【正文】 利用，循環(huán)水系統(tǒng) 只用一小部分水就可產(chǎn)出傳統(tǒng)養(yǎng)殖相同的產(chǎn)量。大約每英畝 一百萬(wàn)加侖的水才能填滿池塘，而且每年需要相等體積的水來(lái)補(bǔ)償蒸發(fā)和滲漏 掉 的水。s capacity to neutralize acidity (hydrogen ions). Bicarbonate (HCO3) and carbonate(CO3) are the predominant bases or sources of alkalinity in most waters. Highly alkaline waters are more strongly buffered against pH change than less alkaline waters. Nitrification is an acidproducing process. As ammonianitrogen is transformed to nitratenitrogen by nitrifying bacteria, hydrogen ions are produced. As hydrogen ions bine with bases such as hydroxide (OH), carbonate and bicarbonate, alkalinity is consumed and the pH decreases. Levels of pH below are dangerous to fish。外文文獻(xiàn)翻譯 Recirculating Aquaculture Tank Production Systems An Overview of Critical Considerations Thomas M. Losordo, Michael P. Masserand James Rakocy Traditional aquaculture production in ponds requires large quantities of water. Approximately 1 million gallons of water per acre are required to fill a pond and an equivalent volume is required to pensate for evaporation and seepage during the year. Assuming an annual pond yield of 5,000 pounds of fish per acre, approximately 100 gallons of water are required per pound of fish production. In many areas of the United States, traditional aquaculture in ponds is not possible because of limited water supplies or an absence of suitable land for pond construction. Recirculating aquaculture production systems may offer an alternative to pond aquaculture technology. Through water treatment and reuse, recirculating systems use a fraction of the water required by ponds to produce similar yields. Because recirculating systems usually use tanks for aquaculture production, substantially less land is required. Aquatic crop production in tanks and raceways where the environment is controlled through water treatment and recirculation has been studied for decades. Although these technologies have been costly, claims of impressive yields with yearround production in locations close to major markets and with extremely little water usage have attracted the interest of prospective aquaculturists. In recent years, a variety of production facilities that use recirculating technology have been built. Results have been mixed. While there have been some notable largescale business failures in this sector, numerous small to mediumscale efforts continue production. Prospective aquaculturists and investors need to be aware of the basic technical and economic risks involved in this type of aquaculture production technology. This fact sheet and others in this series are designed to provide basic information on recirculating aquaculture technology. Critical production considerations All aquaculture production systems must provide a suitable environment to promote the growth of the aquatic crop. Critical environmental parameters include the concentrations of dissolved oxygen, unionized ammonianitrogen, nitritenitrogen, and carbon dioxide in the water of the culture system. Nitrate concentration, pH, and alkalinity levels within the system are also important. To produce fish in a costeffective manner, aquaculture production systems must maintain good water quality during periods of rapid fish growth. To ensure such growth, fish are fed highprotein pelleted diets at rates ranging from to 15 percent of their body weight per day depending upon their size and species (15 percent for juveniles, percent for market size). Feeding rate, feed position, fish metabolic rate and the quantity of wasted feed affect tank water quality. As pelleted feeds are introduced to the fish, they are either consumed or left to depose within the system. The by products of fish metabolism include carbon dioxide, ammonia nitrogen, and fecal solids. If uneaten feeds and metabolic byproducts are left within the culture system, they will generate additional carbon dioxide and ammonianitrogen, reduce the oxygen content of the water, and have a direct detrimental impact on the health of the cultured product. In aquaculture ponds, proper environmental conditions are maintained by balancing the inputs of feed with the assimilative capacity of the pond. The pond natural biological productivity (algae, higher plants, zooplankton and bacteria) serves as a biological filter that processes the wastes. As pond production intensifies and feed rates increase, supplemental and/or emergency aeration are required. At higher rates of feeding, water must be exchanged to maintain good water quality. The carrying capacity of ponds with supplemental aeration is generally considered to be 5,000 to 7,000 pounds of fish per acre ( to pound of fish per gallon of pond water). The carrying capacity of tank systems must be high to provide for costeffective fish production because of the higher initial capital costs of tanks pared to earthen ponds. Because of this expense and the limited capacity of the “natural”biological filtration of a tank, the producer must rely upon the flow of water through the tanks to wash out the waste byproducts. Additionally, the oxygen concentration within the tank must be maintained through continuous aeration, either with atmospheric oxygen (air) or pure gaseous oxygen. The rate of water exchange required to maintain good water quality in tanks is best described using an example. Assume that a 5,000gallon production tank is to be maintained at a culture density of pound of fish per gallon of tank volume. If the 2,500 pounds of fish are fed a 32% protein feed at a rate of percent of their body weight per day, then pounds of feed would produce approximately pounds of ammonianitrogen per day. (Approximately 3 percent of the feed bees ammonianitrogen.) Additionally, if the ammonianitrogen concentration in the tank is to be maintained at mg/l, then a mass balance calculation on ammonianitro