ENVIRONMENTAL PREFERENCES OF YELLOWFIN TUNA (THUNNUS ALBACARES) AT THE NORTHERN EXTENT OF ITS RANGE

We used acoustic telemetry to examine the small-scale movement pattern s of yellowfin tuna (Thunnus albacares) in the California Bight at the northern extent of their range. Oceanographic profiles of temperature , oxygen, currents and fluorometry were used to determine the relation ship between movements and environmental features. Three yellowfin tun a (8 to 16 kg) were tracked for 2 to 3 d. All three fish spent the maj ority of their time above the thermocline (18 to 45 m in depth) in wat er temperatures > 17.5 degrees C. In the California Bight, yellowfin t una have a limited vertical distribution due to the restriction impose d by temperature. The three fish made periodic short dives below the t hermocline (60 to 80 m), encountering cooler temperatures (> 11 degree s C). When swimming in northern latitudes, the depth of the mixed laye r largely defines the spatial distribution of yellowfin tuna within th e water column. Yellowfin prefer to spend most of their time just abov e the top of the thermocline. Oxygen profiles indicated that the tunas encountered oceanic water masses that ranged most often from 6.8 to 8 .6 mg O-2 l(-1), indicating no limitation due to oxygen concentrations . The yellowfin tuna traveled at speeds ranging from 0.46 to 0.90 m s( -1) (0.9 to 1.8 knots h(-1)) and frequently exhibited an oscillatory d iving pattern previously suggested to be a possible strategy for conse rving energy during swimming.