The influence of tidal cycles on the distributional pathway of Penaeus latisulcatus Kishinouye in Shark Bay, Western Australia

Abstract

The means by which Penaeus latisulcatus larvae in Shark Bay migrate from the offshore spawning areas to the inshore nursery grounds have been studied. It is generally accepted that penaeid larvae move at random and are carried into the nursery areas by ocean currents, but this was found to be impossible in Shark Bay. where the general movement of water is against the required direction of migration.

Examination of the tidal flow during the hours of darkness revealed a yearly cycle with flood and ebb flow dominating for alternate halves of the year. This nocturnal tidal cycle and the reduced rate of tidal flow with increasing depth is shown to interact with the vertical diurnal movements of the larvae to provide a mechanism by which larvae can be transported a greater distance inshore at night than offshore during the day for the period March-August. This inshore displacement provides a means by which larvae can be moved inshore against the general movement of water within the embayment. Also the nocturnal tidal cycle during the summer, when ebb flow dominates, is shown to assist the offshore migration of juveniles. The presence of a comparable yearly cycle at Port Lincoln on the South Australian coast seems to indicate that the mechanism is not an isolated occurrence.