Snails and aquatic vegetation in Gezira irrigation canals.

All 38 minor canals in a limited area of the Gezira Irrigated Area were surveyed four times over a 2-year period to determine the aquatic and semi-aquatic vegetation types found and their distribution. A more detailed 2-year study of the correlation between snail density and aquatic vegetation was carried out in 18 of these canals. A total of 22 different species of weed were recorded, with floating and submerged weeds being more widespread and dense in the clear, slow-moving water at the end of the irrigation season in March/April, than in the turbid flood waters in October. The schistosome vector snails Biomphalaria pfeifferi and Bulinus truncatus were found in conjunction with aquatic weeds and B. pfeifferi in particular increased in numbers as weed growth continued. In the second season of the study when mechanical weed clearing and regular focal mollusciciding was being carried out there was a noticeable reduction in weed density but repopulation was rapid. Snail numbers were reduced by mollusciciding and/or weed clearance in four canals, but outside the focal target canal stretches the weed clearance alone did not substantially reduce the snail populations.