Introduction
Tilapia is currently cultured in over 100 countries and increasingly becoming one of the most cherished protein sources globally. The global development of tilapia culture has passed through several phases of improvement over the last two decades. That notwithstanding, water quality management in the main culture system (ponds) for this species is yet to be fully optimised. This study was conducted to assess the water quality dynamics in tropical earthen ponds with and without fish.
Materials and Methods
The study was conducted for three (3) months in four ponds (10×15×1m each) at an experimental fish farm in Kumasi, Ghana. Two treatments i.e. fed pond which comprised two ponds stocked with all male tilapia (38g) and fed twice daily with a commercial feed (CP-30%) and a control- two other ponds without fish/feed. Growth and feed utilization of the fish were assessed at the end of the study. Physicochemical and biological water quality parameters were monitored every four days and every three weeks, respectively. Twice a week, the dissolved oxygen (DO), pH, temperature and conductivity levels were measured in-situ with a multiparameter probe (Hach, Hd40Q) between 7 and 8 am while water samples were taken to the laboratory for turbidity, total suspended solids (TSS), total dissolved solids (TDS), alkalinity, NH3, NH4,ortho-phosphate, NO2, NO3 biological and chemical oxygen demand (BOD & COD), organic matter (OM) and organic carbon (OC) analysis. Furthermore, every three (3) weeks, a 24h O2 monitoring was done in the experimental ponds and sludge accumulation and sludge characteristics were also determined. Data on water quality, growth and feed utilization were expressed as means and standard deviation using Microsoft Excel and graphs generated with GraphPad Prism 5 software. Data on nutrients were subjected to the Mann Whitney test while all physico-chemical and biological parameters were subjected to Students T-test (α =0.05).
Results
Physico-chemical parameters were relatively better in the control treatment ponds than the fed treatment ponds with highly significant differences observed in DO, temperature, TDS, TSS, turbidity, alkalinity and nutrients (NH3, NH4, NO2, NO3). Organic and biological parameters were significantly higher in the fed treatment ponds with BOD, COD, OM, OC and chlorophyll-a recording 22%,20.1%,14%,10.2%,35% respectively than the control treatment pond. Sludge measured was relatively higher in the fed ponds with better sludge characteristics in the control treatment. Water quality parameters recorded in the control ponds were within the recommended ranges for tilapia growth. Diel O2 variation in both treatments decreased along sampling times.