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Ponds may yield more than catfish
By Charmain Courcelle
BILOXI -- Mississippi State University researchers at the Coastal Research and Extension Center in Biloxi are developing water discharge options for the catfish and horticulture industries.
Environmental standards for pond effluents and horticultural runoffs have not been passed yet, but the Environmental Protection Agency has actively considered a national set of regulations to limit the release of nutrients from these operations for several years.
Mississippi Agricultural and Forestry Experiment Station scientists Mark LaSalle and Cecil Pounders, economist Ben Posadas and research assistant Christine Walters are looking for ways to help catfish and horticultural growers meet stricter effluent regulations in the future.
The team is evaluating the use of hydroponic and "sawdroponic" systems as potential pond effluent treatment methods. In hydroponic systems, plants are grown in a bath or flow of nutrient-rich solution. Sawdroponics is a variation of hydroponics that uses sawdust as a soil-free planting medium. In both systems, plants take up available nutrients from the solution for growth.
LaSalle said plants growing in hydroponic and sawdroponic systems could remove excess nutrients found in pond effluents. The treated effluent could then be discharged into the environment or recirculated into the pond production system.
"We think linking the catfish and horticulture industries through hydroponics or sawdroponics will help minimize the environmental impacts from both types of operations," LaSalle said. "Effluent released from ponds could serve as a water and nutrient source for horticultural plants, while the catfish ponds receive water that has been 'cleaned up' to ensure good water quality."
LaSalle and Posadas had previously assessed the use of constructed wetlands to treat pond effluent. The constructed wetlands significantly improved water quality, but they required higher investments and led to higher operating costs for catfish production, Posadas said.
"We found that constructed wetlands cost an extra 7 cents per pound of catfish harvested. This included the cost of constructing and maintaining the wetland," Posadas said. "But there is also a cost from not being able to grow catfish on this productive land."
The group hopes hydroponic or sawdroponic production of a food or landscape crop will offset effluent treatment costs and generate extra revenue for the catfish grower.
LaSalle said adding crop production to a catfish operation requires additional labor, but a catfish grower might be able to partner with a truck farmer to produce food crops for local markets.
In the hydroponic system, LaSalle and colleagues have tried growing tomatoes, beans, Chinese water spinach and lettuce. They are trying to grow rooted plants, including daylilies and mondo grass, in the sawdroponic system, which supports root propagation. The team will evaluate which plants grow best under each system and determine best management practices.
"We have already seen that we have to redesign the sawdroponic system," LaSalle said. "The sawdust we used was too fine, and it prevented water from flowing through. We will try a coarser bark material underneath the sawdust layer next to see if that works better."
The group is also determining what size hydroponic system is required for effluent treatment. LaSalle and Posadas' results from their constructed wetland study suggested an area equal to 25 percent of catfish pond size was required for the greatest improvements in water quality.
"It might be less for hydroponic-based systems," LaSalle said.
To answer this question, the team will collect water entering the sawdroponic setup and water that has passed one-third, two-thirds and all of the way through the plant production system for water quality analysis.
"We are hopeful that these systems will act as a type of filter for aquaculture effluents that will improve water quality in ponds and simultaneously reduce fertilizer and pesticide runoff from horticulture," LaSalle said.
Contact: Dr. Mark LaSalle, (228) 388-4710