Entrainment and development of larval fish assemblages in two contrasting cold core eddies of the East Australian Current system

Abstract

Cyclonic eddies are diverse in their size, age, upwelling and behaviour, which has significant implications for fisheries production and connectivity when they interact with the continental shelf. To ascertain coastal entrainment by eddies, we compared the larval fish community of 2 contrasting cyclonic eddies in 3 depth strata (0-5, 5-50, 50-100 m), and with the adjacent shelf community. The frontal cyclonic eddy was smaller and younger than the mesoscale cyclonic eddy. A larval fish entrainment index, based on the ratio of coastal to oceanic taxa, revealed the relative abundance of coastal larvae entrained into the upper mixed layer of the frontal eddy, consistent with published numerical modelling studies of similar eddies. The frontal eddy had a high abundance of commercially important coastal taxa entrained from the inner shelf. However, the adjacent inner shelf water and putative location for frontal eddy formation had recently been displaced by the East Australian Current, resulting in the larval fish community being dominated by oceanic taxa. The spatial and temporal dynamics of coastal entrainment into the larger, older cyclonic eddy and the adjacent shelf region were revealed by mixtures of coastal and oceanic taxa in each of the depth strata. The larger cyclonic eddy had a higher biomass of zooplankton, indicating the cumulative effects of eddy age and production. Eddies which interact with the shelf enable cross-shelf mixing and may contribute to coastal fisheries.