Incorporation of larval fishes into a developing anti-cyclonic eddy of the Leeuwin Current: timing, sources and pathways

Meso-scale circulation and cross-shelf transport of planktonic biota associated with the formation of an anticyclonic (warm-core) eddy of the poleward-flowing Leeuwin Current (LC) were examined during a month-long, multidisciplinary voyage off south-western Australia. The study was based upon Lagrangian examination of upper ocean circulation and stations for depth-integrated (bongo nets) and depth-stratified (EZ nets) sampling of larval fishes were linked primarily with the locations of satellite-tracked oceanographic drifters. The regional dominance of a modified LC water mass, particularly in the eddy field, indicated strong mixing between LC, shelf and oceanic subtropical surface water and the eddy had physical, chemical and biological signatures reflecting these source waters. High concentration of chlorophyll a (0.5 g L-1) in the eddy was derived, at least in part, from the shelf and was much greater than that in the surrounding ocean. In the study area, the horizontal distributions of larvae of fishes with both neritic and oceanic origins indicated strong onshore-offshore coupling. High concentrations of larval fishes in the eddy (1.0 – 3.0 larvae / m3) contrasted with those in the surrounding ocean. Assemblages were dominated by the larvae of oceanic mesopelagic taxa such as Myctophidae. Larvae of tropical neritic taxa (e.g. Bothidae, Pomacentridae) which were in much lower concentrations (