Movements and geographic distribution of juvenile bluefin tuna in the Northeast Atlantic, described through internal and satellite archival tags

From 2005 to 2010, 136 internal archival tags and 29 pop-up satellite archival tags were used to track juvenile Atlantic bluefin tuna in the Bay of Biscay. Information from 15 pop-up and 5 internal archival tags was recovered. The analysis was adapted for a common treatment of both types of tag data, allowing classification of overwintering distribution patterns, fidelity to the Bay of Biscay feeding area, as well as of horizontal and vertical habitat utilization. Results show substantial geographic dispersion from autumn to spring, with high habitat concentration in the Bay of Biscay during summer, when bluefin tuna inhabit in the mixed layer. Of the individuals that left the Bay of Biscay towards the end of the year, a high percentage returned the next year, suggesting a strong fidelity to the area. Thirty-three percent of records during the overwintering periods revealed residency in the Bay of Biscay and surrounding areas. Half of the fish overwintered in the mid-Atlantic, near the Azores or Madeira Islands, while three (17%) made trans-Atlantic round trips, and one individual travelled to and remained off the eastern coast of the United States. These findings challenge previous assumptions regarding the seasonality and annual movements of bluefin tuna from the Bay of Biscay, while demonstrating extensive spatio-temporal dispersion.