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 relation to the catch by the fishery. A warm-core eddy was identified directly east of Eden with a temperature at 250 m depth of 15 degreesC. At the western edge of this eddy, relatively high levels of fluorescence (chlorophyll a) were recorded together with pigments typical of diatoms, a feature of upwelling communities. The biomass of zooplankton and micronekton was also significantly higher at the western edge of the eddy. Similarly, acoustic data showed relatively high concentrations of backscatter at the margins of the eddy, particularly at the shelf break and slope. These areas had the greatest potential prey biomass; a fact supported by the presence of shelf- and slope-associated prey species in the stomachs of yellowfin tuna caught at the same time. Fishery data for yellowfin tuna (Thunnus albacares) captures showed that catch per unit effort was highest along the shelf and shelf break. Examination of yellowfin tuna catch records from 1988 to 1998 from south-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 presence of a warm-core eddy in the area at this time provided a localized but productive area to which the yellowfin were attracted.