A COMPARISON OF CYPRINID FISH POPULATIONS IN THREE LARGE TRANSKEI RESERVOIRS WITH RESPECT TO ABUNDANCE AND REPRODUCTION

Abstract

Gill net catches of fish in three turbid reservoirs (Lakes Xonxa and Lubisi on the Kei River system and Lake Umtata on the Mtata River) between 1982 and 1985 were dominated by two cyprinids the common carp, Cyprinus carpio, and smallmouth yellowfish, Barbus aeneus. Mean catch per unit effort of carp was low in all three reservoirs (5–6.5 fish 120 m net⁻¹) while smallmouth yellowfish, absent from Lake Umtata, were abundant in Lakes Lubisi and Xonxa (62.9 and 131.5 fish 120 m net⁻¹, respectively). Monthly changes in the gonadosomatic index and gonad state of carp as well as direct observations of spawning fish showed that egg laying in this species occurred only once rising water levels inundated marginal vegetation. Regional weather patterns characterised by heavy mid- and late summer rains lead to late, relatively unsuccessful spawning by carp and may account for their low abundance in local reservoirs. Growth of yellowfish in Lake Lubisi was poorer and size at sexual maturity smaller than in Lake Xonxa. This is a result of the lower water transparency in Lake Lubisi which hampers foraging by these visually orienting zooplanktivorous fish. The yellowfish population in Lake Lubisi is further constrained through the regulation of the influent river by an upstream reservoir which has limited the availability of suitable fluviatile spawning sites.