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. Three sets of pond and wetland experiments were conducted at the Mississippi State University-Coastal Aquaculture Unit (MSU-CAU) located at Mississippi Power Company's Plant Watson power generating station in Gulfport, Mississippi. Twelve quarter-acre ponds and nine constructed wetlands were used during the first experiment conducted in 1994. In the second and third experiments conducted in 1996 and 1997, a total of 11 and 16 quarter-water acre ponds were used. Six of the ponds were used in a replicated fashion to evaluate the effectiveness of constructed marshes toward improving water quality while the remaining ponds were used in a range testing fashion to determine optimal wetland design criteria and operation parameters. Three control ponds, three ponds with standard wetlands (25% of pond size and 2-day retention time), one or two ponds with standard wetland size and variable retention times (½, 1 and 3 days) and one or two ponds with standard retention time (2 days) and variable wetland sizes (15%, 35% and 50% of pond size) were included in the experiments. Analysis of variance was performed using General Linear Model procedure to determine the effects of wetland size and water flow rate on pond and wetland water quality variables, and critical biological and economic variables. Mature constructed wetlands were shown to significantly reduce levels of most parameters at the following rates: total ammonia nitrogen (2-63%), nitrite-nitrogen (29-97%), nitrate-nitrogen (28-80%), total phosphorous (52-95%), total suspended solids (2-76%) and pH ( 0.5-10%). In general, wetlands of standard size and flow rate showed the highest levels of reduction for most parameters monitored at the inlet and outlet ends. Ponds with wetlands had dampened daily fluctuations in dissolved oxygen and reduced levels of photosynthetic pigments compared to control ponds. Levels of nutrients within ponds with wetlands were highly variable among treatments and were within optimal or tolerable ranges for fish production. The added benefits and costs associated with the use of constructed wetlands in intensive and recirculating catfish pond production were estimated by applying the experimental results to a multi-enterprise commercial farm. The points of entry of catfish production and constructed wetlands in a multi-enterprise farm were evaluated by using a mixed integer linear programing model. Assuming present technology, using constructed wetlands in intensive recirculating catfish pond production is not economically viable. The use of constructed wetlands in a 48-water acre catfish farm in a multi-enterprise Mississippi Black Belt farm area would be acceptable to farmers if the annual yield of marketable catfish would increase by at least 5%. Experimental results showed that at intensive stocking densities, expected revenues were not sufficient to justify the costs associated with constructed wetlands. Further testing of this technology needs to be undertaken in order to ascertain the effects on the yields of marketable catfish or other fish species under intensive commercial scale operations.
A copy of the FINAL REPORT can be downloaded in portable document format.