Oceanographic processes governing the dispersal and recruitment of marine larvae off South-Western Australia
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Larval and juvenile recruitment of two Abudefduf damselfish species at Rottnest Island vary both seasonally and inter-annually. Biological and oceanographic factors together support the conjecture that the larvae originate at the Abrolhos Islands some 300 km to the north and are dispersed southwards in the Leeuwin Current. There are very strong links with environmental indices, with enhanced recruitment when the Leeuwin Current is flowing strongly and ocean temperatures are elevated, which generally occurs during La Niña events. This was especially evident during the intense La Niña event which triggered the marine heat wave of early 2011, when the Leeuwin Current was at a near record strength and local water temperatures briefly reached an unprecedented 5°C above long-term background values. Currents along the Western Australian continental shelf seasonally reverse between the northward Capes Current in summer and dominantly southward flow during the winter months. There are, however, frequent shorter-term current reversals throughout the year, and current drifter tracking reveals that drifter buoys released in relatively close proximity can be rapidly entrained into different current systems and moved in opposite directions. This has important implications for commercial and recreational fisheries as the exact timing and location of pelagic spawning events can determine the subsequent movement and ultimate destination of the eggs and larvae, whether retention on the shelf or export into the open ocean and probable loss to the fishery. Against a backdrop of progressively rising water temperatures, the potential for a possible southward range extension of Abudefduf spp. to Rottnest Island in the form of establishing a breeding population there is explored.
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