Water quality dynamics in earthen ponds with and without fish

Tilapia is one of the most cultured fish species globally and over the last two decades, its culture has been intensified. However, water quality management in tilapia pond culture is a major barrier for higher productivity. This study assessed the water quality dynamics in 150 m3 earthen ponds with or without fish over a period of 12 weeks. The two treatments were ponds stocked with fish (fed ponds) and ponds with water but no fish (control pond). The fed ponds comprised two ponds stocked with all-male Nile tilapia (Oreochromis niloticus) sub-adults (Mean mass ~40g) fed twice daily with a commercial feed (30% crude protein) while two other ponds were assigned as control. Physicochemical, biological water quality variables as well as nutrients were monitored in all ponds every four days over the 12 weeks. Dissolved oxygen (DO), pH, temperature and conductivity levels were measured in-situ between 7.00 and 8.00 a.m., while water samples were analysed in the laboratory for turbidity, total suspended solids (TSS), total dissolved solids (TDS), alkalinity, ammonia (NH3), nitrite (NO2), nitrate (NO3), ortho-phosphate, biological and chemical oxygen demand (BOD & COD), organic matter (OM) and organic carbon (OC). Additionally, a 24h DO monitoring as well as sludge accumulation and sludge characteristics were determined every three weeks in the experimental ponds. Mean DO levels over the trial duration was 42% lower in the fed ponds. The increased nutrient loadings from the fish and the supplementary feed presumably increased all biological WQ parameters measured which resulted in a higher sludge accumulated in the fed ponds. The fed ponds consistently recorded pre-sunrise DO levels of 2 mgL-1 during the same period. Generally, the results of the study showed deterioration of water quality was more evident in the fed ponds.