The need for high quality human food grows as the world's population grows. Most of the world's wild fish populations are now at or beyond maximum sustained yield, meaning that additional harvest only reduces the fish available for next year's catch. In addition, there is concern that wild caught fish may sometimes be contaminated with mercury or other pollutants. Aquaculture is seen as a way to produce more fish from a controlled environment. However, traditional extensive aquaculture (ponds, raceways) requires an abundant water supply. The same human pressures that increase the demand for fish also decrease the water available to grow them in. All of these factors encourage the use of recirculation aquaculture, where a large amount of high quality fish can be grown in a small amount of water.
The engineering and biology behind recirculation aquaculture is well established and you will learn the details of these in this course. The economics (essentially competing in price with wild caught fish) is currently the limiting factor preventing broader commercial development of aquaculture. At this time, most successful recirculation enterprises focus on niche markets of high value fish. Recirculation technology is not feeding the world...yet.
Recirculation aquaculture is essentially a life support system for fish. It is generally defined as intensive aquaculture in which the water is reconditioned as it circulates through the system and no more than 10% of the total water volume of the system is replaced daily. In order to compete economically and efficiently use the substantial capital investment in the recirculation system, the fish farmer needs to grow as much fish as possible in his system. The level of intensity in recirculation aquaculture is expressed as weight of fish per unit of water. The upper limit of a system based on the atmospheric oxygen appears to be about 0.5 lb/gal (60 g/L). In systems that use liquid oxygen to boost production this can go to 0.75 lb/gal (90 g/L) or even higher. This is the equivalent of a 10 -12 inch (25 - 30 cm) fish living happily in a gallon (4 L) jar! Obviously, to keep so much fish alive in so little water requires a well designed system. Usually, fish die if overcrowded because they either 1) suffocate (this will occur in hours) or 2) poison themselves with nitrogenous waste (this takes longer). A properly functioning recirculation system must aerate the water in some way, adding oxygen and conversely removing carbon dioxide as well as removing the ammonia that fish excrete as a byproduct of the catabolism of protein. Before these two processes can be conducted efficiently, solid waste (feces and uneaten food ) must be removed from the system. So, three things must occur as the water is reconditioned: 1) removal of solid waste, 2) gas exchange, and 3) removal of ammonia. These last two may be conducted in either sequence or together depending on the system. These three fundamental processes cannot all be effectively executed in the fish tank so the water must be recirculated, or moved, through different modules by a pump.
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