RESOURCE RECYCLING 



The New Greenhouse Effect: Growing 

 Fish and Vegetables Side by Side 



By Carla B. Burgess 



With all the talk about chemicals in 

 our food, excessive use of water and too 

 much agricultural waste, it's easy to 

 wonder if we can sustain ourselves 

 without ruining the world. 



A research group at North Carolina 

 State University has slow-cooked a partial 

 solution in a greenhouse. 



It's called integrated aquaculture- 

 vegeculture. It employs all those concepts 

 that have become politically correct in the 

 90s — "recycling," "organic," and "conser- 

 vation." 



The brainchild of Mark McMurtry, 

 who received his Ph.D. in horticultural 

 science from NCSU, the system involves 

 growing fish and vegetables in a symbiotic 

 relationship. 



"Mark came up with the idea of 

 intermittently applying fish waste to a sand 

 bed and allowing this water to drain back 

 into the fish tank," says Doug Sanders, a 

 N.C. Cooperative Extension Service 

 specialist who is project coordinator and 

 professor of horticultural science at NCSU. 



The pump removes fish feces and 

 uneaten food that accumulates at the 

 bottom of the tank, reducing the ammonia 

 which can kill the fish. The wastewater is 

 pumped into a planted sand bed that 

 filters out the nutrients, simultaneously 

 fertilizing the vegetables. The clean water 

 is aerated and returned to the fish tank. 



"The biofilter is the sand bed with the 

 plants," Sanders says. "It cuts the ammonia 

 and nitrate level in half, which is a 

 reasonable level for the fish." 



The system has produced tilapia, a 

 hardy, disease-resistant fish native to West 

 Africa, and tomatoes, cucumbers, lettuce 

 and basil. 



"We were able to grow all of these 

 crops without any additional nutrients, and 

 sometimes without liming or adjustment of 

 the pH," Sanders says. 



The system is profitable too. 



According to a paper written by 



McMurty, Sanders and Sea Grant aquacul- 

 ture specialist Ron Hodson, a one-half acre 

 integrated greenhouse system can gross up 

 to S240,000, a net profit of about S24 per 

 square yard per year. A conventional 

 vegetable greenhouse nets about 50 cents 

 per square yard per year. 



For each half acre, the integrated 

 system can produce 92,400 pounds of fish, 

 110,000 pounds of tomatoes and 149,600 

 pounds of cucumbers per year. 



It also provides year-round produc- 

 tion of food with minimal water consump- 

 tion. 



"Water is added only to make up for 

 evaporation and plant transpiration," says 

 Sanders, adding that the water is 

 recirculated 100 to 300 times. A given 

 gallon of water is recycled repeatedly, 

 staying in the tank 12 to 38 days. 



The system conserves land resources 

 through its intensified culture. It works in 

 areas with poor soil and climate, and 

 adapts to almost any region or culture. 



It can be expanded for commercial 

 use or scaled down for the backyard. 

 McMurtry has drawn up a plan for high 

 school teachers to use in developing 

 demonstration projects for their students. 



"It teaches symbiosis, re-circulation 

 and conversion of nitrogen," says Sanders. 



And what high-tech equipment would 

 a teacher need? 



"An aquarium, Rubbermaid dishpan 

 and a pump," he says. 



It can also teach small countries how 

 to get more from their fishes and loaves. 



The inhabitants of the small African 

 country, Namibia, might use the system to 

 raise their own native tilapia and produce 

 a vegetable crop to feed their people. 

 Sanders and McMurtry are working on a 

 project that would export the system there. 



But like any new idea, Sanders says it 

 will take more time and experience to 

 provide conclusive results to the public. 



"I think it's something that has real 

 potential," he says. "It will require a careful 

 marketing plan to do this. There is a 

 growing demand for tilapia." 



The research group is working on a 

 new project to develop culture practices 

 for "continually planting and harvesting 

 fish and vegetables," Sanders says. 



Meanwhile, a cooperator in Beaufort 

 County plans to construct a 1/4-acre 

 demonstration system near Bath. 



"While we'll be doing some experi- 

 mentation, their aim is to make it commer- 

 cially feasible," Sanders says. © 



For more information about 

 re-circulating systems, contact 

 Sanders at 919/515-3283, or write 

 IAVS Research Group, Department 

 of Horticultural Science, Box 7609, 

 NCSU, Raleigh, NC 27695-7609. 



12 JULY/AUGUST 1991 



