Empirical Assessment of the Ecological Carrying Capacity for Cage Aquaculture in Ikere Gorge Reservoir, Oyo State

Determining the ecological carrying capacity of inland reservoirs for cage aquaculture is essential for preventing eutrophication and maintaining the productive capacity of these water bodies. This study empirically assesses the ecological carrying capacity for cage fish culture in Ikere Gorge Reservoir, Oyo State, South West Nigeria, using a nutrient mass balance modelling approach. The phosphorus mass balance model was parameterised using field measurements of reservoir hydrodynamics, total phosphorus loading from catchment runoff, existing cage nutrient excretion estimates, and in-situ phosphorus retention coefficients derived from seasonal sampling over 12 months. Water quality parameters including total phosphorus, dissolved oxygen, chlorophyll-a, Secchi disk depth, and total dissolved solids were measured monthly at seven stations distributed across the reservoir. The current trophic state of the reservoir was classified as mesotrophic based on measured phosphorus concentrations and chlorophyll-a values. Modelling results indicated that the maximum allowable cage aquaculture production consistent with maintaining mesotrophic conditions is 84 tonnes per year, compared to existing permitted cage production of 62 tonnes per year. Sensitivity analysis showed that this estimate was most sensitive to assumptions about phosphorus retention efficiency and catchment loading. The study recommends the use of the validated phosphorus mass balance model as a regulatory tool for licensing new cage aquaculture operations in Ikere Gorge Reservoir and calls for mandatory environmental impact assessments for all proposed cage expansions above 20 tonnes per year.

Keywords: carrying capacity, cage aquaculture, phosphorus loading, Ikere Gorge, Oyo State