COASTAL RESEARCH AND EXTENSION CENTER
Use of constructed wetlands in finfish pond culture water recirculation was studied to evaluate their effectiveness in improving water quality, determine optimal design and operating criteria and assess the associated benefits and costs in finfish pond production. The initial experiment was conducted at the MSU-Coastal Aquaculture Unit using 12 six-year-old quarter-acre ponds and nine newly constructed wetlands. The experimental design used in this experiment was as follows: three control ponds (no wetlands), three ponds with standard wetlands (25% of pond size and 2-day retention time), one pond each with standard wetland size and variable retention time (0.5, 1 and 3 days) and one pond each with standard retention time (2 days) and variable wetland size (15, 35 and 50%). Channel catfish (Ictalurus punctatus) fingerlings were stocked at a rate of 5,000 fish per acre in December 1993 and raised for one year. Wetlands were planted in September 1993 with a combination of soft rush (Juncus effusus) and duck potato (Sagittaria lancifolia). Substantial improvements and lesser variations in many water quality variables were observed among ponds with standard wetlands as compared to control ponds. Significant differences in many water quality variables were also noted among ponds with variable wetlands sizes and retention times. Due to the relatively young age and incomplete vegetative coverage in constructed wetlands, there were no definitive trends in the effects of wetland sizes and retention times on water quality. At this stage of determining the effectiveness of using constructed wetlands in finfish pond production, the significant improvements in pond water quality did not lead to significantly observable benefits, namely: higher yields and lesser incidence of off-flavor. Marked differences, however, were observed in pond aeration time and wetland pumping time. The application of the initial experimental results to a 48-acre commercial catfish farming enterprise in the Mississippi Black Belt indicated that the observed additional costs more than offset any limited economic benefits derived. Total revenues from the catfish enterprise with or without constructed wetlands remained at $174,720 per year, $3,640 per production acre or $0.70 per pound harvested. Annual specified costs for the catfish enterprise without constructed wetlands amounted to $146,912 per year, averaging $3,060 per production acre or $0.59 per pound harvested. When constructed wetlands were added to the catfish enterprise, total costs rose by $18,930 annually, $394 per production acre or $0.075 per pound harvested. With the additional costs arising from the use of constructed wetlands, average yield needs to increase by eight percent or 417 pounds per production acre in order for the catfish enterprise to recover the added specified costs amounting to $292 per production acre. As the documentation of the effectiveness of marsh systems in reducing water quality problems progresses, future work could take several directions. First, changes in marsh system function with age need to be evaluated. Second, there is a need to evaluate the potential of improving yields of marketable fish through increased stocking densities. Third, there is a need to investigate the culture of more economically valuable and environmentally sensitive finfish species in order to convert marked improvements in water quality into higher yields and revenues. Fourth, cheaper methods of creating vegetative cover in constructed wetlands need to be evaluated. Finally, it would be desirable to conduct pilot tests of this new technology in commercial scale operations in cooperation with the fish farming industry.
A copy of the FINAL REPORT can be downloaded in portable document format.