Laval and Juvenile Fishes in the Surf Zone Adjacent to the St Lucia Estuary Mouth, KwaZulu-Natal, South Africa

Abstract

Composition, seasonality and developmental stages of larval and juvenile fish in the surf zone adjacent to the St Lucia Estuary mouth, KwaZulu-Natal, were investigated. Samples were collected monthly, from February 1992 to January 1993, at six stations along the beach north of the estuary mouth. A 24-h study was also undertaken in the surf zone when the estuary mouth was closed. In the 12-month study a total of 2931 larvae, representing 88 taxa and 47 families, was collected. The most abundant families were the Sparidae, Haemulidae, Ambassidae, Tripterygiidae and Chanidae, together comprising 64.2% of the total catch. The most abundant species were Pomadasys olivaceum, Ambassis sp. and Rhabdosargus holubi. In the 24-h study a total of 13731 larvae, representing 43 taxa and 22 families, was collected and was dominated by Ambassis sp., Croilia mossambica and Chanos chanos.

Temperature and turbidity were the most important environmental variables related to fish densities of estuary-associated species. Temporal changes in fish densities were most significant for daylnight catches, with densities being significantly higher at night than during the day. Fish species associated with estuaries peaked in abundance in mid autumn and late winter. Spatial trends in fish densities indicated significantly higher densities of estuary-associated species at Stations 2 and 3, with densities decreasing thereafter away from the estuary mouth. Larval stages were predominantly postflexion for all months sampled. The total catch was dominated by marine spawner species, with 36% of all taxa having some degree of estuarine association. Species with some degree of estuarine association dominated the catch in terms of density during both the 12-month study (60%) and the 24-h study (83%). Results from this study indicate that the surf zone adjacent to the St Lucia Estuary functions as a transit route and a nursery habitat for certain estuary- and non-estuary-associated fish species.