Temperature's governing role in the spatial ecology of Atlantic bluefin tuna (Thunnus thynnus) in Ireland

Abstract

Many ecologically and commercially valuable fish taxa are highly mobile, occupying broad spatial distributions across pelagic ecosystems and experiencing substaintal environmental variability through space and time. Understanding how fluctuations in environmental conditions govern their distributions is essential for effective management, particularly under rapid climate change. This thesis examines the role of temperature in shaping the spatial ecology of Atlantic bluefin tuna (Thunnus thynnus; ABFT), a species of considerable ecological and commercial significance. Drawing on multiple datasets from Ireland, I provide the first comprehensive investigation of ABFT in Irish waters, where they have only recently reappeared following decades of overexploitation and subsequent recovery under Atlantic-wide management. This thesis is underpinned by two main datasets: Pop-up satellite archival tag (PSAT) data and catch data from Ireland�s scientific catch-and-release (C&R) fishery. In Chapter 2 I develop habitat suitability models for ABFT across the Northeast Atlantic, using a presence/absence modelling approach. This analysis identifies the environmental drivers of ABFT distribution and highlights seasonal and interannual variation. Furthermore, horizontal migration patterns indicate how marine heatwaves many influence migrations into Irish waters. The second data chapter (Chapter 3) further explores the seasonal residency of ABFT in Irish waters, revealing their residency year-round with shifting distributions. High- and low- resolution PSAT data demonstrates how the vertical behaviour of ABFT in Irish waters is further influenced sea surface temperature (SST) and mixed layer depth. In Chapter 4, I analysis data from Ireland�s C&R fishery to investigate the regional differences in catch patterns, size composition, and seasonal occurrence. By linking these to environmental conditions, particularly SST, I demonstrate regionally distinct responses in both catch rates and size structure. In the final data chapter (Chapter 5), I integrate tagging and catch data, to forecast habitat suitability and catch rates around Ireland under varying climate scenarios. Forecasts indicate an increase in habitat suitability around Ireland, but this does not translate to an increase in catchability. Collectively, this thesis underscores the importance of temperature in shaping ABFT movements across scales and demonstrates how both tagging- and fishery-dependent data can be combined to improve ecological understanding. More broadly, this work highlights the implications of a rapidly warming ocean on ABFT spatial ecology in the Northeast Atlantic and delivers evidence-based insights that can underpin adaptive, climate-responsive management for this ecologically and economically significant species in Ireland and beyond.