Interactions between fish and snails in a Zimbabwe pond, with particular reference to Sargochromis codringtonii (Pisces: Cichlidae)
The impact of fishes on snail densities in a 100 x 12m pond was assessed from October 2000 to May 2002, using a fish exclosure experiment. Snail species included numerically dominant Bulinus globosus (ca 85%) as well as B. tropicus, Biomphalaria pfeifferi, Lymnaea natalensis and Melanoides tuberculata. Six fish species were present in the ponds prior to the start of the experiment; these were Tilapia rendalli, Oreochromis mossambicus, O. macrochir, Clarias gariepinus, Kneria auriculata and an unidentified Labeo species. At the beginning of the experiment 85 Sargochromis codringtonii were introduced into the pond. All fish species were periodically monitored by seine netting and the stomach contents of a sample of these was analysed. For the first five months of the experiment Bulinus globosus numbers were low, both inside the fish exclosures and in the adjacent control areas but, from April 2001 onwards, snail numbers inside the exclosures were consistently higher than those in the control areas. At the end of the experiment the density of B. globosus was 150.5snails m−2 in the exclosures and 4.7snails m−2 in the control areas. The other snail species showed the same trend, but the differences were less pronounced. Sargochromis codringtonii was the only fish species that was found to feed on snails and since this species became successfully established in the pond it was assumed to be responsible for the snail mortality outside the exclosures. Whilst this experimental design did not separate the effects of S. codringtonii from those of the other fish species, it does provide strong evidence that fish are important in the control of B. globosus, an intermediate host for schistosomiasis.