at a stop opening of f/4. Several fish are 

 shown in the illustration, but single fish 

 were used in the experiments. 



Individual fish were used in the trough, 

 and each was tested both in darkness and 

 with infrared radiation. The fish were in- 

 troduced in the light and allowed to swim 

 freely about until they appeared to take up a 

 definite position in relation to the current 

 and the trough. When this position had been 

 maintained for some 20 seconds, it was 

 noted and the visible light turned out. The 

 test situation, darkness or infrared, lasted 

 for !• minute, at which time the visible light 

 was turned on and the position of the fish re- 

 corded. The results of the experiment 

 appear in table 2 . 



There is nothing in the data from either 

 orientation experiment to suggest that the 

 young silver salmon are able to utilize 

 infrared radiation to maintain their position, 

 either with respect to other fish or to en- 

 vironment. The inference is, of course, 

 that the necessary visual stimulation is 

 lacking. 



Fright 



No evidence of a fright reaction was de- 

 tected with the telescope when the fish were 

 subjected to infrared radiation in the experi- 

 ments just described. The matter was also 

 examined specifically in comparison with 

 visible light. The equipment used was the 

 infrared source previously mentioned, a 

 fiashli^t fitted with a Wratten 87 filter of the 

 gelatin type, and an unmodified flashlight. 

 The fish were in the hatchery holding troughs 

 under subdued lights, and several observers 

 were present. A typical fright reaction, with 

 sudden, spasmodic movement and a rapid dart- 

 ing away from the light v/as noted when the 

 visible light was directed on the fish, and quirkt- 

 ly turned on an off. All of the fish in. the beam 

 of light reacted similarly. When the flashlight 



with the gelatin filter was used it was 

 noted that some of the fish reacted as 

 above, but that others appeared completely 

 unconcerned. Similar experiments after 

 substitution of the regular infrared source 

 failed to elicit an observable response. 



The Wratten 87 filter was noticeably 

 lighter than the other filter used, and a 

 distinct red glow in the lens was notice- 

 able upon looking directly into the light. 

 No visible light could be seen m the beam 

 or on the water. It would appear that fish 

 so situated in the tank as to see the glow 

 in the lens reacted to it, while those 

 otherwise situated were unaffected. Walls 

 (1942) notes that many fishes are sensitive 

 to red, and that they "generally seem 

 either to shun red, or to prefer it decidedly'.' 

 Such evidently is not the case with infrared 

 radiation, so far as the silver salmon 

 tested were concerned. 



INFRARED RADIATION AS A TOOL 



Penetration in Water 



Absorption by water is a problem in 

 any use of infrared radiations for the ob- 

 servation of fish behavior . It has been 

 noted in the upper part of figure 1 that 

 penetration of the near -infrared wavelengths 

 differs little from that of visible red, and 

 is considerable. It has been further sug- 

 gested that absorption, per se, is not the 

 sole criterion of suitable penetration, and 

 that the "vlsLial range" of a light is almost 

 double a corresponding Secchi-disk reading. 

 In some situations the Rayleigh effect may 

 be expected to increase the range of ob- 

 servations, and in all conditions visibility 

 is improved by the brightening of the image 

 in the image -converter tabe. Illuminaiion 

 and penetration have proved adequate for 

 work in the 9 inches of water in the main 

 electrical -guiding pool at this laboratory, 

 as illustrated in the photograph in figure 8. 



11 



