A mechanism for near-shore concentration and estuarine recruitment of post-larval 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 three 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 offshore. At the offshore sites, post-larval abundance peaked during the night. In the mouth of the estuary, peak abundances were variable but 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 concentration is caused by the post-larvae being trapped inshore by the tide once they become tidally active. Estimates of the effectiveness of this mechanism are consistent with the observations. The variability of 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 the estuary may be a response to pressure changes. When the pressure change 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 post-larvae can enter nursery grounds without sensing horizontal gradients in estuarine properties such as salinity. Current speeds and post-larval age estimates were used to calculate across-shelf and along-shore advective trajectories and thereby estimate the proximity of effective spawning areas to nearby nursery grounds. The importance of near-shore spawning, limited advection, and local recruitment may have been underestimated in research on long-distance larval advection and adult migrations of P. plebejus.