Environmental preferences of yellow ® n tuna ( Thunnus albacares ) at the northern extent of its range

We used acoustic telemetry to examine the small-scale movement patterns of yellow®n tuna (Thunnus albacares) in the California Bight at the northern extent of their range. Oceanographic pro®les of temperature, oxygen, currents and ̄uorometry were used to determine the relationship between movements and environmental features. Three yellow®n tuna (8 to 16 kg) were tracked for 2 to 3 d. All three ®sh spent the majority of their time above the thermocline (18 to 45 m in depth) in water temperatures >17.5 °C. In the California Bight, yellow®n tuna have a limited vertical distribution due to the restriction imposed by temperature. The three ®sh made periodic short dives below the thermocline (60 to 80 m), encountering cooler temperatures (>11 °C). When swimming in northern latitudes, the depth of the mixed layer largely de®nes the spatial distribution of yellow®n tuna within the water column. Yellow®n prefer to spend most of their time just above the top of the thermocline. Oxygen pro®les indicated that the tunas encountered oceanic water masses that ranged most often from 6.8 to 8.6 mg O2 l , indicating no limitation due to oxygen concentrations. The yellow®n tuna traveled at speeds ranging from 0.46 to 0.90 m s (0.9 to 1.8 knots h) and frequently exhibited an oscillatory diving pattern previously suggested to be a possible strategy for conserving energy during swimming.