MEPS 138:1-14 (1996)  -  doi:10.3354/meps138001

Biomass of zooplankton and micronekton in the southern bluefin tuna fishing grounds off eastern Tasmania, Australia

Young JW, Bradford RW, Lamb TD, Lyne VD

The southern bluefin tuna (SBT) supports a seasonal fishery off the east coast of Tasmania, Australia. The distribution of zooplankton biomass in this region was examined as a means of finding out why the SBT are 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 SBT. Samples of zooplankton and micronekton were collected during the winter SBT fishery seasons in 1992-94. Five net types (mouth opening 0.25 to ~80 m2) with codend mesh sizes ranging from 100 to 1000 um were used. Samples were collected from 4 main hydrographic areas: warm East Australian Current water, cool subantarctic water, the front separating them (the subtropical convergence), and the adjacent shelf. Four depth strata (50, 150, 250 and 350 m) were also sampled. In contrast to our expectations, 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 zooplankton and micronekton. Offshore, there was no significant difference in the biomass of the depth strata sampled, although the biomass of gelatinous 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 and upwelling along the shelf break. This biomass is converted via krill and gelatinous zooplankton to small pelagics such as jack mackerel, and finally to top predators, amongst which is SBT. The SBT, particularly sub-adults, may time their migration eastward to take advantage of the concentrations of prey present at this time of year.

Zooplankton . Micronekton . Biomass . Southern bluefin tuna

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