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African Journal of Marine Science

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Composition, abundance and seasonality of fish larvae in the mouth of Durban harbour, KwaZulu-Natal, South Africa

SA Harris, DP Cyrus

Abstract


Ichthyoplankton samples were collected every six weeks at night on consecutive ebb and flood tides over an 18-month period (June 1991–December 1992) at surface, middle and bottom depths near the entrance of Durban Harbour to investigate the composition, abundance, seasonality and developmental stages of fish larvae in the harbour. In all, 8 797 fish larvae, representing 144 species and 64 families were collected. The Clupeidae and Gobiidae were the dominant families, representing 30 and 15% of the total catch respectively. The most abundant larvae were the blueline herring Herklotsichthys quadrimaculatus, which contributed 29.7% of the total catch. Larvae of estuarine-independent species dominated the total catch, both in terms of density (78%) and number of species (81%). In all, 28 species dependent on estuaries at some stage in their life cycle were recorded; of these 13 were species totally dependent on estuaries. Temperature and salinity accounted for 31% of the variation in larval densities of estuarine-dependent species. Turbidity was a significant variable for estuarineindependent species, larval densities of the abundant species being negatively correlated to turbidity. Larval
density peaked mainly in August 1992 (winter), with a mean larval density of 118 larvae.100m-3. Larvae of estuarine-associated species were mainly at the flexion and postflexion developmental stages, whereas most larvae
of estuarine-independent species were at preflexion and flexion stages. Larval densities of certain estuarineassociated species (e.g. Argyrosomus sp.) were significantly higher in bottom samples, mainly on flood tides but
also on ebb tides, suggesting that selective tidal stream transport is a recruitment mechanism used by these species. The impact of harbour development is shown by the dominant marine component of the larval fish assemblage in the harbour. However, despite the seminatural estuarine environment of Durban Harbour, the high species diversity of fish larvae in the system indicates that the harbour is in a relatively good ecological condition.



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