71 



AERATION AND TEMPERATURE CONTROL IN TANKS 



Aeration and temperature control are important in 

 the operation of carrying and holding tanks. A series 

 of experiments was run in Minnesota to determine the 

 effect of these factors on the mortality of fish. It 

 was shown that fat-headed minnows exposed over a period 

 of hours to non-aerated waters die with increasing 

 rapidity as higher temperatures are used. The most 

 rapid loss occurs at temperatures above 65 E. Fat- 

 headed minnows can be kept in well-aerated water for 

 several hours at relatively high temperatures without 

 loss, and greater numbers of minnows can be carried at 

 all temperatures in aerated water than in non-aerated 

 water. 



These experiments showed that safe operation of 

 minnow tanks requires close adherence to the following 

 1 imi ts : 



a. Water in non-aerated tanks should be kept at 

 65° P. or lower. 



b. When tanks are aerated with running water, a 

 continuous flow of not less than 1 gallon per minute 

 for each 25 gallons of water in the tank should be 

 maintained. The water should reach the tank from 

 pressure jets placed well above the water level. Each 

 tank should have a minimium of two pressure jets and at 

 least one jet for every 25 gallons of water in the 

 tank. 



c. When oxygen is used for aeration, it should 

 be dispersed into the water through carbornndum tips 

 or a perforated oxygen-release tube. When oxygen or 

 other forms of aeration are used, the equipment should 

 be operated so as to maintain a minimum of three parts 

 per million of dissolved oxygen under operating condi- 

 tions. 



d. A popular practice when handling minnows is 

 to fill the truck tank at the bait store with cold 

 water (often around 50° P.). While this water is 

 hauled to the pond, it does not have time to warm up 

 very much. The minnows are seined from a warm-water 

 pond and placed in the cold water of the tank with only 

 a few minutes of tempering. The shock is not great 

 enough to kill the fish at once, but within several 

 hours a large percentage will be dead. A minnow should 

 not be subjected to more than a 10° change of tempera- 

 ture unless the change isvery gradual. Proper temper- 

 ing requires 20 minutes for every 10 change. A pocket 



