Food composition and selectivity of Synodontis zambezensis (Pisces: Mochokidae) in Lake Kariba, and the ecological implications

Abstract

The diets of 441 catfish Synodontis zambezensis from three different localities in Lake Kariba were analysed. S. zambezensis was found to be primarily a molluscivore, actively selecting the pulmonate Lymnaea natalensis and chironomid larvae in 59% and 30% of the cases, respectively. Although highly selective, S. zambezensis changed its prey preference according to availability of prey species. Some changes in diet were associated with the emergences of insects such as the flying form of the termite Aletes sp. The range of food items found in the stomachs suggested that the fish can also use other food items effectively. The Percent Similarity Index showed significant temporal differences in diet. Evidence showed that S. zambezensis was a benthic feeder but also fed at the water surface. No dietary shifts from the smallest fish (12 cm) to the largest fish caught (38 cm) were observed. However, specimens under 20 cm total length were few and this could have influenced the results. S. zambezensis is an 'oral sheller' that removes gastropod shells prior to swallowing the soft tissue and either discards or swallows the smaller shells (< 10 mm height). The results showed that the S. zambezensis does not have a strict food regime and this gives it a better change of becoming widely distributed. There is a high possibility that the food composition will vary as the food abundances and distributions in the lake shift and S. zambezensis is likely to be little affected. Ecological it is important that the squeaker is feeding on a resource which is hardly utilized in the lake.