Movements and diving behavior of Atlantic bluefin tuna Thunnus thynnus in relation to water column structure in the northwestern Atlantic

We analyzed the movements and diving behavior in relation to water column structure of 35 electronically tagged Atlantic bluefin tuna (176 to 240 cm in length at tagging) during their spring-fall period of occupancy of the Gulf of Maine, Canadian Shelf, and neighboring off-shelf waters from 1999 to 2005. Tagged fish arriving in this study region in March-April initially occupied weakly stratified off-shelf waters along the northern Gulf Stream. As waters over the continental shelf warmed in June, the fish shifted onto the shelf. Sea surface temperatures occupied were relatively constant in both off- and on-shelf waters (April-September monthly medians varying from 16.1 to 19.0 degrees C). Dives made in the stratified waters of the shelf during summer and fall were significantly more frequent (up to 180 dives d(-1)) and fast (descent rates up to 4.1 m s(-1)) than in weakly stratified off-shelf waters occupied during spring, defining dives as excursions below tag-derived estimates of the surface isothermal layer depth (ILD). The duration and depth of dives also decreased significantly in association with changing water column structure, from medians in off-shelf waters during April of 0.45 h and 77.0 m, respectively, to 0.16 h and 24.9 m in August. Analyses of tag measurements of internal body temperature and feeding proxies suggest that both the horizontal movement of the fish onto the continental shelf and the dives made from the warm surface layer into colder waters at depth represent a balance between the maintenance of preferred ambient temperatures and foraging opportunities.