Distribution of juvenile bluefin tunas in the North Atlantic: comparison between light based geolocations from internal and pop-up satellite archival tags.

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This study aims at comparing light based geolocations from internal and pop-up satellite archival tags implanted on juveniles Bluefin tunas, and to discuss their potential consequences for characterizing bluefin tuna migrations and habitat utilization. From 2005 to 2010, 136 juvenile bluefin tunas were released in the Bay of Biscay with internal archival tags and 29 ones with miniaturized pop-up satellite archival tags. Archived information was recovered from 3 individuals tagged internally and from 15 individuals tagged with PSATs. For both types of tags, an SST-assisted Kalman filter was applied to the raw light-based geolocations. The seasonality of the distribution and movements of the juvenile bluefin tunas were analyzed. We observed that during autumn months (start of the overwintering migration and maximal latitudinal error in light-based geolocation) the standard deviation of the geolocations is lower in average for internal archival tag data than for pop-up tags. The higher spatial dispersion of pop-up tag data position estimates is discussed in relation to the accuracy of the characterization of different migration patterns and habitat use. Despite the difference between both tagging types, these position estimates are considered reliable enough to build a habitat utilization index and to characterize winter migrations.