A MECHANISM FOR NEAR-SHORE CONCENTRATION AND ESTUARINE RECRUITMENT OFPOSTLARVAL PENAEUS-PLEBEJUS HESS (DECAPODA, PENAEIDAE)

Vertical migratory behaviour of post-larval Penaeus plebejus Hess was monitored every 2 h for 96 h during both the full and new moon at thre e adjacent sites in south-east Queensland, Australia: in the mouth of the Nerang River; 2 km offshore (20 m depth); and 12 km offshore (50 m depth). Coastal currents were also measured at stations 4 and 17 km o ffshore. At the offshore sites, post-larval abundance peaked during th e night. In the mouth of the estuary, peak abundances were variable bu t were always within 3-4 h of the start of the flood tide, regardless of day or night. Post-larval densities were 10-12 times greater in the mouth of the estuary than offshore. We suggest that this coastal conc entration is caused by the post-larvae being trapped inshore by the ti de once they become tidally active. Estimates of the effectiveness of this mechanism are consistent with the observations. The variability o f the along-shore currents together with the varying distances between estuaries will result in a variable number of tidal cycles over which post-larvae can accumulate in the near-shore zone before being swept into an estuary on the flood tide. This variability is consistent with the observed differences in abundance between successive tidal cycles . The mechanism that initiates the movement of the post-larvae into th e estuary may be a response to pressure changes. When the pressure cha nge at the bottom becomes a significant fraction of the total pressure , we suggest that post-larvae change from a diurnal vertical-migration pattern to a tidal vertical migration. This mechanism explains how po st-larvae can enter nursery grounds without sensing horizontal gradien ts in estuarine properties such as salinity. Current speeds and post-l arval age estimates were used to calculate across-shelf and along-shor e advective trajectories and thereby estimate the proximity of effecti ve spawning areas to nearby nursery grounds. The importance of near-sh ore spawning, limited advection, and local recruitment may have been u nderestimated in research on long-distance larval advection and adult migrations of P. plebejus.