P .2 System Boundaries cies: nile or red tilapia (Oreochromis niloticus or Oreochromis spp) and a number of hydroponically growing herbs and greens. While the choice between Nile and red tilapia produces different mass yields per square meter, they each produce virtually identical economic yields per square meter. The tilapia family are the most commonly cultured fish in the world. They are stress tolerant and are a commercially desirable food. Nile tilapia are well adapted to feeding primarily on plant-based feeds such as grain and soybeans and can also consume algae and zooplankton. Research performed at the University of the Virgin Islands has worked to refine and optimize a tilapia-basil system and according to the model presented in this publication, it has a potential average annual gross economic productivity of $268.48 per square meter of installed capacity.5 Other options for aquaculture include a fish polyculture modeled after a Chinese system. It cultivates four species of carp that each occupy a different ecological niche: “the grass carp eats large plant material and grass clippings, the silver carp eats algae, the bighead carp eats zooplankton, and the black carp eats snails and other detritus.”6 The overall productivity may be lower than a highintensity system with commercial feeds, but it has the potential for high net economic productivity and environmental impact minimization due to the elimination of feeding, operations, and maintenance costs. Tilapia and carp can productively coexist with each other within a fish polyculture and have nearly identical temperature requirements.7 Tilapia and carp can both consume dried leafy vegetables as supplemental feed including comfrey, spinach, and vegetable amaranth. The fish in the aquaponics system can therefore serve an additional purpose of converting unmarketable or spoiled vegetables into usable and high quality calories. Adding animal manure to the aquaponics system increases primary productivity of phytoplankton and zooplankton—which in turn serve as additional supplemental feed. Therefore, the system can also accept a limited amount of the manure produced from the livestock portion of the operation.8 We also investigated the possibility of including European Eels (Anguilla anguilla) or American Eels (Anguilla rostrata) in the system. Globally, eel is in high demand and a combination of factors have led the International Union for the Conservation of Nature (IUCN) to list European Eels as critically endangered and American Eels as threatened. Under these circumstances, closed-loop cultivation becomes desirable. Until recently, it was not possible to grow young eel fingerlings, or glass eels, in captivity. This meant that all eel farming was necessarily dependent on wild-caught fish, which did nothing to reduce the worrisome pressure on the wild population. In the last year, new developments in specialized feed for baby eels have made it possible to cultivate these fish in captivity for the first time. When more information becomes available regarding the costs and productivity eels bred fully in captivity, this could prove to be a profitable and desirable alternative for the Polydome system. 5 Rakocy, J., R.C. Shultz, D.S. Bailey, E.S. and Thoman. 2004. Aquaponic production of tilapia and basil: comparing a batch and staggered cropping system. Acta Horticulturae. Vol. 648. p. 63–69. 6 Van Gorder, Steven D. 2000. Small Scale Aquaculture: A hobbyist’s guide to growing fish in greenhouses, recirculating systems, cages, and flowing water. Breinigsville, PA: Alternative Aquaculture Association, Inc. 7 Ibid. 8 Ibid 64 Pagina 63
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