Short communications An enclosure study to evaluate the effect of Sargochromis codringtoni on snail populations in an irrigation canal Jakob Brodersen 1 , Moses J. Chimbari 2 ‡ & Henry Madsen 1 † 1 Danish Bilharziasis Laboratory, Jaegersborg Allé 1D, 2920 Charlottenlund, Denmark 2 Blair Research Laboratory, P.O. Box CY 573, Causeway, Harare, Zimbabwe Received 5 July 2001. Accepted 11 January 2002 The cichlid fish, Sargochromis codringtoni has been suggested as an alternative to the use of molluscicides for controlling schistosome intermediate host snails. To determine the effect of the fish on Bulinus globosus and Biomphalaria pfeifferi populations in irrigation canals, an enclosure experiment was carried out. The presence of fish was found to significantly reduce the numbers of both B. globosus and B. pfeifferi. Key words: Sargochromis codringtoni, snail control. M olluscivorous fish may have a significant impact on snail populations (McKaye et al. 1986; Brönmark & Weisner 1996) and could there- fore be considered as potential snail control agents. Experiments with Sargochromis codringtoni in Zimbabwe have indicated that this cichlid fish has the potential to control snails (Moyo 1995; Chimbari et al. 1996; Chimbari et al. 1997a,b). However, all experiments have been carried out in lentic habitats or in laboratory aquaria. The pur- pose of this study was to investigate the efficiency of Sargochromis codringtoni in controlling popula- tions of the intermediate schistosomiasis host snails, Bulinus globosus and Biomphalaria pfeifferi, under lotic conditions in an irrigation canal in southern Zimbabwe. The experiment was conducted in an unlined irrigation canal in the Chisumbanje Irrigation Scheme, about 100 km from Chiredzi in the south- eastern parts of Zimbabwe. The canal was about 4 m wide and the bottom-substratum consisted mainly of clay and mud. Willow herb (Ludwigia stolonifera) grew along the banks. Water was very turbid and the water flow ranged from zero to about 15 cm/s measured at the surface in the middle of the canal. Catfish (Clarias sp.) and breams (different species of Cichlidae) were observed in the canal. The snails Melanoides tuber- culata and Lymnaea natalensis were present in the canal although in relatively low numbers. Ten enclosures (l: 0.75 m, w: 0.75 m and h: 1.0 m) were constructed as a wooden frame supporting a 50 % black shade-cloth net. Mesh size was 3 mm. The mesh allowed free movement of water but restricted fish and snails of the sizes used for the experiment within the enclosures. The enclosures were placed close to one side of the canal, where there was some growth of L. stolonifera. Two days after placing the enclosures in the canal, 200 Bulinus globosus and 27 Biomphalaria pfeifferi were introduced into each enclosure. The snails were collected from the Chimanimani irrigation scheme, Zimbabwe. Five days after introduction of snails, one specimen of S. codringtoni with a standard length (SL), i.e. length from tip of snout to end of caudal peduncle, in the range 109– 130 mm (weight: 40–62 g) was introduced into each of five enclosures while the rest were left as controls without fish. The fish used in the experi- ment had been caught in Lake Kariba using hook and line and driven to De Beers Research Labora- tory in Chiredzi where they were maintained in outdoor ponds for one month. This method of capture ensured a good survival of fish during transport. Prior to the experiment the fish were transferred to aquaria and only healthy specimens were used in the experiment. 37(2): 255–258 (October 2002) † Author for correspondence: hm@bilharziasis.dk ‡ Present address: University of Zimbabwe, Lake Kariba Research Station, Kariba, Zimbabwe