from this tow, but a low concentration compared to the three collections at station 

 8 with heavy concentrations. One each of three species of myctophids were taken 

 in this sample as were two specimens of a gonostomatid Pollichthys mauli . One or 

 two specimens of Pollichthys mauli were also in each of the other three samples but 

 there were no myctophids. 



DISCUSSION 



Numerous workers have noted the similarity between the vertical 

 distribution patterns of mesopelagic animals and the diel vertical movements 

 of sound scattering layers. Backus et al. (1968), using a submersible, recently 

 demonstrated that a myctophid Ceratoscopelus maderensis is indeed responsible 

 for a peculiar 12-kHz scattering layer known as "Alexanders Acres", recorded 

 regularly in slope water off New England. 



Many pelagic fish have gas-filled swimbladders which can resonate 

 when struck by sound waves of the proper frequency. This resonant frequency 

 of the associated gas bubble varies primarily with the depth and bubble 

 size. 



The resonant frequency, f^, of a swimbladder is approximated by 

 Minnaert's equation (Minnaert, 1933) as 



where fp is in cycles/sec, R is the radius, in cm, of a sphere equal in volume 

 to that of the swimbladder, / is the ratio of specific heats of the swimbladder 

 gas, P is the ambient pressure in dynes/cm'^ and p is the density of seawater 

 in gm/cm^. To account for the energy losses in the resonating fish-swimbladder 

 system, Andreeva (1964) modified the above equation to 



where jX-^ represents the real part of the complex shear modulus of fish tissue. 

 At depths greater than 200 meters, where 3 y P » 4 ;/,, , the effect of the fish 

 tissue can be neglected. Using equation (1), Gold and Van Schuyler (op.cit.) 

 computed swimbladder volumes for two scattering layers detected by means of 

 explosive measurements in Area Bravo. The frequency characteristics of these 

 layers, and associated bubble sizes and volumes are summarized in Table V. 



25 



