019. Significant annual variations in both the number and size of fish captured were reported across the Irish coast, with the Northwest emerging as the most productive region, where catch rates per fishing trip were nearly double those observed in the South of Ireland. Fish arrived slightly later in the season in the South but were on average larger than those in the Northwest. Catch rates were highest at intermediate surface temperatures between 12.5 and 15°C, whereas larger fish were caught at the start of the fishing season in warmer waters, with fish size declining as the season progressed into cooler autumn months. Given the highly migratory nature of ABFT, these regional and seasonal patterns in relative abundance and size structure provide insight into population dynamics in Irish waters. The relationship with temperature also highlights the potential for climate change to alter future distribution patterns, with possible consequences for the long-term viability of the recreational fishery, and the coastal communities that benefit from it.