Ichthyoplankton in two meso-scale Leeuwin Current eddies: preliminary results

In many coastal oceans, eddies and jets occur predictably under certain conditions, and are a common mechanism for entraining coastal water across the continental shelf. Eddies may therefore act as transport mechanisms for planktonic organisms and larvae. In October 2003, a cruise was undertaken on the RV Southern Surveyor to investigate the dynamics of two eddies located more than 500km off the Western Australian coast. Primary production patterns, nutrient cycling, oceanographic and biological characteristics of both eddies (one cold-core upwelling, and one warm-core downwelling) were investigated over a 23 day period, using oblique and depth stratified plankton tows, continuous physiochemical samplers (“Seasoar”), sediment traps and other methods. Larval fish were collected from plankton samples taken in the centre, body and perimeter of each eddy, during both day and nighttime, with both oblique bongo, and depth stratified EZ Net tows. Preliminary analysis shows that the larval fish assemblage was mostly composed of offshore and oceanic families, such as the Myctophidae, Phosichthydae and Gonostomatidae. The cold-core eddy showed few differences between its centre, body and perimeter in terms of larval density, and species assemblage. The warm-core eddy, however, showed much lower densities of larvae in the centre, compared to both the body and perimeter, and to the cold-core eddy. This was particularly apparent at night, suggesting that the dynamics of the centre of the warm-core eddy were affecting the presence, and/or the vertical migration of larger larvae in the eddy at night