School fidelity and homing synchronicity of yellowfin tuna, Thunnus albacares

Abstract

Thirty-eight yellowfin tuna (Thunnus albacares) were tagged with coded ultrasonic beacons between 6 March and 4 December 1996 near two buoys off the western coast of Oahu, Hawaii. Two to four tuna were captured, tagged, and released on the same day in as rapid succession as possible in an effort to tag members of the same school. Automated “listening” monitors attached to the buoys recorded when these marked individuals entered within a radius of ≤1.1 km of the buoys during a 13 mo period. Twenty-seven of the tuna returned to the site of tagging. The mean number of returns was 4.2 per tuna (max. = 17), and visits ranged from 1 to 910 min (median = 2.7 min, mean = 40.1 min). The intervals between successive returns varied from 1 to 257 d (median = 3.0 d, mean = 17.4 d). Seventy-three percent of the tuna returned together with tunas tagged on the same day, exceeding the frequency of returns of tuna tagged on another day or arriving alone. This social cohesion is supported by the pattern of return visits by five tuna tagged on 6 March at Monitoring Station R. Two or more of these tuna arrived together on 24 of 35 d when tagged tuna were detected. All five individuals visited R on 11 April, a month after tagging, three arrived together 5 mo later on 4 August, and three returned 6 mo later on 1 December 1996. Tuna often arrived at the same time of day, e.g. Individuals 1 and 3 visited R at 09:15 hrs on 12 April and at 09:00 hrs 8 mo later. The returns were also site-specific. The 22 tuna tagged at R made 182 return visits to R (92.4%) and only 15 visits to Monitoring Station K (7.6%), 10 km away. An allegiance of tuna to one school, a predilection for returning to the site of tagging, and precise timing when visiting sites, are consistent with tuna having migratory pathways consisting of “way-points” that are visited with temporal regularity.