Abstract
Spawning behaviour of Atlantic bluefin tuna (Thunnus thynnus) was investigated using electronic satellite tags deployed in the western Mediterranean spawning ground, around the Balearic Islands (years 2009-2011). All the fish were tagged underwater and released within schools. In general, the fish tagged in the same year/school displayed common migratory trends. Following extended residency around the Balearic Islands, most tagged tuna crossed the Strait of Gibraltar heading for the North Atlantic. Discrepancies between the migratory tracks reconstructed from this and previous electronic tagging studies suggest that the bluefin tuna Mediterranean population may comprise distinct units exhibiting differing migratory behaviours. The diving behaviour varied between oceanic regions throughout the migratory pathways, the shallowest distribution taking place in the spawning ground and the deepest at the Strait of Gibraltar. A unique diving pattern was found on the majority of nights while the fish stayed at the spawning ground; it consisted of frequent and brief oscillatory movements up and down through the mixed layer, resulting in thermal profiles characterized by oscillations about the thermocline. Such a pattern is believed to reflect recent courtship and spawning activity. Reproductive parameters inferred from the analysis of vertical profiles are consistent with those estimated in previous studies based on biological samples.
Highlights
In spring, Atlantic bluefin tuna, Thunnus thynnus (Linnaeus, 1758), perform long seasonal reproductive migrations between feeding areas in the Atlantic Ocean and spawning grounds, either in the Gulf of Mexico or the Mediterranean Sea
Most electronic tags implanted on Atlantic bluefin tuna have been deployed in the north-western Atlantic Ocean, off the east coasts of the United States of America (US) and Canada [4,5,6,7,8,9,10,11,12,13,14,15,16,17]
Electronic tagging experiments conducted in the eastern Atlantic Ocean and Mediterranean Sea are scarce and their results less conclusive [18,19,20,21,22,23,24,25,26,27], even though the eastern stock size is around 10 times that of the western population [28,29,30]
Summary
Atlantic bluefin tuna, Thunnus thynnus (Linnaeus, 1758), perform long seasonal reproductive migrations between feeding areas in the Atlantic Ocean and spawning grounds, either in the Gulf of Mexico (western stock) or the Mediterranean Sea (eastern stock). The continued decline of Atlantic bluefin tuna spawning biomass between the 1970s and 2000s has raised great concern regarding the sustainability of the resource and led to serious questions on the efficacy of current fishery management [1,2,3], though the most recent assessments show signs of biomass increase, especially in the eastern stock. Electronic tagging experiments conducted in the eastern Atlantic Ocean and Mediterranean Sea are scarce and their results less conclusive [18,19,20,21,22,23,24,25,26,27], even though the eastern stock size is around 10 times that of the western population [28,29,30]
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