Horizontal and vertical distribution of larvae of Pacific bluefin tuna Thunnus orientalis in patches entrained in mesoscale eddies

Nine high density larval populations (patches) of Pacific bluefin tuna Thunnus orientalis were detected and 7 patches were tracked with reference buoys for 28 to 171 h in the northwestern Pacific Ocean in May-June 2004 to 2008. Buoy trajectories and surface current velocities and direc- tions measured by acoustic Doppler current profiler data showed close agreement. Growth rates for the sampled larvae (3.0 to 9.9 mm standard length (SL), 4 to 17 d after hatch), estimated by the daily modes of SL, correspond to growth rates estimated from otolith daily ring analysis. These results indi- cate that the same populations were sampled by tracking the buoy on almost all the sampling days. At fine- (100s of m to km) and mid-scale observations (~15 to 30 km range), patches consisted of a number of cohorts which had different distributions within the patch. The larval spatial structure was studied using variograms. At the fine scale, age-specific sills, ranges and spatial dependence of patches were similar; at the mid scale, these indexes showed stability during the trackings. Horizon- tally, larvae formed patches within an approximate 10 km range and advected together during the larval stage. Larvae were only distributed in the mixed layer and diel vertical movement was not clearly observed. Patches were entrained in mesoscale eddies (~100 to 500 km diameter) which prop- agated westward. Such mesoscale eddies are known to coalesce with the Kuroshio current. The spawning area and the recruitment fishing grounds are thereby linked by the Kuroshio. Results sug- gest that cohorts have a stable spatial structure after fertilization (i.e. during advection, while entrained in mesoscale eddies). Therefore, the positional relationship between spawning events and mesoscale eddies is concluded to be important for the recruitment process.