Report On An Experimental Solar-heated Aquaculture System

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The rationale for recirculating and filtering the pond water has to do with the tendency of fish at high population densities to emit growth-inhibiting metabolites. When concentrations of these natural chemicals are high, as they normally would be in a heavily stocked culture system such as ours, growth may virtually cease, though health, external appearance and feeding of the fish are normal. This situation may of course be corrected by enlarging the pond or thinning the fish population, but a more practical course is to remove chemically the growth-inhibiting compounds by means of a bacterial titter analogous to the subsand filters familiar to aquarium hobbyists. In this type of filter, water is pumped from the pond through a bed of oystershell or other calcareous material and back Into the pond The shell acts physically to remove particulate matter and chemically to buffer pH, but its principal function is as a substrate for the growth of bacteria which chemically break down the growth-inhibiting substances. This type of filter was first used in practical fish culture in Japan in the early 1950's, and has since been applied in a variety of aquaculture systems, where it has increased production by factors of 10—1,000 over stagnant ponds.

The problem I encountered in using an external filter system of 55-gallon drums had mainly to do with the fact that the small 12-volt bilge pump used to circulate the water was not powerful enough to push the liquid through three drums full of crushed oystershell and through a solar heat collector. The solution to this problem is a simple one, but will require some modifications to the tank: A section of the fish container will be walled off with some kind of screening structure so as to contain the crushed oystershell, yet still allow the water to enter. The liquid will then be pumped from the filter through the solar panel and back to the tank. In this way, the pump won't be required to work so hard, since it Will have only to push the filtered water through the solar collector.

Speaking of solar collectors, the one I'm using is a "Solarator", a commercially manufactured unit designed for heating swimming pools. This product was described in my article in MOTHER NO. 25. The manufacturer claims that it will warm up to 10,000 gallons of water . . . but, perhaps significantly, he doesn't state to what temperature. At any rate, I've found that the Solarator adds very little to the heat in my 1,400-gallon tank. When the sun is shining the thermometer will rise as much as five degrees in one day whether the solar panel is being used or not. Since the device seemed to be making no contribution at all to the tank's warmth, I finally stopped using it altogether. I intend to install a larger panel of a different type when finances permit.

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