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 results of numerical modelling to investigate the dispersal of snapper Pagrus auratus eggs and larvae in Shark Bay, Western Australia. A 3-dimensional hydrodynamic model was coupled with a 2-dimensional Lagrangian particle-tracking program to simulate passive transport of these planktonic particles. Modelling analysed the separate and combined effects of the region's principal hydrodynamic forces of tide, wind and the Leeuwin Current. Model runs simulated a 30 d period to recreate residual transport on time scales encompassing early snapper development prior to settlement. Hydrodynamic conditions during winter were tidally dominated and exhibited minimal residual flows. Eggs and larvae were retained within residual eddies that were coincident with the 4 main spawning areas. No connectivity between these areas was observed. The results provided further evidence of the existence of a number of proximate but geographically discrete spawning populations of snapper in Shark Bay waters. Hydrodynamic retention provides a mechanism that partly explains existing data indicating that snapper stock-structure in the region is complex. Such information is fundamental to determining the most appropriate geographic scale at which local snapper populations should be managed.