Yellowfin tuna (Thunnus albacares) aggregations along the shelf break off south-eastern Australia: links between inshore and offshore processes

In May 1996 the biological oceanography of the main yellowfin tuna longline fishing waters off southern New South Wales, Australia, was examined in re lation to the catch by the fishery. A warm-core eddy was identified directl y east of Eden with a temperature at 250 m depth of 15 degreesC. At the wes tern edge of this eddy, relatively high levels of fluorescence (chlorophyll a) were recorded together with pigments typical of diatoms, a feature of u pwelling communities. The biomass of zooplankton and micronekton was also s ignificantly higher at the western edge of the eddy. Similarly, acoustic da ta showed relatively high concentrations of backscatter at the margins of t he eddy, particularly at the shelf break and slope. These areas had the gre atest potential prey biomass; a fact supported by the presence of shelf- an d slope-associated prey species in the stomachs of yellowfin tuna caught at the same time. Fishery data for yellowfin tuna (Thunnus albacares) capture s showed that catch per unit effort was highest along the shelf and shelf b reak. Examination of yellowfin tuna catch records from 1988 to 1998 from so uth-eastern Australia showed highest catches in 1996, similar to 75% of the catch coming from the western edge of the eddy. We conclude that the prese nce of a warm-core eddy in the area at this time provided a localized but p roductive area to which the yellowfin were attracted.