The influence of selected environmental parameters on the distribution of the dominant demersal fishes in the Kariega Estuary channel, South Africa
The Kariega Estuary channel was sampled using an otter trawl and the demersal fish catch analysed using the PRIMER statistical package. A biological-environmental (BIOENV) analysis was undertaken using the catch per unit effort ichthyofaunal data from the spring/summer period of 1996/97 and concurrent water temperature, salinity and turbidity data. No strong correlations between the ichthyofauna and recorded physico-chemical parameters were documented, although temperature did show a weak correlation to demersal fish distributions. A second sampling programme undertaken in 2004 was specifically designed to test the influence of sedimentary characteristics on the distribution patterns of the dominant demersal species. Analyses showed that sediment size composition and organic content was strongly correlated (r = 0.70) to the distribution of the soles Solea bleekeri and Heteromycteris capensis, as well as the goby Glossogobius callidus. S. bleekeri and G. callidus showed a preference for the muddier middle and upper reaches of the Kariega Estuary channel, whereas H. capensis and Psammogobius knysnaensis were more abundant in the sandier lower reaches and mouth region of the estuary. The exact reasons for these preferences are unknown but are likely to be related to preferred prey availability, foraging success and predator avoidance in the different types of sediment. Other species caught in the otter trawl were not significantly correlated to sedimentary characteristics.
Keywords: estuaries, demersal fishes, salinity, temperature, sediments