The schistosome intermediate host, Bulinus nyassanus, is a 'preferred' food for the cichlid fish, Trematocranus placodon, at Cape Maclear, Lake Malawi.

Over the last decade, transmission of the schistosome parasites of humans has increased in parts of Lake Malawi, possibly because over-fishing has led to a decline in the numbers of molluscivorous fish. The stomach contents of 51 wild-caught Trematocranus placodon from the Cape Maclear area of Lake Malawi have now been investigated and compared with the field abundance of snail species at two sites, close to where the fish were caught. The snails found in the fish stomachs were identified to genus or species on the basis of their shell fragments and/or opercula. The sizes of the prosobranchs consumed by the T. placodon were determined from their operculum sizes whereas the Bulinus nyassanus ingested by the fish were categorized as small (<4 mm) or large (> or =4 mm) on the basis of their shell fragments. The proportion of the Bu. nyassanus ingested that were categorized as large increased with fish size. Furthermore, the relative abundance of Bu. nyassanus in the guts of the fish was considerably higher than in the field, indicating that the T. placodon had a 'preference' for Bu. nyassanus over the Melanoides species that dominated the snail fauna. Small specimens of Bellamya species were also consumed in a greater proportion than found in field.