A review of factors influencing fish utilization of South African estuaries

The distribution and abundance of teleosts found in southern African estuaries are determined by both biotic and abiotic variables. Factors which have already been shown to have an influence on estuarine-associated fishes are latitude, seasonality catchment size, estuary size, axial salinity gradients, habitat diversity, estuary type, mouth depth and degree of marine influence, timing of the open phase, the ability of species to adjust to salinity and temperature fluctuations, dissolved oxygen levels, turbidity, available food resources, the occurrence and severity of floods, trapping of organic material by impoundments, nearshore marine conditions, physical constrictions within estuarine systems, predation, competition, parasite loads, reproductive condition, juvenile cohort abundance, and habitat degradation as a result of pollution or dredging. The influence of several of the above variables on ichthyofaunal diversity and abundance are reviewed, with particular emphasis on salinity fluctuations as a primary factor affecting fish communities. A summary diagram of the biotic and abiotic variables affecting fishes, as well as a conceptual model on the influence of high turbidity on the foraging strategies of selected teleosts, is presented.