Tracking adult North Atlantic bluefin tuna (Thunnus thynnus) in the northwestern Atlantic using ultrasonic telemetry

Ultrasonic, depth-sensitive transmitters were used to track the horizontal and vertical movements, for up to 48 h, of 11 adult (136 to 340 kg estimate d body mass) North Atlantic bluefin tuna (Thunnus thynnus Linnaeus). Fish w ere tracked in October 1995, September and October 1996, and August and Sep tember 1997 in the Gulf of Maine, northwestern Atlantic. The objective was to document the behavior of these fish and their schools in order to provid e the spatial, temporal, and environmental information required for direct (i.e. fishery-independent) assessment of adult bluefin tuna abundance using aerial surveys. Transmitters were attached to free-swimming fish using a h arpoon attachment technique, and all fish remained within the Gulf of Maine while being followed. Most of the bluefin tuna tagged on Stellwagen Bank o r in Cape Cod Bay land followed for at least 30 h) held a predominately eas terly course with net horizontal displacements of up to 76 km d(-1). Mean ( +/- SD) swimming depth for all fish was 14 +/- 4.7 m and maximum depth for individuals ranged from 22 to 215 m. All but one fish made their deepest ex cursions, often single descents, at dawn and dusk. In general, adult bluefi n tuna spent < 8% of their time at the surface (0 to 1 m), < 19% in the top 4 m, but > 90% in the uppermost 30 m. Mean (+/- SD) speed over ground was 5.9 km h(-1) but for brief periods surpassed 20 to 31 km h(-1). Sea surface temperatures during tracking were 11.5 to 22.0 degrees C, and minimum temp eratures encountered by the fish ranged from 6.0 to 9.0 degrees C. Tagged b luefin tuna and their schools frequented ocean fronts marked by mixed verte brate feeding assemblages, which included sea birds, baleen whales, basking sharks, and other bluefin schools.