Variations in abundance of Synodontis zambezensis (Pisces: Mochokidae) Peters 1852, in the inshore fishery of Lake Kariba

Abstract

In Lake Kariba Synodontis zambezensis Peters 1852 dominated catches in a survey done in 1990–1993 using Lundgren multimesh monofilament gill-nets. S. zambezensis had a catch of 43.8% fresh weight followed by Serranochromis codringtonii (13.8%), Hydrocynus vittatus (9.6%) then Schilbe intermedius (6.5%). The other species did not contribute more than 5% individually. At nine of the 13 stations sampled S. zambezensis made up more than 40% of the total catches by fresh weight. Its index of relative importance (IRI) was 54%, followed by Brycinus lateralis 20% and Barbus unitaeniatus 8%. Other inshore species contributed less than 18% all combined. The catches of S. zambezensis increased with depth as those of the other inshore species decreased. The highest catch per unit effort (CPUE) occurred at 12–20 m and lowest at 3–6 months for the months May to November. The depth variation was similar at all stations analysed ( P = 0.066). However, the sampling month was the factor that most determined the depth zone with the highest CPUE. The CPUE was found to vary seasonally showing a peak in April, the lowest catches occurred from August to October. The appearance of high catches in the shallower depths coincided with the breeding period. The patterns of distribution were different for size and CPUE. The large fish occurred in the shallower zones. The size also varied with position of station in relation to the dam wall. The smallest fish occurred near the riverine end of the lake. It appears that S. zambezensis has increased considerably in abundance since the establishment of Lake Kariba and may still be increasing. Its success is attributed to the fact that it is not captured by fishermen's gill-nets, its ecological niche is robust and it is not extensively predated.