on a table bolted solidly to the concrete floor. 

 Although this method of mounting proved satis- 

 factory, and in spite of the special lining, below 

 50 c .p. s . there were limitations because of 

 standing waves . This situation limited the 

 usefulness of the tank in testing fish in the lower 

 frequency range . 



In addition to the problem of standing 

 waves, early in the exploratory testing we dis- 

 covered that light was a very effective stimulus. 

 Original plans called for testing fish in the 

 experimental tank with soundproof covers on 

 all compartments. The covers were sound- 

 proofed with rubberized horsehair (fig. 4) in 

 the same manner as the tank. It was believed 

 that in this way, the surface of the water could 

 be eliminated as a barrier 'for reflection of 

 sound waves, and that light could also be dis- 

 missed. After several tests, it became obvious 

 that when the covt.rs were on the tanks there was 

 little movement of the fish. This lack of move- 

 ment was noticeable in both the sound tests and 

 the controls, suggesting that darkness might 

 be acting as a depressant to the fish within the 

 tank. 



In an attempt to verify this possibility, a 

 test was made in a lightproof tank, where fish 

 could be viewed by means of infrared light and 

 and infrared viewer. It had been previously 

 determined that infrared light does not act as a 

 stimulant to young salmon (EXincan, 1956) . Fish 

 in total darkness in this lightproof tank demon- 

 strated no movement whatsoever when the sides 

 of the tank were subjected to heavy pounding. In 

 the same tank, under incandescent lights, pound- 

 ing immediately set the fish into fast and erratic 

 swimming movements. Each of these observa- 

 tions was made through a panel of one-way glass. 

 This simple test convinced us that the tank covers 

 were affecting the results of the sound tests, and 

 therefore they were eliminated. 



In addition to these findings, 

 Fields and Finger (1954)^/ demonstrated 

 that young salmon respond without train- 

 ing by actively seeking darker areas to 

 avoid light. It appeared that such a phen- 

 omenon might also be influencing our tests. 

 Bright spots of lig^t from windows in the 

 laboratory were often concentrated in the 

 tank. The fish were always reluctant to 

 pass through these bright spots and leave 

 the "protection" of the darker areas. 

 Because of this, all windows were covered 

 with black cloth and light fixtures of the 

 fluorescent type were installed 4 feet above 

 the tank. With the light fixture at this 

 height, the light intensity, as measured by 

 and underwater photometer, was approx- 

 imately the same in each of the five 

 compartments. 



After the adjustment in the problem 

 of lighting, we felt prepared to begin 

 systematic testing. Because of the length 

 of the tank, frequencies of less than about 

 500 c.p.s. had to be tested with a pulsing 

 technique — 3 pulses with a duration of 

 400 milliseconds in 5 seconds. This method 

 allowed for the dying-off of the sound waves 

 from one pulse before the next pulse was 

 begun, thus eliminating to some extent the 

 standing -wave problem in the low -frequency 

 range. Exploratory testing had shown, 

 however, that even when using pulsed 

 sound, accurate testing below 50 c.p.s. 

 was not possible. 



As in the investigations at Leetown 

 (Burner and Moore, 1953), three basic 

 assumptions were made prior to testing: 

 (1) If the fish were unaffected by, or in- 

 different to, sound waves, they would move 

 within the tank and between compartments 

 in a pattern similar to that of the controls; 



2/ Fields, Paul E., and Gary L. Finger, 1954. The reaction of five species of young 

 Pacific salmon and steelhead trout to light. Univ. of Washington School of Fish. Tech. 

 Rept. No. 7, 24 pp. (Processed.) 



