Horizontal and vertical movements of juvenile bluefin tuna (Thunnus thynnus), in relation to oceanographic conditions of the western North Atlantic, determined with ultrasonic telemetry

We employed ultrasonic trans­ mitters to follow (for up to 48 h) the hor­ izontal and vertical movements of five juvenile (6.8–18.7 kg estimated body mass) bluefin tuna (Thunnus thynnus) in the western North Atlantic (off the eastern shore of Virginia). Our objective was to document the fishes’ behavior and distribution in relation to ocean­ ographic conditions and thus begin to address issues that currently limit pop­ ulation assessments based on aerial surveys. Estimation of the trends in adult and juvenile Atlantic bluefin tuna abundance by aerial surveys, and other fishery-independent measures, is con­ sidered a priority. Juvenile bluefin tuna spent the major­ ity of their time over the continental shelf in relatively shallow water (gen­ erally less then 40 m deep). Fish used the entire water column in spite of rela­ tively steep vertical thermal gradients (≈24°C at the surface and ≈12°C at 40 m depth), but spent the majority of their time (≈90%) above 15 m and in water warmer then 20°C. Mean swimming speeds ranged from 2.8 to 3.3 knots, and total distance covered from 152 to 289 km (82–156 nmi). Because fish gen­ erally remained within relatively confined areas, net displacement was only 7.7–52.7 km (4.1–28.4 nmi). Horizontal movements were not correlated with sea surface temperature. We propose that it is unlikely that juvenile bluefin tuna in this area can detect minor horizontal temperature gradients (gen­ erally less then 0.5°C/km) because of the steep vertical temperature gradi­ ents (up to ≈0.6°C/m) they experience during their regular vertical move­ ments. In contrast, water clarity did appear to influence behavior because the fish remained in the intermediate water mass between the turbid and phytoplankton-rich plume exiting Ches­ apeake Bay (and similar coastal waters) and the clear oligotrophic water east of the continental shelf. Manuscript accepted 6 July 2001. Fish. Bull. 100:155–167 (2002). Horizontal and vertical movements of juvenile bluefin tuna (Thunnus thynnus), in relation to oceanographic conditions of the western North Atlantic, determined with ultrasonic telemetry Richard Brill Michael Arendt Pelagic Fisheries Research Program Jon Lucy Joint Institute for Marine and Sea Grant Program Atmospheric Research Virginia Institute of Marine Science School of Earth and Ocean Science Technology College of William and Mary University of Hawaii at Manoa Gloucester Point, Virginia 23062-1346 Honolulu, Hawaii 96822 E-mail address: [email protected].