Ichthyoplankton interchange in the mouth region of a southern African estuary

Abstract

Migration patterns of ichthyoplankton in the seasonally open Swartvlei estuary [South Africa] were monitored during 1986/87 with particular emphasis on exchange between the estuary and sea. Four groups of fishes are recognised in the area according to breeding and recruitment strategies. Group 1 consists of species (e.g. Rhabdosargus holubi and Monodactylus falciformis) which spawn at sea, enter the estuary mainly as postlarvae, and congregate along the margins or on the bottom where water current speeds are reduced. Group 2 species (e.g. Spondyliosoma emarginatum and Etrumeus whiteheadi) also spawn at sea, their larvae are swept into the estuary on the flood tide, but are returned to the marine environment on the ebb tide. Breeding and larval development of Group 3 (e.g. Hippocampus capensis and Syngnathus acus) occur within the estuary, although some eggs and larvae are lost to the sea following opening of the mouth. Group 4 larvae (e.g. Psammogobius knysnaensis and Caffrogobius spp.), which hatch from demersal eggs in the estuary, leave the system on the ebb tide before returning several weeks later as postlarvae. Dial ichthyoplankton density changes in the lower reaches of the Swartvlei estuary revealed that movements of larvae and postlarve between the estuary and marine environment occurred mainly during twilight/nocturnal hours. The above data, together with a hydrodynamic model of the estuary, has enabled the quantification of ichthyoplankton exchange over specific 24 h periods.