Tidal exchange of two decapod larvae Palaemon pacificus (Caridea) and Upogebia africana (Thalassinidae) between the Swartkops River estuary and adjacent coastal waters

Abstract

Two 24-h plankton and seine-netting studies were carried out in the Swartkops estuary mouth, one during a spring-tide, the other on a neap-tide in order to determine larval movements of the prawns Palaemon pacificus and Upogebia africana in and out of the estuary. Two size classes of the former were caught, namely zoea 6 and adults. No zoeae 1, 2, 3, 4 or 5, post-larvae or juveniles were caught. Zoea 1 of U. africana are flushed out of the estuary into Algoa Bay on the nocturnal ebb-tide with very few returning on the subsequent flood-tide. Net fluxes were 97,1% and 99,9%. Larval life is then spent at sea after which the pre-postlarvae invade the estuary to reach the estuarine nursery areas. For P. pacificus this was during zoea 6, mainly on the nocturnal flood-tides. Net fluxes of P. pacificus zoea 6 were 95,8% and 100%. Most larval movement occurred at peak velocities as they utilize the tidal currents in dispersion and recruitment. Some P. pacificus zoea 6 are washed out again on ebb-tides, but lateral movement to the banks during slack-tide appears to be a mechanism to prevent this occurring. P. pacificus adult movement in the mouth region was better cor-related with nocturnal activity than with tidal movements.