Ichthyoplankton assemblages and primary production in meso-scale eddies associated with the Leeuwin Current, Eastern Indian Ocean

Along the coast of Western Australia, where upwelling is largely suppressed by the southward flowing Leeuwin Current, meso-scale eddies may be significant for the input of nutrients and overall productivity of the region. Eddies forming near the continental shelf also provide a potential mechanism for the entrainment of coastal water, and may therefore act as offshore transport mechanisms for planktonic organisms. In October 2003, a cruise was undertaken on the RV Southern Surveyor to investigate the dynamics of two eddies, located about 500km offshore, that had been generated off the Western Australian coast. Primary production patterns, nutrient cycling, oceanographic and biological characteristics of both eddies (one cold-core and one warm-core) were investigated over a 23 day period, using a wide range of methods including a continuous physiochemical sampler (“Seasoar”). Larval fish were obtained from the centre, body and perimeter of each eddy using oblique bongo net tows and depth-stratified EZ net tows. The larval fish assemblage was mostly composed of oceanic families, such as Myctophidae, Phosichthydae and Gonostomatidae. The cold-core eddy showed few differences between its centre, body and perimeter in terms of larval density and larval fish assemblage structure. 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