Vertical migration of postlarval penaeid prawns in two Australian estuaries: the effect of tide and day/night

Knowledge of the vertical migration behaviour of postlarval penaeid prawns is important in understanding postlarval migration from offshore spawning grounds to estuarine nursery areas. We sampled throughout several 24-h periods in two estuaries in Australia to assess the distribution of postlarvae in the water column. We used four trawl nets: one net in the top 0.5 m of the water column and three nets one above the other, from the seabed to 0.72 m above the seabed. Penaeus plebejus was abundant in catches in the Nerang River, southern Queensland, while Penaeus semisulcatus, Penaeus esculentus and Penaeus merguiensis were caught in the Embley River, northern Queensland. For all species, the highest catches at the surface were on night flood tides. Many postlarvae in the Embley River were caught in nets above the seabed on ebb tides, suggesting that net migration patterns cannot be determined simply by sampling surface waters. The vertical distribution of penaeid postlarvae is determined primarily by tide and day/night; however, water turbidity and cloud cover, factors that reduce light at the seabed, are also likely to be important. All these aspects of postlarval behaviour should be included in hydrodynamic models of larval advection from spawning grounds to inshore nursery areas.