Comparative analysis of the limnological stock assessment, productivity and potential fish yield of Ojirami and Oguta lakes, Nigeria

Abstract

Ojirami and Oguta Lakes, located in Edo and Imo States respectively, were sampled four times (May, August and November 2005 and February, 2006) for their physico-chemical and biological characteristics, primary productivity and potential fish yield. Except for the very low productivity of Oguta lake (about 10μmhos cm-1), the concentrations of the 19 physico-chemical parameters measured were largely typical of Nigerian inland waters and are suitable for water supply, fisheries and irrigation. Pelagic Primary (photosynthetic) Production (PPP) in Oguta lake (6.75gO2 m^-1d^-1) was about 10 times higher than in Ojirami reservoir which has PPP level of only 0.63 gO2 m^-1d^-1. Enhanced water transparency in Oguta lake (3.85± 0.68m) compared to Ojirami (0.71 0± .14m) largely favoured higher PPP in the former lake. The observed PPP fall into the high and low classification, respectively. Both lakes harbour a wide array of pelagic plankton . The phytoplankton population included species of the green, the blue greens and the diatoms, while the zooplankton comprised of the Copepods, Cladocerans and rotifers. Potential fish yield (PFY) was the inverse of PPP in both lakes. Using the Morpho-Edaphic Index (MEI) technique, the PFYs for Ojirami and Oguta were 49.6kg ha^-1 and 15.8kg ha^-1, respectively. The lower PFY for Oguta is due to low conductivity (mean= 10 μhos cm^-1) and higher mean depth (4.5m) compared to Ojirami that has a conductivity of 158 μmhos cm^-1 and a mean depth of 2.3m. In general, this study, though preliminary, has provided for the first time valuable scientific data for Ojirami reservoir and has also updated, to some extent, the physico-chemical characteristics of Oguta Lake. For both lakes, information, on PPP and PFY are provided for the first time.

Key words: Limnological stock assessment, productivity, potential fish yield, Ojirami and Oguta Lakes, Nigeria.