for counting the test fish in the various com- 

 partments. 



Along both sides of the deck opening, or 

 well, overhead garage -door rails were mounted 

 for hanging the underwater pen. These rails ex- 

 tended 14 feet beyond the raft, and were held by 

 cables running from the afterdeck forward over 

 a large tripod, as shown in figure 5. Side -to - 

 side motion of the rails was controlled by 

 cables extending from the forward corners of 

 the raft to the ends of the rails. The under- 

 water pen was constructed of small cedar 

 strips hung on two larger main stringers. The 

 entire pen (2 by 2 by 12 feet) was covered with 

 1/4-inch mesh galvanized hardware cloth, ex- 

 cept for the top . Under normal weather 

 conditions, the top of the pen was about 4 

 inches above the surface. Each of the six com- 

 partments formed a 2 -foot cubicle separated by 

 a sliding curtain of nylon bobbinette . The pen 

 could be rolled forward of the raft and back into 

 the well by means of garage -door hangers and 

 rails . 



For use in the open water, a watertight 

 container was fabricated out of heavy aluminum 

 sheeting for submerging the electromagnetic 

 transducer (fig. 6). The barium titanate crystal 

 transducer was watertight, and submerging this 

 unit presented no problems . 



The move from tank testing to open -water 

 testing required additional equipment and 

 facilities. Several problems were encountered 

 that had not been evident in the laboratory . The 

 main difference was the background noises in 

 the lake, which were always present to some 

 degree. Frequent measurements of the ambient- 

 noise levels showed them to be highly variable 

 in both quantity and quality. This variability 

 was quite closely associated with wind force and 

 the resulting surface condition of the lake . In 

 addition, directional noises from both known and 

 unknown sources also proved worthy of consider- 

 ation. These findings made it essential to 

 evaluate the noise conditions prior to each 

 systematic test. Without this knowledge, the 



results of the tests could be mislead- 

 ing. 



The sounds that were picked up 

 throu^ the hydrophone were monitored 

 with earphones. A recorder was incorp- 

 orated in the circuit, as shown in figure 7, 

 to make a permanent record of these 

 sounds . 



It was noted, while listening to re- 

 cordings of the same frequency made at 

 two different times, that the background 

 noises were often of different magnitude; 

 at times it was almost impossible to 

 recognize the frequency being introduced 

 into the water. This finding demonstrated 

 that it was not always possible to measure 

 the effects of a frequency on the distribu- 

 tion of fish, without considering the back- 

 ground noise at the time of testing. 



For further analysis of the problem, 

 the recorded noises were fed into a record- 

 ing voltmeter. This provided us with a 

 permanent visual record of the noise levels 

 in the water. The effects of various back- 

 ground noises could be seen superimposed 

 on the primary frequency introduced by the 

 transducer (fig. 8). It was evident from 

 these findings that there were times when 

 accurate testing of fish in the open water 

 was not practicable. In the majority of 

 cases, however, testing could be done with 

 little concern for ambient noises in the 

 water. Occasionally, temporary cessations 

 in testing were necessary, when inboard 

 motorboats or boats with large outboard 

 motors were passing near the raft. 



OPEN- WATER PROCEDURE 



In testing from the floating labora- 

 tory, 25 fish were introduced into the first 

 compartment of the underwater pen, the 

 pen was rolled out to a position in front of 

 the raft, and the transducer was submerged 

 between the raft and the underwater pen. 



