Hydrobiologia 416: 181-191, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. Distribution of freshwater snails in the man-made Oyan Reservoir, Ogun State, Nigeria Iteanyi Emmanuel Ofoezie Institute of Ecology and Environmental Studies, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria Received 12 January 1999; in revised form 6 July 1999; accepted 6 August 1999 Key wordy: Freshwater snail, man-made reservoir, snail ecology, snail distribution, tropics, Nigeria Abstract The factors influencing patterns of snail distribution in Oyan Reservoir, a typically medium sized man-made reservoir in southwest Nigeria, were investigated once a month, for 28 months (August 1990-November 1992). During each monthly visit, seven stations were sampled for relative snail density, vegetation cover and physical and chemical properties of the lake water. Statistical analyses were performed to determine the influence of these factors on snail abundance and distribution. Seven aquatic snail species were found in the reservoir, namely Bulinus globosus, B. forskalii, B. truncatus, Lymnaea natalensis, Indoplanorbis exustus, Potadoma freethi and Potadoma moerchi. The relative proportion of these species was 17.6% (B. globosus), 16.1% (B. forskalii), 0.2% (B. truncatus), 12.9% (L. natalensis), 23.4% (/. exustus) and 29.8% (both Potadoma spp. combined) of 28 183 snails collected over the study period. The seasonal trends of the species were variable but basically bimodal with peaks in August (rainy season) and November/December (early dry season). These trends were similar to the water level and rainfall patterns, suggesting that seasonal changes in snail density were probably influenced by these factors. Spatial distribution of each snail species was similar along each shoreline but significantly different across the E-W transect. Similar trends occurred for conductivity, dissolved oxygen and biochemical oxygen demand suggesting that these factors may have affected snail distribution across shorelines. Ten species of aquatic vegetation were identified, six of which correlated positively and significantly with various snail species. The most important of the associating vegetation was Impatiens irvingii which correlated with all snail species and served as an indicator plant for Indoplanorbis exustus, an introduced snail species in Nigeria. Introduction Ecological changes resulting from the damming of rivers and streams to create man-made reservoirs are well documented (Jordan et al., 1980; Hunter et al., 1982; Pike, 1987). In endemic tropical Africa, such changes have given rise to biotopes more favourable than hitherto for the breeding of intermediate hosts and vectors of important human diseases, e.g. malaria and schistosomiasis (Waddy, 1975;Gilles, 1980; Hunter et al., 1982, 1993; Jewsbury & Imevbore, 1988). In Nigeria, more than a thousand reservoirs were established since 1973, when following the devastating effect of the sahel drought, the Federal Government adopted a bold policy of large scale irrigation and water conservation. The adverse health implication of these reservoirs have since manifested in many parts of the country (Betterton et al., 1988; Ndifon, 1991; Ofoezie et al., 1991, 1997). Reports from many other parts of Africa and southeast Asia point to similar trends (Hira, 1969; Paperna, 1970; Talla et al., 1990). While disease control cannot be carried out at the expense of water resource development, a balance could be struck by incorporating disease mitigating and control safeguards from design to operational stages of such projects (Pike, 1987). For snail trans- mitted diseases, e.g. schistosomiasis, this will require a good knowledge of snail distribution over space and time. Although such information is given in the liter- ature, this is mainly concerned with snail distribution at the regional and sectoral levels (Webbe, 1965; Ok-wuosa & Ukoli, 1980; Betterton, 1984; Thomas & 181