Hydrodynamic modelling of snapper Pagrus auratus egg and larval dispersal in Shark Bay, Western Australia: reproductive isolation at a fine spatial scale

Ichthyoplankton data collected between 1997 and 2000 were combined with the resultsof numerical modelling to investigate the dispersal of snapper Pagrus auratus eggs and larvaein Shark Bay, Western Australia. A 3-dimensional hydrodynamic model was coupled with a2-dimensional Lagrangian particle-tracking program to simulate passive transport of these planktonicparticles. Modelling analysed the separate and combined effects of the region’s principalhydrodynamic forces of tide, wind and the Leeuwin Current. Model runs simulated a 30 d periodto recreate residual transport on time scales encompassing early snapper development prior tosettlement. Hydrodynamic conditions during winter were tidally dominated and exhibited minimalresidual flows. Eggs and larvae were retained within residual eddies that were coincident with the4 main spawning areas. No connectivity between these areas was observed. The results providedfurther evidence of the existence of a number of proximate but geographically discrete spawningpopulations of snapper in Shark Bay waters. Hydrodynamic retention provides a mechanism thatpartly explains existing data indicating that snapper stock-structure in the region is complex. Suchinformation is fundamental to determining the most appropriate geographic scale at which localsnapper populations should be managed.