3.1 Distribution and abundance of freshwater snails
Two thousand and twenty-seven (2,027) freshwater snails were collected from the four sampling sites during the study period. Identification based on shell morphology revealed ten species in order of increasing abundance: Bulimulus tenuissimus (d’Orbigny, 1835) 12(0.6%),Bulinus globosus (Morelet1886), 12(0.6%), Monach cantian a (Montagu, 1803) 13(0.6%), Lymnaea natalensis (Linnaeus, 1758) 14(0.7%), Gabbiella humerosa (Martens, 1879) 14(0.7%),Drymaeus multilineatus (Say, 1825) 19(9%), Lanistes varicus (Morelet, 1860) 22(1.1%), Cleopatra bulimoide s (Olivier, 1804) 66(3.3%) Bellamya unicolor (Jousseame, 1886) 302(14.9%) and Melanoides tuberculata (Muller, 1774) 1,553(76.6%). M. tuberculata was found to be the most dominant fresh water snail species accounting for 76.6% of the total collected from the dam, followed by B. unicolor with 14.9%. Both M. tuberculata and B. unicolor accounted for 91.5% of fresh water snails collected from the dam between April and October 2019. Over the entire malacological study period (April to October, 2019), the highest number of fresh water snails were recorded in the site of human activity (670 snails) followed by 482 from vegetation cover, 442 from lotic water and 433 from lentic water (Table 1).
The mean snail abundance varied significantly across the four sampling sites within the same period and duration of sampling (April to October): F (3, 24) = 4.82, P = 0.01. Post-hoc comparison test indicated that there were significantly more fresh water snails at site with human activity (M = 95.7) compared to lentic water (M = 61.9) and lotic water (M = 63.1) of the dam from April to October 2019 (Table 2). However, the mean snail abundance at sites with vegetation cover (M = 68.9) did not significantly differ from either site with human activities, or from lentic or lotic water sites. In the sampling sites,M. tuberculata was the most frequently encountered freshwater snail; B. unicolor was the second most dominant species in each site followed by C. bulimoide s.
There was a progressive decrease in number of snails collected in all sites with exception of lotic water where the numbers fluctuated after the first sampling in April. The peak period with the highest number of fresh water snails collected in areas with human activity, vegetation cover and lentic water was the month of August. However, in lotic water site, the highest number of snails was collected in the month of May. On the other hand, the least number of snails collected from sites with human activity, vegetation cover and lentic water was in October, while lotic water site recorded its least number of snails in July. This result suggests that sites with human activity, vegetation cover and lentic water share similar snail distribution dynamics with regards to time/season of the year, as shown in Figure 1.