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.

An increase in human urinary schistosome transmission in southern Lake Malaŵ i has been suggested to be associated with decreased density of molluscivorous fishes due to illegal seine-net fishing from the shore. In addition, the increased density of snails (Melanoides spp.) through the invasion of an Asian morph could have changed the predators' prey choice. At Chembe village, the intermediate host snail of urinary schistosomes, Bulinus nyassanus, constitutes55% of the total gastropod fauna. This study was designed to compare crushing resistance of the intermediate host snails Bulinus globosus and B. nyassanus with that of Melanoides tuberculata, which dominates the gastropod fauna. A crush value index (CVI) as an indicator of potential prey value was expressed as the ratio of benefit (weight of snail tissue) to cost (crush resistance of snail shell). Bulinus globosus had the highest CVI. Using shell height as measure of snail size indicated that B. nyassanus had higher CVI than M. tuberculata within the size range of snails consumed by Trematocranus placodon, one of the molluscivore fishes. This may be one of the reasons that B. nyassanus is a preferred prey of T. placodon. In spite of this preference, the reduced population of T. placodon has not been able to control the population of B. nyassanus because of its apparent opportunistic feeding on large numbers of M. tuberculata.