Preliminary Observations on the Family Mormyridae in Oyan Dam Lake (Nigeria)

This study was carried out to investigate the abundance, distribution, diversity and condition factor of Mormyrids in Oyan Dam lake, Abeokuta North Local Government Area of Ogun State, Nigeria. The fish were sampled from the catches of the fishermen in Oyan Dam lake, which used gillnets of 30-80 mm laterally stretched mesh size. The fish were sampled monthly between February and September 2011, at two different sites (Imala and Ibaro). The results revealed that a total number of 100 fishes of six species belonging to the family Mormyridae were collected. Numerically, catches were dominated by Mormyrus rume (34%), Hyperopisus bebe (30%) and Petrocephalus bane (24%). Together, these species comprised 88% of all collected individuals. Marcusenius ihuysi was rare and least in number representing 1% of the fish caught. The results showed that only four species were recorded in the dry season, namely Mormyrus rume, Hyperopisus bebe, Petrocephalus bane and Mormyrus deliciosus while five species were recorded in the wet season particularly in September. Mormyrus deliciosus, was not recorded in the wet season. In terms of body weight, the highest mean value was recorded for Mormyrus deliciosus (435.71 ± 95.71), followed by Mormyrus rume (143.82 ± 5.55) and Hyperopisus bebe (123.5 ± 5.26) while Petrocephalus bane was the least one (95.00 ± 8.56). The results also showed that the condition factor during the wet season (4.18) was higher than the condition factor of the dry season (3.51). Simpson’s index was used to determine the species richness, while Shannon-Wiener’s Index was used to evaluate species diversity; species evenness was equally evaluated using Pielou’s Index. The indices of diversity included (d) = 0.31 and 0.23, H = 1.18 and 1.53 and E = 0.851 and 0.853 respectively for the two sites at Oyan Dam lake. The estimates from these indices indicated a low fish species composition and richness, also unevenness in the population of Mormyrids in Oyan Dam lake. Therefore, it will be advisable to regulate the activities of fishers in the water body to allow future increase in the abundance of this species.