BIOMASS OF ZOOPLANKTON AND MICRONEKTON IN THE SOUTHERN BLUEFIN TUNA FISHING GROUNDS OFF EASTERN TASMANIA, AUSTRALIA

The southern bluefin tuna (SET) supports a seasonal fishery off the ea st coast of Tasmania, Australia. The distribution of zooplankton bioma ss in this region was examined as a means of finding out why the SET a re attracted to this area. We examined whether there was a particular area or depth stratum that supported significantly greater amounts of potential feed, directly or indirectly, for SET. Samples of zooplankto n and micronekton were collected during the winter SET fishery seasons in 1999-94. Five net types (mouth opening 0.25 to similar to 80 m(2)) with codend mesh sizes ranging from 100 to 1000 mu m were used. Sampl es were collected from 4 main hydrographic areas: warm East Australian Current water, cool subantarctic water, the front separating them (th e subtropical convergence), and the adjacent shelf. Four depth strata (50, 150, 250 and 350 m) were also sampled. In contrast to our expecta tions, the biomass in the subtropical convergence was no greater than that in the 3 other areas. Rather, it was the shelf, albeit with some inconsistencies, that generally had the greatest biomass of both zoopl ankton and micronekton. Offshore, there was no significant difference in the biomass of the depth strata sampled, although the biomass of ge latinous zooplankton in the surface waters increased during the study period. We suggest that the higher biomass on the shelf is the result of increased nutrients derived from a mixture of subantarctic water an d upwelling along the shell break. This biomass is converted via krill and gelatinous zooplankton to small pelagics such as jack mackerel, a nd finally to top predators, amongst which is SET. The SET, particular ly sub-adults, may time their migration eastward to take advantage of the concentrations of prey present at this time of year.