Fish abundance in the Wilderness and Swartvlei lake systems: changes relative to environmental factors

Abstract

Fish communities throughout the Wilderness and Swartvlei lake systems were sampled in the winters of 1991 and 1993 using gill and seine nets. Gill net catches indicated substantial increases in the abundance of large juveniles and adults of the majority of marine fish species in both systems. Seine net catches indicated increases in the abundance of juvenile Liza richardsonii, but a decline in the abundance of other fish species. The recruitment of most fish is unlikely to have been negatively affected by the recorded fluctuations in salinity or turbidity. The senescence of macrophytes, however, may have increased exposure of recruiting Monodactylus falciformis and Rhabdosargus holubi to piscivorous birds and fish, The artificial breaching of estuaries during optimum fish recruitment periods in 1991 and 1992 may have contributed to the recorded increases in the abundance offish in 1993. A longer duration tidal phase in the Swartvlei system during 1992 and 1993, compared to the Wilderness lake system, did not result in greater abundance of fish sampled. There appears to be no justification for the artificial maintenance of permanently tidal conditions in the Swartvlei and Touw River estuaries on the grounds of benefit for the maintenance of viable fish communities.