Turtles on the edge: movement of loggerhead turtles (Caretta caretta) along oceanic fronts, spanning longline fishing grounds in the central North Pacific, 1997-1998

Nine juvenile loggerhead sea turtles tracked during 1997 and 1998 in the ce ntral North Pacific by satellite telemetry all travelled westward, against prevailing currents, along two convergent fronts identified by satellite re motely sensed data on sea surface temperature (SST), chlorophyll and geostr ophic currents. These fronts are characterized by gradients in sea surface height that produce an eastward geostrophic current, with gradients in surf ace chlorophyll and SST. Six of the turtles were associated with a front ch aracterized by 17 degrees C SST, surface chlorophyll of about 0.2 mg m(-3), and eastward geostrophic current of about 4 cm s(-1), while the other thre e turtles were associated with a front with 20 degrees C SST, surface chlor ophyll of about 0.1 mg m(-3), and eastward geostrophic flow of about 7 cm s (-1). These results appear to explain why incidental catch rates of loggerh eads in the Hawaii longline fishery are highest when gear is set at 17 degr ees C and 20 degrees C, SST. Further, from the seasonal distribution of lon gline effort relative to these fronts, it appears that the surface longline fishing ground lies largely between these two fronts during the first quar ter and well to the south of the 17 degrees C front, but including the 20 d egrees C front, in the second quarter. These findings suggest seasonal or a rea closures of the longline fishery that could be tested to reduce inciden tal catches of loggerheads. Finally, these results illustrate the insights which can be achieved by combining data on movement of pelagic animals with concurrent remotely sensed environmental data.