Estuarine, Coastal and Shelf Science (2001) 53, 221–235 doi:10.1006/ecss.2001.0803, available online at http://www.idealibrary.com on Horizontal Trends in Larval Fish Diversity and Abundance Along an Ocean-Estuarine Gradient on the Northern KwaZulu-Natal Coast, South Africa S. A. Harris a,c , D. P. Cyrus a and L. E. Beckley b a Coastal Research Unit of Zululand, University of Zululand, Private Bag X1001, KwaDlangezwa, 3886, KwaZulu-Natal, South Africa b Oceanographic Research Institute, P.O. Box 10712, Marine Parade 4056, Durban, South Africa Received 15 September 2000 and accepted in revised form 23 April 2001 The structure of the larval fish assemblages along an ocean-estuarine gradient in the St Lucia region on the northern KwaZulu-Natal coast of South Africa was examined using a combination of univariate, distributional and multivariate techniques. The data was comprised of a full annual set of ichthyoplankton samples taken from three types of environment: nearshore coastal waters, surf zone and within the St Lucia Estuary itself. The mean monthly densities of each species in each environment were used in the species matrix, and the mean monthly values of salinity, temperature and turbidity were used in the physical variables matrix. The mean species diversity and eveness index was significantly higher in the nearshore waters than the surf zone and estuary. The patterns of relative species abundances in each environment (K-dominance curves) showed that the estuarine environment was dominated by a few species in large numbers, the surf zone was intermediate, and the nearshore coast was the most diverse. Classification and multidimen- sional scaling (MDS) ordination analyses of larval fish densities grouped together into three main clusters based on the three different environments. The species similarity matrix (inverse analysis) clustered into four groups at the 10% similarity level. The MDS analysis of the same matrix showed that the groups separated out more or less according to the type of environment they occur in, and hence the level of estuarine dependence of the various species. Species belonging to each assemblage showed similarities with regards to their reproduction modes and/or preference to a particular physical condition. Some species were restricted to one environment, whilst others were common to two or all three environments. The occurrence of partially estuarine-dependent species in all three environments suggests that ocean-estuarine coupling is an important process for the recruitment success of these species. The ‘ best fitting ’ physical variable which explained the larval fish community patterns was turbidity on its own, with the relationship of larval densities to the physical variables being species-specific. The present study indicates that a number of factors must play a role in determining the structure and composition of larval fish assemblages occurring in different types of environments along an ocean-estuarine gradient. It is suggested that these distinct assemblages might be considered indicators for the different environments which they inhabit.  2001 Academic Press Keywords: fish larvae; ocean-estuarine gradient; type of environment; multivariate analysis; species diversity; abundance patterns; estuarine-dependence; physical conditions Introduction The linkage by larval and juvenile fishes between nearshore coastal waters and estuarine nurseries is referred to as ‘ ocean-estuarine coupling ’ (Shaw et al., 1988). Several studies have examined physical pro- cesses along an ocean-estuarine gradient in relation to larval transport (Norcross & Shaw, 1984; Shaw et al., 1985; Koutsikopoulos et al., 1991), density distri- butions, length frequency data and growth rates (Shaw et al., 1985, 1988), and vertical distribution patterns in relation to physical factors (Norcross, 1991). Both passive and active mechanisms are involved in the process whereby larval and juvenile fishes recruit to estuarine nursery areas and physical variables, such as temperature, salinity, turbidity and current have been correlated to the recruitment response (Norcross & Shaw, 1984; Boehlert & Mundy, 1988; Miller, 1988). Juvenile and adult fish communities of tropical estuarine and inshore environments in northern Australia have different community structures, despite having species common to both environments (Blaber et al., 1994). These differences were attributed mainly to the physical characteristics of current speed and turbidity in each environment. Larval fish assemblages c Current address: Geographic Environmental Solutions, P.O. Box 1750, Paulshof, Sandton, 2056, Johannesburg, South Africa 0272–7714/01/080221+15 $35.00/0  2001 Academic Press