Potential mechanisms of influence of the Leeuwin Current eddy system on teleost recruitment to the Western Australian continental shelf

The Leeuwin Current (LC), an oligotrophic, warm current that flows south (poleward) along the shelf-break off the west coast of Australia and then east along the south coast, is recognized as a key factor affecting fisheries production in the region, but the mechanisms for this influence have not been determined. Recruitment strength of the globally significant western rock lobster (Panulirus cygnus) stock is correlated to interannual variations in the strength of the LC. While this relationship has been based on a 2-decade time-series of P. cygnus recruitment data, the important teleost species of the region rarely have recruitment data for more than a few years; yet this group is nonetheless economically, socially and politically important. Furthermore, there is little knowledge of the egg- and larval-stage dynamics for the majority of these teleosts. Previous and new information on those aspects of the LC system that could theoretically impact on recruitment of shelf teleosts were identified to provide a basis for developing a conceptual model of how the LC could affect recruitment. The potential impacts of the LC system, which entrains shelf water, were examined with reference to retention/loss of teleost eggs and larvae and positive/negative influences on feeding conditions for larvae. Owing to the lack of early-life-history information for many teleosts in Western Australia, this was undertaken for generalized shelf species whose eggs are spawned on the shelf and whose larvae must settle on the shelf to access favourable nursery habitat. The results indicate that the LC system most likely contributes a net negative impact on success of teleost eggs and larvae. Larvae of shelf teleosts entrained and trapped in the warm-core (WC) eddies that form from the LC and then propagate offshore would contribute little to recruitment. Given that larval teleosts predominantly feed on copepods and that these were much less abundant in the WC eddy than is typical of shelf waters, the general larval feeding conditions in the WC eddy were inferior to those on the shelf. Any larvae that escaped from the eddy that were able to orientate towards the shelf and had sustained swimming capabilities would incur significant energetic penalties when attempting to return to the shelf. Furthermore, flow of the LC onto the shelf could dilute the concentrations of phytoplankton and zooplankton, negatively impacting feeding conditions for larvae that remain on the shelf. Clarification of the timing and geographical locations of interactions between the LC and shelf waters relative to spawning behaviour of shelf teleosts is required before the potential negative impacts on recruitment can be adequately quantified. However, because fisheries management issues cannot (always) await detailed understanding of biophysical effects on recruitment, the conceptual model of potential effects was developed here to provide immediate improvements for interpretation of stock assessment information, for which recruitment variability is often a key uncertainty. Finally, an improved understanding of the effects of mesoscale oceanography on fish stocks will increase the ability for fisheries managers to discuss climate-change implications with stakeholders.