Horizontal trends in larval fish diversity and abundance along an ocean-estuarine gradient on the northern KwaZulu-Natal coast, South Africa

The structure of the larval fish assemblages along an ocean-estuarine gradi ent in the St Lucia region on the northern KwaZulu-Natal coast of South Afr ica was examined using a combination of univariate, distributional and mult ivariate techniques. The data was comprised of a full annual set of ichthyo plankton samples taken from three types of environment: nearshore coastal w aters, surf zone and within the St Lucia Estuary itself. The mean monthly d ensities of each species in each environment were used in the species matri x, and the mean monthly values of salinity, temperature and turbidity were used in the physical variables matrix. The mean species diversity and evene ss index was significantly higher in the nearshore waters than the surf zon e and estuary. The patterns of relative species abundances in each environm ent (K-dominance curves) showed that the estuarine environment was dominate d by a few species in large numbers, the surf zone was intermediate, and th e nearshore coast was the most diverse. Classification and multidimensional scaling (MDS) ordination analyses of larval fish densities grouped togethe r into three main clusters based on the three different environments. The s pecies similarity matrix (inverse analysis) clustered into four groups at t he 10% similarity level. The MDS analysis of the same matrix showed that th e groups separated out more or less according to the type of environment th ey occur in, and hence the level of estuarine dependence of the various spe cies. Species belonging to each assemblage showed similarities with regards to their reproduction modes and/or preference to a particular physical con dition. Some species were restricted to one environment, whilst others were common to two or all three environments. The occurrence of partially estua rine-dependent species in all three environments suggests that ocean-estuar ine coupling is an important process for the recruitment success of these s pecies. The 'best fitting' physical variable which explained the larval fis h community patterns was turbidity on its own, with the relationship of lar val densities to the physical variables being species-specific. The present study indicates that a number of factors must play a role in determining t he structure and composition of larval fish assemblages occurring in differ ent types of environments along an ocean-estuarine gradient. It is suggeste d that these distinct assemblages might be considered indicators for the di fferent environments which they inhabit. (C) 2001 Academic Press.