1 10 FISH HATCHERY MANAGEMENT 



(3) If the problem is caused by too much supplemental feed, drastically 

 reduce or eliminate feeding until the anaerobic condition is corrected. 

 Drain off foul bottom water. Refill the pond with fresh water and add P^Or, 

 to induce phytoplankton growth. 



(4) Summer stratification of ponds often is inevitable. During its early 

 stage of development, when cool anaerobic water is less than 20-25% of the 

 total pond volume and upper waters have a moderate growth of green 

 plants, top and bottom water can be thoroughly mixed. Aerate the pond 

 with special equipment or an air compressor, or vigorously stir it with an 

 outboard- powered boat or with a pump. If the layer of anaerobic water is 

 more than ^ the total pond volume, drain off the anaerobic water, refill 

 with fresh water, and fertilize to re-establish the phytoplankton bloom. 



(5) Low dissolved oxygen may result from excessive application of 

 organic fertilizers, which overstimulates plankton production. Treat this 

 problem as in example (l), above. Two to six parts per million potassium 

 permanganate (KMnO,) 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 fac- 

 tors acting simultaneously. In such cases, a combination of treatments may 

 be needed. If a substantial amount of foul bottom water exists, the pond 

 should never be mixed, because the oxygen deficit in the lower water layer 

 may exceed the amount of oxygen available in the surface layer. Drain off 

 the anaerobic water and replace it with fresh water from a stream, well, or 

 adjacent pond. An effective technique is to pump water from just below 

 the 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 are 

 most effective in small ponds or when several are operated in a large pond. 

 More powerful aerators such as the Crisafulli pump and sprayer and the 

 paddlewheel aerator supply considerably more oxygen to ponds than the 

 spray- type surface aerators. However, Crisafulli pumps and paddlewheel 

 aerators are expensive and must be operated from the power take-off of a 

 farm tractor. The relative efficiency of several types of emergency aeration 

 appears in Table 15. 



ACIDITY 



Fish do not grow well in waters that are too acid or too alkaline, and the 

 pH of pond waters should be maintained within the range of 6.5 to 9. The 

 pH of water is due to the activity of positively charged hydrogen ions 

 (H ), and pH is controlled through manipulation of hydrogen ion concen- 

 trations: if the pH is too low (acid water), H^ concentrations must be de- 

 creased. 



The treatments for low pH (liming) were discussed on pages 108-109. 

 The principle involved is to add negatively charged ions, such as carbonate 



