Temperature and salinity as primary determinants influencing the biogeography of fishes in South African estuaries

Relationships between key fish taxa and various abiotic factors in South African estuaries have been examined. Of the physico-chemical parameters recorded, temperature and salinity appeared to be the main factors affecting the distribution and occurrence of estuarine fishes. Tropical estuarine-associated species appear to prefer warm, turbid, brackish water conditions with many species being largely restricted to subtropical estuaries. The decline in sea temperatures as well as upwelling events limits the dispersal of tropical species into warm-temperate estuaries. Some eurythermal tropical species, however, do extend into warm-temperate estuaries. Warm-temperate estuaries are characterised by cool, more saline waters with low turbidity and are dominated by endemic taxa. Some warm-water endemic species are common in both warm-temperate and subtropical estuaries but are generally not a major component of the fish community of cool-temperate estuaries. Other endemic species appear to prefer cooler waters and occur in both warm- and cool-temperate estuaries but are uncommon in subtropical systems. Temperate species also occur in warm-temperate systems but are generally absent from subtropical estuaries. Widespread or cosmopolitan species occur in all biogeographic regions.