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

Abstract

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 a s 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 m') w~t h codend mesh sizes ranging from 100 to 1000 pm were used. Samples were collected from 4 main hydrographic areas: warm East Australian Current water, cool subantarctic water, the front separating them (the subtrop~cal 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 s~gnificant 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 bionlass 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 tune of year

Acoustic characterisation of micronekton distribution in French Polynesia

“…Percentage is related to drained weight and dry weight (in parentheses). The conversion factors used to convert wet weight to dry weight came from Young et al (1996a Log(sa+l) total integrated on the whole water column SSL, IMSL. TSL, LSL, NSL, LAS, Presence (2)/absence (1) of the structural descriptors SSS, SAG, NUL of the scattering structures during a whole prospecting SSLl.5, hlSLI.5, TSLl.5, LSL1~5, NSL1.5, Presence (2)/absence (1) of the structural descriptors LASI.5, SSSI.~, NUL1.5 of the scattering structures by 100 m strata from surface to 500 m terns (Swartzman et al 1992, Simard et al 1993, Pelletier & Parma 1994, Coyle et al 1998) and stock assessments (Sullivan 1991, Guillard et al 1992, Petitgas 1993.…”

“…Fishes, molluscs and crustaceans (size-class of 1 to 10 cm) of the micronekton are the main tuna prey (Blackburn 1968, Sund et al 1981. Micronekton distribution and composition have been studied in some parts of the Pacific (Blackburn 1968, Young et al 1996a but remain poorly studied in the central South Pacific; the principal investigation was carned out by Legand et al (1972) but, unfortunately, micronekton spatial distribution was mainly described in the vertical plane and from pelagic trawls. However, micronekton biomass and composition studies are known to be biased by this type of sampling (Power 1996), particularly for micronektonic fishes, which are difficult to catch with existing techniques (Roger 1994).…”

Effect of sinking spring phytoplankton blooms on lipid content and composition in suprabenthic and benthic invertebrates in a cold ocean coastal environment

“…The range in mean lipid content for all calcareous taxa was 0.8 to 5.9% (13 Crustacea and the mud star Ctenodiscus crispatus), while the range for non-calcareous taxa was 0.3 to 1.3% (6 polychaete worm, shucked bivalve, holothurian and chaetognath taxa). Assuming that dry weight is 20% of wet weight for calcareous taxa (Billones et al 1999) and 3.4% of wet weight for noncalcareous taxa (Young et al 1996), the range in total dry weight-specific lipid content (lipid % of dry weight, % DW) for all calcareous taxa becomes 3.8 to 29%, and for the non-calcareous taxa 8.5 to 37%. Thus, on a dry weight basis, the ranges in total lipid content of the calcareous and non-calcareous taxa overlap to a great extent.…”

Inter-annual plasticity of squid life history and population structure: ecological and management implications

“…Under such conditions the dominant large zooplankter, krill (Nyctiphanes australis), and pelagic fish, jack mackerel (Trachurus declivis) disappear from the shelf . In contrast, when nutrient-rich water dominates the shelf, primary and secondary productivity are higher (Harris et al 1987;Young et al 1993) and numbers of predators increase (Young et al 1996). The oceanographic variability evident within Great Oyster Bay makes this region an ideal area to study environmentally induced individual and population variability in loliginid squid.…”