Tracking Atlantic bluefin tuna from foraging grounds off the west coast of Ireland

Abstract

Pop-up archival tags (n=16) were deployed on Atlantic bluefin tuna off the west coast of Ireland in October and November 2016 (199 to 246cm Curved Fork Length, CFL), yielding 2799 days of location data and 990 and 989 days of depth and temperature time-series data respectively, including downloaded archives from three recovered tags. Most daily locations (96%, n=2,651) occurred east of 45°W, the current stock management boundary for Atlantic bluefin tuna. Key open ocean habitats occupied were the Bay of Biscay and the Central North Atlantic, with two migratory patterns evident: an east-west group and an eastern resident group. Five out of six tags that remained attached until July 2017 returned to the northeast Atlantic after having migrated as far as the Canary Islands, the Mediterranean Sea and the Central North Atlantic. Tracked bluefin tuna exhibited a diel depth-use pattern occupying shallower depths at night and deeper depths during the day. Four bluefin tuna visited known spawning grounds in the central and western Mediterranean Sea, and one may have spawned, based on recovered data showing oscillatory dives transecting the thermocline on 15 nights. These findings demonstrate the complexity of the aggregation of Atlantic bluefin tuna off Ireland and, more broadly in the northeast Atlantic, highlighting the need for dedicated future research to conserve this important aggregation.