364 U.S. BUREAU OF FISHERIES 



fish in no, 4 trough were found to weigh approximately 0.3 gram less 

 than those in the other troughs. From this time on the slower 

 growth of the larger lots became more and more noticeable, until on 

 July 10 it was necessary to discontinue the lot in trough no. 4 on 

 account of an outbreak of bacterial gill disease. 



The average individual weight of the fish in each lot on July 

 5 was as follows: Trough no. 1, 14.26 grams; no. 2, 13.27 grams; 

 no. 3, 11.15 grams; and no. 4, 8.89 grams. The mortality up to the 

 time of the outbreak of the gill disease in trough no. 4 was in all 

 cases too small to be of any significance. The results show clearly 

 that overcrowding not only retards the growth of the fish but also 

 increases their susceptibility to disease. 



A series of experiments were carried out at the Leetown hatchery 

 to determine the amount of oxygen removed from the water in 

 troughs containing various numbers and sizes of fingerling trout. 

 The complete results cannot be given here, but a few examples 

 may be of interest. The troughs were supplied with water having 

 a temperature of 54° F. at the rate of 5.17 gallons per minute. 

 In one trough containing 1,500 brook trout with an average individ- 

 ual weight of 13.45 grams, 5.78 parts per million of oxygen or 

 60.02 percent of the total amount in the water was removed by the 

 fish. In another trough containing 20,461 rainbow fingerlings 

 with an average individual weight of 0.24 grams, the amount of 

 oxygen removed was only 1.4 parts per million or 12,5 percent of 

 the total amount. In a third experiment 3,900 black spotted trout 

 fingerlings, averaging 2.33 grams in weight, removed 1.88 parts 

 per million of oxygen, or 18.02 percent of the total amount present. 



Feeding greatly increased the consumption of oxygen, as shown 

 in the case of a trough containing 1,500 brook trout, with an average 

 weight of 13.45 grams. On the morning of November 28 after these 

 fish were fed 300 grams of food, 7.26 parts per million of oxygen 

 were consumed in this trough. At 4:05 p.m., several hours after 

 feeding, the consumption of oxygen in the same trough was only 4.72 

 parts per million, or 44.74 percent. At this time the fish were again 

 given 300 grams of food, and the oxygen consumption rose to 8.55 

 parts per million, or 81.04 percent of the total amount present. It 

 is evident from these experiments that in overcrowded troughs there 

 is a distinct possibility that during or shortly after feeding the 

 oxygen content of the water may drop to dangerously low levels 

 even though at other times the supply may be more than sufficient 

 for the needs of the fish. 



A self -cleaning device for use in circular pools has been developed 

 by Mr. Surber, which it is believed will greatly simplify the opera- 

 tion of this type of pool. The device consists of a large sleeve, 

 which is attached to the outlet pipe and extends for a short distance 

 above the surface of the water. At the bottom there is a small 

 opening between the sleeve and a slojDing flange which rests on the 

 bottom of the pool. The width of this opening can be easily ad- 

 justed according to the size of fisli in the pool. Excrement and waste 

 material are drawn through the opening by the water flowing 

 through the outlet pipe, thus automatically keeping the pool clean 

 and in good sanitary condition. 



