Effects of on-farm residues on pond water quality, productivity, and fish growth in an aquaculture ecosystem in Ghana

The effects of selected on-farm wastes comprising groundnut peel (GNP), cocoa pod husk (CPH), rice bran (RB) and a standard feed (wheat bran - groundnut peel mixture) which constituted treatments in this study on water quality, plankton composition and tilapia growth at three application rates of 150 mg/I, 75 mg/I and 32.5 mg/I, have been assessed in aquatic ecosystems. The ecosystem consisted of a pond in which open-ended cylindrical concrete tanks were placed on one end to enclose the pond bottom. In these tanks, effects of inputs were assessed using multivariate analyses in comparison to no-input controls. The investigation tested the hypothesis that effects of on-farm wastes when applied singly to freshwater ponds impact differently on the water quality, phytoplankton, zooplankton and tilapia growth. The degradation rate constants of the on-farm wastes and their medium term effects on water quality were also assessed in glass aquaria under laboratory conditions. Following the determinations of the degradation constants, interactions and relative effects among water quality parameters; phytoplankton and zooplankton variables and the specific growth rate of tilapia were evaluated using the multiple regression approach. Comparative evaluations of plankton composition, periodicity and diversity for the various treatments in tanks were also studied Degradation rate constants K as determined from short-term biological oxygen demand (BOD) measurements for the inputs applied at application rates of 32.5 mgfl, 75 mg/I and 150 mg/I, were groundnut peel (GNP): 0.052, 0.058 and 0.065 per day; cocoa pod husk (CPH): 0.420, 0.037 and 0.057 per day and Rice bran (RB) 0.032, 0.036 and 0.057 per day respectively. Application of the different on-farm wastes resulted in changes in water physico-chemical parameters especially, that related to dissolved oxygen (DO), ammonia and phosphate. DO levels were low in all treatments at rates of 150 mg/I. Significant changes were also detected in total alkalinity levels (p<0.05). However, total alkalinity decreased with decreasing application rates. There was no significant difference in pH among treatments. GNP treatments recorded higher ammonia values at 150 mg/l. Although phosphate levels were high at rates of 150 mg/l which lowered C:P ratios below 20:1 in all tanks, it did not result in increases in blue green algae Phytoplankton counts differed for the various treatments showing the pattern of population density as GNP’CPH>Control>Stanfeed>RB with chlorophyceae and euglenophyceae dominating Phytplankton periodicity was not different for most of the treatments except in GNP Zooplankton (dominated by copepoda cladocera and ostracoda) had low diversity at higher than lower input rates Multiple regression analyses of water quality and plankton predictors both separately and jointly, on tilapia growth indicated that, generally water quality variables in groundnut peel and cocoa pod husk treatments explained over 58% of the variance, but less than 20% was explained in rice bran treatment. Over 50% of the variance was explained in stanfeed as against almost 73% in the controls by water quality variables. Nitrogenous compounds especially ammonia, nitrite and nitrate together with phosphate were the main water quality predictors that influenced growth in the different treatments. Stepwise addition of plankton variables to the regression, however, resulted in changes in the significant water quality that affected growth. Zooplanktons were significantly affected more by phytoplankton variables than by water quality in all treatments except the control. The results suggested that factors relating to water quality and plankton in semi- intensive culture express their effects on tilapia growth differently through both direct and indirect regulation of food organisms or improvement in the medium for the fish.

Source: KNUST