Ecological Carrying Capacity Assessment for Cage Aquaculture Expansion in Nigerian Freshwater Reservoirs

This research developed and validated an ecological carrying capacity assessment methodology for the expansion of cage aquaculture in freshwater reservoirs in Nigeria, addressing the critical absence of science-based spatial planning tools for inland aquaculture siting. Nigeria's major reservoirs, including Kainji, Shiroro, and Jebba, have been identified as candidate sites for cage culture expansion, yet nutrient loading limits and ecosystem resilience thresholds have not been quantified. The study applied a coupled hydrodynamic-biogeochemical modelling approach using ELCOM-CAEDYM to simulate nutrient dynamics under varying cage density and feeding intensity scenarios in Kainji Reservoir, Niger State. Model inputs were derived from 18 months of field monitoring data covering water temperature profiles, current velocity, dissolved oxygen stratification, phosphorus loading, chlorophyll-a, and fish production metrics from existing cage units. Scenario simulations tested cage expansion from current levels (10 units) to 50, 100, and 200 units under different spatial configurations. Results indicated that the reservoir could accommodate expansion to 50 cage units before phosphorus loading exceeded the mesotrophic threshold, beyond which eutrophication risk escalated sharply. Spatial clustering of cages in the reservoir's least-flushed southern basin created localised nutrient hotspots even at low total cage density. Current velocity played a critical role in nutrient dispersion, with cage siting in areas above 0.05 m/s substantially reducing localised accumulation. The study delivers an original spatially explicit carrying capacity framework and recommends its adoption as a mandatory environmental planning tool by regulatory authorities overseeing cage aquaculture licensing in Nigerian reservoirs. Keywords: carrying capacity, cage aquaculture, hydrodynamic modelling, Kainji Reservoir, eutrophication