Studies on the current state of the limno-chemistry and potential fish yield of Lake Volta (Yeji sector) after three decades of impoundment

English

This study aimed at providing information on the current state of one of the relatively high fish production areas of Lake Volta the Yeji sector. As in its early life, the water of Lâke Volta was found to be oligotrophic, With Iow concentrations of nitrates, nitrites and phosphates which ranged between 0.51 and 0.82, 0.02 and 0.05, and 0.34 and 0.41 mg 1-1 , respectively. Sodium was the dominant ion, With a mean concentration of 12.1 mg I A . Generafly, the ionic dominance pattern recorded was Ca>Na>K>CI>Mg. Considerable increase in turbidity is regarded as a major limiting factor affecting primary production and resultant potential fish yietd. As a result of turbidity, oxygen saturation levels were high (>100 %) at the surface, possibly leading to the restriction of primary production at the air-water interface. The difference in temperat.ure between the surface and bottom was narrow (1.7 OC), With practically no thermal stratification in the Yeji sector of the lake. This suggests mixing of waters between the surface and deeper layers, thereby enhancing oxygen availability in sufficient concentrations for fish life clear to the bottom. Based on the morpho-edaphic model, the potential fish yield estimate of the lake has declined from 32.8 kg ha -l in 1974 to 29.0 kg ha I in 1995/96. The decline is attributed to limnological changes in the lake, notably increased turbidity.

Ofori-Danson, P. K.; Ntow, W. J.