EFFLUENT QUALITY DURING HARVEST OF CHANNEL CATFISH FROM WATERSHED PONDS

Water samples were collected twice daily during the draining and harve sting of three ponds containing channel catfish (Ictalurus punctatus) during January 1992. Samples were analyzed for selected water quality variables. For each tonne of fish produced, an average of 9.2 kg nitro gen, 0.57 kg phosphorous, 22.5 kg biochemical oxygen demand, and 530 k g settleable solids were discharged. Approximate chemical budgets for organic matter, nitrogen, and phosphorous were prepared. Of the organi c matter, nitrogen, and phosphorous added in feed, averages of 3.1%, 2 8.5%, and 7.0%, respectively, were released in effluents; 18.5%, 30.7% , 31.8%, respectively, were removed in fish. Of the nitrogen, phosphor ous, and biochemical oxygen demand discharged, 50% was discharged in t he last 15-20% of effluent discharged. Of the settleable solids discha rged, 50% was released in the last 5% of effluent discharged. These fi ndings suggest that the best way to minimize the pollution potential o f aquaculture pond effluents is to harvest ponds as quickly as possibl e, and either to not discharge water during the seining phase or to di scharge this highly contaminated water into a settling basin or retent ion pond. It also appears feasible to allow effluents to flow untreate d into the environment during the preseining phase of draining, becaus e concentrations of potential pollutants are low then.