Management to minimize the environmental impacts of pond effluent: harvestdraining techniques and effluent quality

An experiment was conducted to evaluate the amount of wastes discharged fro m fish ponds during harvest and to assess pond draining schemes and harvest techniques that may reduce waste loading in effluent water. Nile tilapia ( Oreochromis niloticus) were cultured for 113-119 days in twelve 200 m(2) po nds with fertilization and supplementary pelleted feed. There were four tre atments with three replicates each: (A) ponds were treated with teaseed cak e at a rate of 10 ppm to partially anaesthetize tilapia 1.5 h prior to harv est by three seinings without draining ponds; (B) ponds were limed at a rat e of 75 ppm calcium hydroxide 24 h prior to harvest and drained by sequenti al complete draining, and then tilapia were collected from a harvesting pit ; (C) ponds were drained by sequential complete draining and tilapia were c ollected from a harvesting pit; and (D) ponds were drawn from 100 to 50 cm and tilapia were harvested by two seinings, followed by complete draining a nd collection of the remaining tilapia from a harvesting pit. Treatment D i s the common practice in Thailand. The harvest efficiency was 97% in treatm ent A. Comparing the harvest efficiency for the first two seinings, there w ere no significant differences between treatments A and D. Liming in treatm ent B resulted in significantly higher concentrations of 5 day biochemical oxygen demand, total settable matter, total solids and total phosphorus (TP ) in the water discharged from the last 25 cm, depth than those at both 100 -50 and 50-25 cm depths, while there were no significant differences in eff luent quality parameters among different depths in both treatments C and D. Concentrations of all effluent quality parameters in the water discharged from the last 25 cm depth in treatment B were higher than those in treatmen ts C and D. Seining in treatment D resulted in the highest concentrations o f an effluent quality parameters except TP at middle depth. Compared with t he common draining practice, adoption of the sequential complete draining s chemes (treatments B and C) caused 33-86% reductions for different effluent quality parameters except total nitrogen and TP in the discharged wastes. The present experiment showed that the use of teaseed cake to anaesthetize tilapia could allow effective harvest by seining, without draining the pond . Alternatively, the discharge of potential pollutants from ponds into the environment could be reduced by sequential complete draining of ponds and c ollecting fish from harvesting pit (treatments B and C), and can be further reduced by modification of sequential partial draining of pond water level to 25 cm above pond bottom and seining fish without further draining. The present experiment demonstrated that appropriate management in pond drainin g and fish harvest could minimize the environmental impacts of pond effluen ts. (C) 2001 Elsevier Science. B.V. All rights reserved.