Tracking bluefin tuna reproductive migration into the Mediterranean Sea with electronic pop-up satellite archival tags using two tagging procedures

Thirteen adult bluefin tuna were tracked with elec- tronic pop-up satellite tags during their reproductive migration towards Mediterranean spawning grounds as they entered the Strait of Gibraltar. Fish were caught in tuna traps and tagged either underwater, with the aid of a modified spear gun, or on the deck of the boat. Fish tagged on board initially showed a shallower behavior than those tagged in the water. The pattern of horizontal movements was also different between both groups. Shortly after tagging, the eight fish tagged in the water entered the Mediterranean Sea. Six of these fish reached the spawning ground located south- west of the Balearic archipelago before headin g back for the Atlantic, whereas the other two traveled far- ther east, reaching its easternmost longitudes between Formentera and Sardinia and the South Tyrrhenian Sea, respectively. In contrast, two out of the five fish tagged on board never entered the Mediterranean Sea, and another one did enter the Medi terranean when the reproductive season was already over. These results suggest an impact of the tagging procedure on the post-release behavior of bluefin tuna. Exclu ding the tags that popped-off east of the Strait of Gibra ltar, bluefin tuna stayed in the Mediterranean Sea for 22– 28 days. Analysis of the median depth indicated a shallow behavior during both day and nighttime throughout the return phase of the fish from the Mediterranean Sea to the Atlantic Ocean with the exception of the area around the Strait of Gibraltar, where they showed a deeper behavior that coincided with a marked vertical gradient in the currents.Versión del editor2,044