Combining Mariculture and Seawater-Based Solar Ponds

Solar ponds have been thoroughly studied as a means to produce electricity or heat, but there may be comparable potential to use solar ponds to produce optimized environments for the cultivation of some aquaculture crops. For this, conventional brine-based solar ponds could be used. This strategy would probably be most suitable at desert sites where concentrated brine was abundant, pond liners might not be needed, and the crop produced could be shipped to market. Generally, a heat exchanger would be required to transfer heat from the solar pond into the culture ponds. Culture ponds could therefore use either fresh or marine water. In contrast, this paper explores what we name seawater-based solar ponds. These are solar ponds which use seawater in the bottom storage zone and fresh water in the upper convective zone. Because the required temperature elevations for mariculture are only about 10°C, seawater-based solar ponds are conceivable. Seawater-based ponds should be very inexpensive because, by the shore, salt costs would be negligible and a liner might be unnecessary. An initial paper described the design and preliminary experience with two 16 m2 seawater-based solar ponds adapted for mariculture during the winter of 1986-1987 (reference [1]). Subsystems designed for air injection, salt gradient maintenance, filtering to remove ammonia, feeding, and maintenance of water clarity were detailed. Typical temperature and salinity gradients and month-long temperature elevation performance were also presented. This paper presents follow-up experimental results. During Jan. and Feb. 1986, operation of the two seawater-based solar ponds with no cultivation in them produced sustained bottom temperatures averaging 25.5°C. During this period, the ambient air averaged 13.8°C and the overnight low averaged 8.9°C. Elevation of seawater to 20–28°C would be extremely useful for winter mariculture along the entire southern coastline of the United States. In contrast, during the winter of 1987–1988 formal growth comparison experiments were conducted with striped bass, (Morone saxatilis), growing within two replicate solar ponds and within two replicate, conventional, control ponds. Over six winter months the solar pond bottom temperatures averaged 6°C warmer than the ambient air. Fish weights in the solar ponds increased by a cumulative average of 1105 percent compared with 172 percent for fish in the control ponds. These results are in line with other studies of the influence of temperature on the growth rate of striped bass. Management of the solar ponds involved a simple daily routine. These two experiments, therefore, demonstrate the profound potential of combining suitably designed seawater-based solar ponds with mariculture in winter to raise water temperature and accelerate growth. The solar ponds could be used either (a) as a warm water source or (b) with cultivation directly in the solar ponds. With either approach seawater-based solar ponds can potentially be very inexpensive. Both strategies deserve continued study because they have distinct advantages. For case (b), more involved research will generally be required since the solar ponds and cultivation practice for a specific species must both be adjusted to work together.