Estonian Journal of Ecology, 2010, 59, 2, 147157 doi: 10.3176/eco.2010.2.05 147 Seasonal distribution and richness of fish species in the Badagry Lagoon, south-west Nigeria Olufemi O. Soyinka a ! , Minasu P. Kuton a , and Caroline I. Ayo-Olalusi b a Department of Marine Sciences, University of Lagos, Nigeria b Nigerian Institute for Oceanography and Marine Research, V. Island, Lagos, Nigeria ! Corresponding author, soyinka.olufemi@gmail.com Received 12 March 2009, revised 13 November 2009 Abstract. We recorded 37 fish species representing 21 families in the Badagry Lagoon, south-west Nigeria. Adult stages of 13 (35.14%) of them were freshwater, 7 (13.51%) estuarine, and 19 (51.35%) near-shore marine species. The seasonal variation in the fish catch composition of the lagoon was attributed to the seasonal fluctuation in salinity. Availability of food organisms was also noted to affect the seasonal distribution of the fish fauna. Other physical features such as temperature and pH varied slightly, while the concentration of dissolved oxygen was higher in the dry season than in the rainy season. The highest species richness was observed in September with 11 species, while the lowest with the smallest abundance was observed in February with 5 species. The most abundant species during their occurrence were Cynoglossus senegalensis (7.92%), Tilapia mariae (7.17%), and Ilisha africana (6.04%). Species richness and total abundance were found to increase during the rainy season. Key words: fish species, rainy season, dry season, salinity, lagoon. INTRODUCTION Several publications are available on the fish fauna and fisheries of the south- western lagoons of Nigeria. The United Nations Food and Agricultural Organization Report (FAO, 1969) gives a survey of the fisheries of the Badagry Lagoon. Ezenwa & Kusemiju (1985) studied the reproductive biology of the catfish, Chrysichthys nigrodigitatus, from the Badagry Lagoon. Solarin & Kusemiju (1991) documented the day and night variations in the fish and shellfish caught in the Badagry creeks. Ajado & Edokpayi (2003) worked on the comparative racial study of Clarias gariepinus (Burchell, 1882) from the Niger River and the Badagry Lagoon. Lagoons of south-western Nigeria are characterized by seasonal fluctuation in salinity (Olaniyan, 1957; Hill & Webb, 1958). On this basis Williams (1962) discussed the seasonal distribution of the fishes in relation to their salt tolerance. Olaniyan (1969) suggested salinity as an ecological factor in the Lagos Lagoon. While salinity may be an important ecological factor in the distribution of the fish fauna, Fagade & Olaniyan (1974) postulated that the availability of the food of the fish species can also influence their distribution. Other works on the fisheries of the Badagry