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Leeuwin current

Larval transport along the coast of South-Western Australia

Alan Pearce (PhD candidate; supervisors Dr Peter Fearns and Prof. Merv Lynch)

Because of the southward-flowing warm Leeuwin Current, there is an interesting mix of tropical and temperate marine species off south-western Australia. By comparison, the faunal composition off the west coasts of South America and southern Africa are dominated by cool-water species because of the cool north-flowing currents and upwelling systems operating in those regions.

The Leeuwin Current flows most strongly during the autumn, winter and early spring months, but can transport tropical species (both fish and invertebrates) into southern waters throughout the year. A case in point is the annual recruitment of tropical (reef) fish larvae along the southern coast of Rottnest Island, which has been studied for many years by Dr Barry Hutchins of the WA Museum. Two species of damselfish in particular have been monitored for over 2 decades, and clearly show a pulse of new recruits arriving at Parker Point each autumn, having been brought down (probably from the Houtman Abrolhos Islands) in March/April. The abundance of new arrivals varies from year, some of the inter-annual fluctuations being related to the Leeuwin Current and El Nino-Southern Oscillation (ENSO) events while others are more likely to be associated with biological factors such as spawning strength, larval predation and mortality.

Larval dispersal

The oceanic processes responsible for the southward transport are being examined in a PhD project titled “A re-assessment of oceanic processes off south-western Australia in relation to larval recruitment”. This essentially involves a comprehensive review of oceanic processes and water properties off Western Australia between Shark Bay and Cape Leeuwin, including a re-analysis of both historical and new ocean current and temperature data, with application to larval dispersal and transport. Because of the relative paucity of current data along our coast, the analysis has been complemented with hydrodynamic modelling (in collaboration with Dr Ming Feng and colleagues from CSIRO, using BRAN) to study the alongshore transport between the Abrolhos Islands and Rottnest Island. The results confirm that the highest settlement at Rottnest based on the ocean currents alone tends to occur during the autumn months, and many details of the actual larval trajectories and travel times have been elucidated – these model results are supported by the re-analysed historical current and temperature data. The results have been submitted to a journal for publication.

The diagram below shows typical larval dispersal trajectories by season from the hydrodynamic model. Note the wide dispersal out into the Indian Ocean in January (summer) an October (spring) compared with the largely shelf-retained larvae in autumn and winter. The pair of joined boxes at 28°S represent the larval source at the Abrolhos Islands while the box at 32°S is around Rottnest Island where some of the larvae recruit.