Horizontal movements and depth distribution of large adult yellow®n tuna (Thunnus albacares) near the Hawaiian Islands, recorded using ultrasonic telemetry: implications for the physiological ecology of pelagic ®shes

We measured the horizontal and vertical movements of ®ve adult yellow®n tuna (Thunnus albacares, estimated body mass 64 to 93 kg) near the main Hawaiian Islands, while simultaneously gathering data on oceanographic conditions and currents. Fish movements were recorded by means of ultrasonic depth-sensitive transmitters. Depth±temperature and depth±oxygen pro®les were measured with vertical conductivity±temperature±depth (CTD) casts, and the current-velocity ®eld was surveyed using an acoustic Doppler current pro®ler (ADCP). Large adult yellow®n tuna spent '60 to 80% of their time in or immediately below the relatively uniform-temperature surface-layer (i.e. above 100 m), a behavior pattern similar to that previously reported for juvenile yellow®n tuna, blue marlin (Makaira nigricans), and striped marlin (Tetrapturus audax) tracked in the same area. In all three species, maximum swimming depths appear to be limited by water temperatures 8 C° colder than the surface-layer water temperature. Therefore, neither large body mass, nor the ability to maintain elevated swimming-muscle temperatures due to the presence of vascular countercurrent heat exchangers in tunas, appears to permit greater vertical mobility or the ability to remain for extended periods below the thermocline. In those areas where the decrease in oxygen with depth is not limiting, the vertical movements of yellow®n tuna, blue marlin and striped marlin all appear to be restricted by the effects of water temperature on cardiac muscle function. Like juvenile yellow®n tuna, but unlike blue marlin and striped marlin, adult yellow®n tuna remained within 18.5 km of the coast and became associated with ̄oating objects, including anchored ®sh-aggregating devices (FADs) and the tracking vessel. Like juvenile yellow®n tuna, large adult yellow®n repeatedly re-visit the same FAD, and appear able to navigate precisely between FADs that are up to 18 km apart. The median speed over ground ranged from 72 to 154 cm s. Neither speed nor direction was strongly in ̄uenced by currents.