The demography and management of large cyprinids in a reservoir on the Orange River, South Africa

Abstract

The population dynamics of smallmouth yellowfish, Barbus holubi, and Orange River labeo, Labeo capensis, was studied in a large reservoir from 1978 to 1983. These two species made up over 90% of the catches in a fleet of experimental gillnets, but there were considerable fluctuations in catch and in the size and species composition of the catch. These variations largely resulted from changes in environmental conditions caused by hydrological management. Yellowfish breed in the inflowing Orange River, which is regulated through a large reservoir upstream. Spawning success is variable but generally good, and depends on time of spawning which is influenced by pattern of water release. The juveniles disperse well throughout the 80-km-long lake and sub-adults are generally most abundant near the dam wall where the water is clearest. Sharp decreases in water transparency caused by the short retention time of flood waters in the two impoundments greatly reduced the carrying capacity of the lake for yellowfish, and resulted in declining body condition and large-scale mortalities. This species could probably withstand intensive exploitation in the lake. Orange River labeo breed throughout the lake, but conditions for spawning and early survival are variable, leading to variable reproductive success. This species is relatively stationary, but dispersal in the lake is enhanced when decreased water transparency reduces food production. Although growth is very poor at times of high turbidity, mortality remains low and the ultimate size reached by individual fish varies little. Therefore, this species cannot sustain an intensive fishery, but lends itself well to exploitation on an accumulated stock basis. A fishery for labeo should complement a commercial fishery for yellowfish.