Larval fish assemblages and particle back-tracking define latitudinal and cross-shelf variability in an eastern Indian Ocean boundary current

Empirical and modelled data were used to examine the influence of the Leeuwin Current on larval fish assemblages along the Western Australian continental shelf and adjacent eastern Indian Ocean (22°S-34°S) during the late austral autumn. Larval fish assemblages, comprising >200 taxa from 114 neritic and oceanic teleost families, displayed significantly distinct latitudinal and cross-shelf variability, which was linked to meso-scale features of the Leeuwin Current, specifically anticyclonic eddies. Results of Lagrangian particle back-tracking showed good connection between the shelf and a developing eddy situated at 27°S; connectivity was highest in the 0-20 m depth stratum where >80% of the particles entering the eddy were derived from the shelf. This was supported by the high abundance (mean: 84 larvae m -2) of larval neritic taxa, especially small anchovy Engraulis australis larvae in the eddy. The lack of a significant difference in the size structure of these larvae between the shelf and eddy indicated continuous connection between these waters. In contrast, connectivity between the shelf and an older eddy further south was much lower (<40% of particles in the 0-20 m stratum) and the larval fish assemblage was dominated by meso-pelagic species. Further, the distribution of Myctophidae larvae (Diaphus spp.) highlighted areas of enhanced onshore transport in the south. The study has shown that, in the alongshore dominated Leeuwin Current system, cross-shelf transport can be similarly important in dispersal processes.