Seasonal variability of high-latitude foraging grounds for Atlantic bluefin tuna (Thunnus thynnus)

Aim: Predicting spatial and temporal changes in species distributions is difficult for highly mobile species, making effective management challenging. We aim to understand the seasonal variability in habitat suitability for Atlantic bluefin tuna (Thunnus thynnus; ABFT) by using tracking data to create species distribution models. Location: North-east Atlantic Ocean. Methods: Forty-nine ABFT were tagged with pop-up archival satellite tags along the west coast of Ireland during successive autumns from 2016 to 2021. Four thousand two hundred and sixteen daily locations were used to build a habitat model to describe ABFT environmental preferences and habitat suitability. For each location, 100 'pseudoabsence' locations were simulated using correlated random walks. Results: Tags had a mean retention rate of 237 +/- 107 days, resulting in 11,602 cumulative days at liberty, and a mean daily distance travelled of 51 +/- 143 km. ABFT showed two primary migration routes from Ireland, travelling to the Bay of Biscay or across the Mid-Atlantic ridge in the winter. However, in 2019 six individuals travelled northwards towards Iceland, likely in response to a marine heatwave. Eight ABFT entered the Mediterranean in May/June for putative spawning, with six returning to Irish waters a year later. Five environmental covariates best described ABFT habitat, with habitat suitability highest at sea surface temperatures (SST) of 15-18 degrees C, a mixed layer depth between similar to 200 and 300 m, high relief bathymetric features (i.e. shelf breaks and seamounts) and frontal regions as revealed by non-linear preferences of eddy kinetic energy and SST standard deviation. Main Conclusions: In the last decade, ABFT have been re-establishing historic foraging grounds in higher latitudes, for example, Irish and Nordic waters. Our model shows the extent of suitable habitat in these regions and how recent ocean warming may have contributed to northern expansions. Understanding the drivers behind these reappearances is fundamental to successful future management.