A comparison of the ichthyofaunas in two permanently open eastern Cape estuaries

Abstract

The Kowie and Great Fish estuaries are situated less than 30 km apart, yet they differ considerably in terms of riverine inflow, turbidity, food resources and habitat availability. The ichthyofauna of the two estuaries were sampled using plankton, seine and gill nets. A greater ichthyofaunal richness (R) was recorded in the Kowie estuary and this is attributed to the wider range of habitats and greater degree of marine influence in this system. In contrast, all three sampling gears revealed an approximate 3:1 ratio between fish abundance in the Great Fish and Kowie estuaries. The higher abundance of fishes in the Great Fish estuary is partly attributed to the large organic and nutrient inputs into this system when compared with the Kowie system, and the influence of these inputs on estuarine primary and secondary production. Individual fish species are affected differently by turbid water conditions. Indications from this study were that piscivorous fishes (e.g. Lichia amia) which rely mainly on visual foraging methods were adversely affected by the high turbidity conditions within the Great Fish estuary, whereas piscivores (e.g. Argyrosomus hololepidotus) which rely mainly on non-visual methods were unaffected. Macrobenthic predators (e.g. Pomadasys commersonnii) and detritivo-rous fish species (e.g. Mugil cephalus) also appear to be unaffected by high suspensoid levels and were usually more abundant in the Great Fish than in the Kowie estuary. The length-frequency distributions of some of the dominanl fish species occurring in both estuaries are presented.