Analysis of the Relationship between Phytoplankton Biomass and Fish Yield in Semi-Intensive Catfish Ponds in Oyo State

Semi-intensive catfish culture in Oyo State, South West Nigeria, relies on both supplementary feeding and natural pond productivity, yet the quantitative contribution of phytoplankton-based natural food chains to fish yield in this production system has not been empirically established. This study analyses the relationship between phytoplankton biomass, measured as chlorophyll-a concentration, and catfish production outcomes in 24 semi-intensive ponds in Ibadan and Ogbomoso over two production cycles. Phytoplankton biomass, species composition, zooplankton abundance, dissolved oxygen, nutrient concentrations, and fish growth rates were monitored monthly. Catfish yields at harvest were correlated with average seasonal phytoplankton biomass using regression analysis, and path analysis was used to disentangle direct and indirect effects of phytoplankton on fish yield mediated through dissolved oxygen and zooplankton intermediaries. Results showed that ponds maintaining chlorophyll-a concentrations of 40 to 80 micrograms per litre produced significantly higher fish yields than those with lower or higher concentrations. Above 80 micrograms per litre, yield declined, associated with dissolved oxygen crashes during night-time respiration. Path analysis revealed that 34% of the total effect of phytoplankton biomass on catfish yield was indirect, mediated through zooplankton abundance serving as a natural food source for juvenile fish. The study provides quantitative benchmarks for phytoplankton management in semi-intensive catfish ponds and recommends the adoption of chlorophyll-a monitoring as a routine pond management tool for commercial catfish farmers in Oyo State.

Keywords: phytoplankton, chlorophyll-a, catfish yield, semi-intensive pond, Oyo State