June 22, 2010
Last week I had the pleasure of attending the 12th annual International Aquaponics and Tilapia Aquaculture Course in St. Croix at the University of the Virgin Islands (UVI). I was able to meet and learn from many wonderful people who traveled from about 21 U.S. states and 18 countries including Canada, Mexico, six Caribbean islands, Peru, Argentina, the United Kingdom, Nigeria, Singapore, and Saipan, a tiny island in the middle of the Pacific Ocean. Course participants ranged from commercial and aspiring farmers to backyard hobbyists, non-profit and international development workers, aquaculture extension specialists, academics, entrepreneurs, and investors.
Throughout the week, the course instructors emphasized that integratedfarming systems, such as aquaponics, can be more environmentally sustainable, resilient and potentially more profitable than monoculture of either fish or plant species. Course lectures included tilapia biology, broodstock spawning, fry cultivation and growout, plant propagation and hydroponics, integrated pest management, system design, and commercial considerations. Field work followed in-class lectures for hands-on learning activities.
Aquaponics is essentially a method for boosting profits from aquaculture (i.e., fish farming) by capturing excess nutrients inherent in fish waste to raise plants as a secondary revenue crop. In this regard aquaponics borrows heavily from hydroponics — a method for raising plants in a soil-less, nutrient-rich water, but it differs in one key respect. Hydroponics is performed with microbiologically clean water where all inputs come from fertilizers, while in aquaponics “we keep our system dirty” says the course’s lead instructor and UVI professor, Dr. Jim Rakocy, repeating the mantra this leader in the aquaponics movement has developed during his 30 years of research.
As we learned, a “dirty system” is not a bad thing — it refers to the greenish-brown scum of beneficial bacterial communities that accumulate on all surfaces of the system — from the walls of fish tanks to the undersides of the plant grow beds and even in the PVC tubing. The main aquaponic system at UVI has not been shut down or cleaned in eight years, aside from daily removal of fish waste solids. These bacterial communities are instrumental in nitrification, or the breaking down of fish waste and uneaten fish feed from ammonia, a chemical toxic to fish, into nitrate, an essential plant nutrient. For fruiting plants a decrease in nitrate is often desirable to retard vegetative growth, which can be accomplished by allowing denitrification to occur in anoxic or oxygen-poor regions of the tank system.
In the UVI aquaponics system, tilapia, a mild-flavored fish from Africa, are stocked in aerated tanks at about 100 fish per cubic meter. Tilapia are fed three times a day on a diet of fish feed and the nutrient-rich wastewater from growing tanks is continually pumped through a series of physical and biological water treatment steps including tanks for nitrification, denitrification, and plant biofiltration. The effluent leaving these treatment steps is clear and ready for direct recycling back into the fish-rearing tanks. Less than 1% of the system water is lost daily, mainly attributed to evaporation, transpiration from plants, tilapia splashing out water during feeding, and waste solids removal.
In the year-round Caribbean growing season – enough to make any farmer jealous – leafy greens like Bibb lettuce can grow to harvestable size in three to four weeks, and tilapia grows to a harvestable size of one and a half pounds in roughly 24 weeks. Over the course of a year, UVI produces 20,000 pounds of both tilapia and a wide assortment of vegetables that are sold to local restaurants and through direct sales at a farm store and market. Seeing the volume of plant matter produced, which is a free by-product of fish production and often a better cash crop than fish, turned many course participants into converts (if they weren’t already) and made them eager to apply what they had learned on their own farm.
This year the record UVI course attendance of 92 participants could be attributed to both growing interest in aquaponics (see recent New York Times article), and news that Dr. Rakocy will be retiring in November. His legacy will hopefully continue in the many commercial, research, and backyard aquaponics systems that have been and will be built using the Rakocy-UVI model.
One area of aquaponics that piqued my interest was the similarity between fish wastewater treatment and human wastewater treatment, and the potential for bringing sanitary engineering students into aquaculture. I was also interested in studying other fish species, such as hybrid striped bass, koi, or catfish, that may be better suited to winter conditions in a Baltimore greenhouse than tilapia, a more tropical fish. Overall, my goal is to continue cultivating aquaponics in Baltimore as a model for how we can apply systems thinking and ecology to farming.