Fish Hatchery Management - fisheries & aquaculture

ll0FISH HATCHERY MANAGEMENT(3) If the problem is caused by too much supplemental feed, drasticallyreduce or eliminate feeding until the anaerobic condition is corrected.Drain off foul bottom water. Refill the pond with fresh water and add P20.;to induce phytoplankton growth.(4) Summer stratification of ponds often is inevitable. During its earlystage of development, when cool anaerobic water is less than 20-25"[ of thetotal pond volume and upper waters have a moderate growth of greenplants, top and bottom water can be thoroughly mixed. Aerate the pondwith special equipment or an air compressor, or vigorously stir it with anoutboard-powered boat or with a pump. If the layer of anaerobic water ismore than ] the total pond volume, drain off the anaerobic water, refillwith fresh water, and fertilize to re-establish the phytoplankton bloom.(5) Low dissolved oxygen may result from excessive application oforganic fertilizers, which overstimulates plankton production. Treat thisproblem as in example (l), above. Two to six parts per million potassiumpermanganate (KMnOr) may be added to oxidize decaying organic matter,freeing the available oxygen for the pond fish.Quite often, oxygen depletion is caused by two or more of the above factorsacting simultaneously. In such cases, a combination of treatments maybe needed. If a substantial amount of foul bottom water exists, the pondshould never be mixed, because the oxygen deficit in the lower water layermay exceed the amount of oxygen available in the surface layer. Drain offthe anaerobic water and replace it with fresh water from a stream, well, oradjacent pond. An effective technique is to pump water from just belowthe surface of the pond and spray it back onto the water surface with force.Small spray-type surface aerators are in common use. These aerators aremost effective in small ponds or when several are operated in a large pond.More powerful aerators such as the Crisafulli pump and sprayer and thepaddlewheel aerator supply considerably more oxygen to ponds than thespray-type surface aerators. However, Crisafulli Pumps and paddlewheelaerators are expensive and must be operated from the power take-off of afarm tractor. The relative efficiency of several types of emergency aerationappears in Table 15.ACIDI'TYFish do not grow well in waters that are too acid or too alkaline, and thepH of pond waters should be maintained within the range of 6.5 to 9. ThepH of water is due to the activity of positively charged hydrogen ions(H'), and pH is controlled through manipulation of hydrogen ion concentrations:if the pH is too low (acid water), H'concentrations must be decreased.The treatments for low pH (liming) were discussed on pages 108-109.The principle involved is to add negatively charged ions, such as carbonate