translucent solar-energy absorbing cylinder with a capacity up to 1,000 gallons (3,785 liters) that is filled with water and seeded with more than a dozen species of algae and a complement of microscopic organisms (Figures 39–1, 39–2, and 39–3). Within these cylinders, phytoplankton-feeding fishes and omnivorous fishes are cultured at very high densities. The species selected depend upon climate, region, and market opportunities: the range of species we have studied is broad, including African tilapia (Tilapia spp.), Chinese carps (Cirrhinus molitorella, Ctenopharyngodan idellus, Hypophthalmichthys molitrix, and Aristichthys nobilis), and North American catfish (Ictalurus spp.) and trout (Saluelinus spp.).

Dense populations, up to one actively growing fish per 2 gallons (7.6 liters), produce high levels of waste nutrients beyond the capability of the ecosystem to take up. The module recycles these nutrients in the following ways: nutrients released from the decomposed waste material are absorbed by the fish, the plankton, and the crop plants rafted on the cylinder surface. In addition, partially digested algae that floculate out and settle to the bottom are periodically discharged through a valve to fertilize and irrigate the surrounding ecosystem under restoration. The root systems of the vegetable crop plants take up the nutrients before they reach toxic levels and also capture detritus; they function as living filters by purifying the water.

These modules can yield more than 250 pounds (113.5 kilograms) of fish annually in a 25-square-foot (2.3-square-meter) area, depending on species and supplemental feeding rates. At the same time, each unit can produce 18 heads of lettuce weekly, i.e., more than 900 heads per year (Zweig, 1986). Tomatoes and cucumber crops

FIGURE 39–1 Solar aquaculture ponds. A dense population of fishes can be seen in the module in the foreground. Photo by J.Todd.

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