Original Contributions to the Modelling of Dissolved Oxygen Dynamics in Tropical Cage Aquaculture Systems under Variable Wind and Temperature Conditions in Nigeria

Dissolved oxygen dynamics in tropical cage aquaculture systems are governed by complex interactions among fish metabolic demand, atmospheric reaeration, photosynthetic production, and thermally driven stratification, yet mechanistic models of these dynamics calibrated to the specific meteorological, thermal, and biological conditions of Nigerian inland cage systems have not been developed. This study makes original contributions to dissolved oxygen dynamics modelling in tropical cage aquaculture through the development and validation of a mechanistic mass balance model for dissolved oxygen in cage systems, incorporating fish bioenergetics, atmospheric reaeration, photosynthetic oxygen production, sediment oxygen demand, and thermocline depth as forcing variables. The model was developed using data from a two-year intensive monitoring programme covering eight cage sites across Kainji Lake in Niger State and Oguta Lake in Imo State, incorporating 15-minute interval dissolved oxygen profiling, meteorological measurements, phytoplankton biomass, water temperature profiles, and fish biomass records. An original sub-model is contributed for predicting cage microenvironment dissolved oxygen as a function of stocking density, feeding rate, wind speed, and lake stratification depth, enabling real-time prediction of critical oxygen conditions. Model validation against independent time-series data yielded root mean squared errors of 0.41 mg/L for surface dissolved oxygen and 0.68 mg/L for hypolimnetic dissolved oxygen. Scenario modelling demonstrated that wind speed below 1.5 m/s combined with stocking density above 30 kg/m3 creates high-risk dissolved oxygen crash conditions in Nigerian tropical cage systems. The study provides an original decision support tool for cage site selection, stocking management, and emergency aeration response in Nigerian cage aquaculture.

Keywords: dissolved oxygen modelling, cage aquaculture, tropical lakes, stocking density, bioenergetics