III-65 



The swim-bladders of bathypelagic fishes are not generally spherical 

 but are more nearly prolate spheroids. Furthermore, they probably do not be- 

 have strictly as "free" bubbles but are constrained by the body of the fish. Scat- 

 tering by a prolate spheroidal bubble can be shown to have a resonant frequency 

 which varies as p^'^ if the bubble size and shape remain constant. If the gas 

 content remains constant and the bubble is free to compress in all directions, 

 then the resonant frequency varies as p^'^, as for the spherical bubble. How- 

 ever, if the bubble is constrained so that it will not shorten its major diameter, 

 but will compress by becoming "slimmer, " then the resonant frequency varies 

 very nearly as p. The latter possibility is likely, because the structure of the 

 fish leaves it freer to compress in this way. 



The above two possible modes of behavior for the swim-bladder require 

 considerably different biological mechanisms. There is considerable controversy 

 as to whether such behavior is anatomically at all possible, and if so, which, if 

 either, exists in the types of marine organisms picked up in dragging operations 

 in the deep scattering layer. Further details are given in Hill (Ref. Ill- 15). 



No experimental studies have been made of single specimens over a 

 sufficiently great frequency range to permit the evaluation of simplified theo- 

 retical models. Some scattering cross sections have been reported by Albers 

 (Ref. III-l). No frequencies are given, but presumably these are the resonant 

 cross sections. The values given by Albers are 



Shrimp (Palaemonetes) 



Shrimp (Panaens) 



Scup 



Squid 



Sea Bass 



Hersey and Backus (Ref. Ill- 15) have studied the relationship between 

 peak sound scattering frequency and depth during the vertical migration of deep 

 scattering layers. The relationship that would hold if the swin>-i) ladder and its 

 gas were responding passively to changes in ambient pressure (III-39) was ob- 

 served during both a sunrise and sunset migration. This seems to imply that the 

 gas content of the swinHsladder is kept constant, even though this causes the fish 

 to be at neutral buoyancy only at one point of its depth range . 



The relationship between variation in pressure and frequency of maxi- 

 mum scattered intensity that would be expected if the swim-bladder contents were 

 being adjusted to maintain neutral buoyancy was observed only during sunset mi- 

 gration. Thus, it seems that some bathypelagic fish are able to absorb swim- 

 bladder gas at a rate which would allow them to maintain neutral buoyancy during 



S-7001-0307 



2.32 • 



10"^ 



6.56 • 



10-^ 



3.8 ■ 



10-* 



5.33 • 



10-* 



4.4 • 



■ 10-^ 



