Fish community structure response to major habitat changes within the littoral zone of an estuarine coastal lake

Synopsis The littoral environment and fish fauna of Swartvlei, an estuarine lake, was monitored for four years during which major habitat changes occurred. Initially (1979) the zone was dominated by the submerged macrophytes Potamogeton pectinatus, Chara globularis and Lamprothamnium papulosum. This plant community was replaced by filamentous algal mats during 1980 and with the disappearance of these mats in 1981 the littoral zone was transformed into a sandy habitat. There was a highly significant decline in the numbers of fishes in the littoral zone between the macrophyte and sand phases but no significant decrease in fish biomass between the two phases. Analysis of gill net catches revealed an increase in the CPUE of the family Mugilidae between the macrophyte and sand phases but a decline in the CPUE of vegetation associated species such as Monodactylus falciformis and Rhabdosargus holubi over the same period. The increase in mullet stocks during the sand phase was attributed to epipsammic micro-algal production and the input of allochthonous detritus during the 1981 floods. The three fish species diversity indices used in this study showed minor fluctuations between the habitat phases and these variations were related to changes in the equitability of distribution between the individual species within each habitat type. The numbers of fish species recorded during the macrophyte, algal mat and sand phases varied by less than 20%. The resilience of estuarine fishes to major alterations in their environment was illustrated by the fact that all fish species recorded at the beginning of 1979 were present at the end of 1982, despite major habitat and food resource changes.