For migratory animals, understanding where individuals are in space and time is a cornerstone for conservation. Atlantic bluefin tuna (ABT) are a migratory teleost fish with a high commercial value. Their distribution in the North Atlantic has fluctuated over time, and as a result a key research goal over the past two decades has be discerning trends in both their general distribution and migratory habits. This thesis investigates the contemporary spatial ecology of ABT in the northeast Atlantic.

This thesis represents the most comprehensive analysis of the spatial ecology of ABT in the northeast Atlantic to date. In Chapter 2 I combine datasets from multiple sources (citizen science, ecotour operators, fisheries surveys, and commercial fisheries) to demonstrate that ABT have increased in relative occurrence off the UK and Ireland since 2014. This increase is likely linked to an increase in the eastern stock of ABT, and in part to hydroclimatic forcing. In Chapter 3, I show that ABT present in foraging aggregations off the coast of Ireland form two migratory ecotypes, one that migrates to the Central Atlantic and another that conducts dominant east-west movements. For all ABT tracked from Ireland, the Bay of Biscay is identified as an important foraging ground, no tracked ABT migrated to the English Channel (hereafter “the Channel”) and individuals from both ecotypes visit known spawning grounds in the Mediterranean Sea. In Chapter 4, I identify a direct relationship between data quantity and the accuracy and precision of geolocation for tracked ABT using Wildlife Computers’ proprietary software (GPE3) and hardware (MiniPATs). To aid the field of biologging. I provide researchers and manufacturers with quality control processes to reduce the risk of drawing conclusions based on inaccurate or imprecise data. The ramifications of the findings from this chapter are considerable as they will aid all researcher undertaking PAT-based research requiring route reconstruction from drawing erroneous conclusions. In Chapter 5, I contrast the migratory behaviours observed in Chapter 3 by analysing a dataset generated from 50 ABT tagged with PATs at foraging grounds on the Celtic Deeps and off southwest England. Generally, these ABT did not migrate to waters off the coast of Ireland but did overlap spatially on foraging grounds in the Central Atlantic and Bay of Biscay and on eastern-stock spawning grounds in the
Western and Central Mediterranean Sea. Finally, the piece of work outlines putative spawning in four Mediterranean Sea regions.

Together, these chapters indicate that ABT have returned to waters off the UK and Ireland for the first time in a generation. The ABT that now, once again, visit these waters (1) comprise at least two migratory ecotypes, (2) all spend most of the time east of the 45°W meridian, (3) visit known spawning grounds in the Mediterranean Sea when large enough, and (4) exhibit tagging-site fidelity in subsequent years. Between July and November large ABT forage off the Irish coast and smaller ABT forage in waters off southwest England. However, outside of this period, ABT tracked from the UK and Ireland mix extensively on High Seas foraging grounds and in the Bay of Biscay, specifically. These results have some parallels with the migratory behaviours of ABT tracked from the USA, where similar size-specific migration patterns have been documented. Finally, the results detailed from tracking ABT shown here have important implications for conservation through management of the commercial fishery. Previous assumptions of foraging aggregations off the UK and Ireland being comprised of ABT that roam freely throughout eastern and western management regions are dispelled and, instead, I show that the individuals responsible for the recovery of ABT in the northeast Atlantic, remain almost completely in eastern stock management regions.