mass of zooplankton in the ocean is many times the mass of 

 fish. Also, since the mass of a body varies as the cube of 

 any linear dimension, whereas the surface varies as the 

 square, a given mass of small organism such as zooplankton 

 present a much greater surface from which scattering might 

 take place than an equal mass of fish. Thus the extensive 

 and frequently continuous distribution of the deep scattering 

 layer suggests that pelagic zooplankton may be the basic 

 cause of the phenomenon. Yet, if zooplankton scatter sound 

 like solid or liquid particles, most of them are too small to 

 be effective scatterers of 18-kc. sound, which has a wave 

 length of about 3 inches in water, because the scattering 

 efficiency of an object smaller than the wave length is pro- 

 portional to(d/\) 4 ,where \ is the wavelength of the sound 

 signal, and d is the circumference of the scattering object. 

 Therefore, solid or liquid particles are inefficient scatterers 

 if less than about one inch, so an unreasonably high concen- 

 tration of such organisms is required to produce the observed 

 scattering. Also the intensity of the scattering from small 

 organisms is highly dependent upon the frequency of the 

 sound but, in work off California, such frequency dependence 

 has been shown not to exist. Thus, although the small zoo- 

 plankton such as the copepods are probably not the scatterers, 

 larger, forms such as the euphausiids, which often reach the 

 length of about one inch, may possibly cause the scattering. 



Very small air bubbles in water are known to be excellent 

 scatterers of sound because a gas has a markedly different 

 density and compressibility than water. For this reason, 

 gas bubbles have an "effective diameter" for scattering 

 sound of many times their actual physical diameter. Scattering 

 is especially great if the bubble is of a certain critical size so 

 that it is resonant. The diameter of a resonant air bubble 

 in water for 18-kc. sound is about 1.5 millimeters at 150 

 fathoms, about 2.0 millimeters at 265 fathoms, and 2.5 milli- 

 meters at 415 fathoms. The size of a resonant gas bubble 

 enclosed in a marine organism would vary somewhat from 

 these figures because it would depend upon the elastic proper- 

 ties of the organism and the type of gas. In any event, if 

 minute bubbles are enclosed in the migrating organisms or 

 if they are excreted by these organisms and exist for a short 

 interval before being dissolved, they would probably adequately 

 account for the phenomenon of deep scattering. However 

 there is no record of the presence of such bubbles in vertically 

 migrating types of pelagic organisms. 



As might be expected, there appears to be some correlation 

 between the depth of the deep scattering layer and factors 



90 



