Schilbe

Abstract: The distribution and abundance of the five species of Schilbeidae in Lake Kainji were studied. The gill net samples showed that Eutropius niloticus was the most abundant and very common species in the lake. Physailia pellucida although greatly under-estimated by the gill nets was the next abundant and fairly common species. Schilbe mystus was not as abundant as the two previous species but was found to be common. Schilbe uranoscopus and Siluranodon auritus were very rare in the Lake. The spatial distribution showed that E. niloticus and P. pellucida occurred in high numbers in the open lake and the shorelines, while S. mystus had its highest number in the riverine areas of the Lake. An upward trend in the abundance of E. niloticus and P. pellucida was indicated while a decrease in the abundance of S. mystus within the lake was noted.

Abstract: Schilbe mystus (L.) and Eutropius niloticus (Ruppel) (fm. Schilbeidae) from rivers of the Volta Basin were screened at 18 enzyme loci for electrophoretically detectable variation. The mean observed heterozygosity was 0.001 in S. mystus and 0.018 in E. niloticus. The possible causes of these rather low levels of genetic variation are considered in relation to natural and human influences in the Volta Basin river system. The results also suggest that the species S. mystus consists of several distinct biological species and that E. niloticus may have evolved from within the Schilbe group.

Abstract: Understanding of migration patterns is essential in the interpretation of hydro-acoustic stock assessment data of partly demersal partly pelagic fish stocks. In this paper we provide this kind of information for some species that were common in the Mwanza Gulf of Lake Victoria in the 1980s, before and after the upsurge of introduced Nile perch (Lates niloticus). Detritivorous haplochromines and Nile tilapia (Oreochromis niloticus), both stay near the bottom during day and night. Feeding seems to occur predominantly during the day. The zooplanktivorous haplochromines and dagaa (Rastrineobola argentea) dwell near the bottom by day and migrate towards the surface during the night. They seem to follow their prey, zooplankton and lake-fly larvae. Piscivorous nembe (Schilbe intermedius) show similar migration patterns to zooplanktivorous fishes, but their behaviour cannot be unambiguously explained by pursuit of prey. Nile perch to some extend migrate into the column at night, though the majority remains near the bottom. Feeding takes place during day and night.

Abstract: An 8-month survey of the Upper Zambezi River, its associated floodplain and marginal upland habitats yielded 16 catfish (Siluriformes) species, among which Schilbe intermedius comprised over half of the 3534 specimens. Generally catfishes were most abundant in floodplain and river channel habitats, but three species (Clariallabes platyprosopos, Synodontis macrostoma, S. nigromaculatus) were most abundant in the river channel, and three species (Amphilius uranoscopus, Chiloglanis neumanni, Leptoglanis rotundiceps) were most abundant or restricted within small tributary streams. Diet analysis revealed low pairwise dietary overlaps, but there was no statistically significant pattern of community-wide niche segregation based on prey type. The 16 catfishes fell into four size/trophic guilds: large carnivores (Clarias gariepinus, C. ngamensis), medium-sized carnivores (C. stappersii, C. theodorae, Parauchenoglanis ngamensis, Clariallabes platyprosopos, Schilbe intermedius), medium-sized omnivores (Synodontis leopardinus, S. nigromaculatus, S. woosnami, S. sp. cf. woosnami), and small omnivores (S. macrostigma, S. macrostoma, Amphilius uranoscopus, Chiloglanis neumanni, Leptoglanis rotundiceps). No evidence of reproductive activity was detected in any of the catfishes during the survey period of falling and low water conditions. Although catfishes are not as highly valued for food as cichlid fishes, three species are significant components of local commercial and subsistence fisheries: the two large Clarias by virtue of their size, and Schilbe by virtue of its great abundance. Abundances of the heavily exploited Clarias gariepinus and C. ngamensis stocks appear to be lower in the Upper Zambezi compared with the Okavango floodplain system. ? 1996 The Fisheries Society of the British Isles