76 MAINTAINING FISHES 



much easier at temperatures of 55° to 65° F. than at 

 higher temperatures. The metabohc rate is lowered 

 and loss of scales reduced in some species. Cooler 

 water has a higher oxygen-holding capacity and the 

 rate of reproduction of putrefying bacteria is re- 

 duced. 



In warm weather lower temperatures are most easily 

 maintained by the use of ice. It is, of course, of value 

 to utilize an insulated tank. If the temperature at 

 which fishes are to be transported is different from 

 the temperature at which they are being held, they 

 must be slowly and carefully acclimated to the new 

 temperature. Failure to acclimate fishes to a new tem- 

 perature may result not only in acute symptoms but 

 also in delayed symptoms. 



Oxygen 



When fishes are crowded, a shortage of oxygen is 

 most apt to cause early mortality, and even the 

 crudest transport facilities include means of aeration. 

 One may use compressed air, compressed oxygen, 

 baffles that agitate the water as a result of movement 

 of the transport unit, circulation of the water, and 

 mechanical agitation. 



Compressed air, in addition to oxygenating the 

 water, probably washes out a considerable quantity of 

 ammonia and carbon dioxide. But a large compressor 

 is needed to maintain a high oxygen level, and in 

 some cases warming of the water by the continuous 

 flow of warm air is a problem. Compressed oxygen is 

 in some respects convenient but expensive. It is also 

 questionable if it has the washing ability of com- 

 pressed air since a lesser flow is commonly used. It 



