Estuarine typology and the structuring of fish communities in South Africa

Some 190 South African estuaries, covering all biogeographic provinces within the region, were classified into three types based on a combination of mouth condition and estuary size (surface area). The fish communities of the estuary types within each zoogeographic region were described and compared. Multivariate analyses revealed that each estuary type contained somewhat distinct fish communities. In addition, the study identified common patterns in species richness and ichthyofaunal composition. Open estuaries have relatively high species richness; this is a reflection of a permanent or near-permanent connection with the sea which allows access into these estuaries by all marine migrant species within the region. Intermittent connection with the sea limits the recruitment and utilisation of closed estuaries by marine migrant species; this results in reduced species richness in moderate to large closed estuaries. Small closed estuaries exhibit the lowest species richness and this is probably a result of their limited habitat and increased isolation from the sea. The key fishes that utilise estuaries could also be categorised into a number of groups based on their relative importance within each estuary type. Some species are largely restricted to predominantly open systems. Other taxa, while important in predominantly open estuaries, also occur in moderate to large closed systems. Some estuarine-associated species are well represented in all estuary types but exhibit a greater importance in closed estuaries. This study has shown that South African fish communities not only reflect estuarine typology but also respond to these differences in a consistent manner that spans all zoogeographic regions. The prevalence of similar patterns in other parts of the world suggests that estuarine typology is a major driver in the structuring of global estuarine fish communities.