502 



HERSEY AND BACKUS 



[CHAP, 13 



1952 ; Herdman, 1953 ; Gushing, Lee and Richardson, 1956 ; and Weston, 1958). 

 The question is often asked if density discontinuities themselves may not 

 reflect sufficient sound to be recorded as a scattering layer. In one promising 

 situation in the North Sea this was carefully investigated by Weston (1958) 

 who demonstrated that the sharp density gradient at the level of a scattering 

 layer was not the scattering agent. Weston admits that in some extreme cases 

 this might be possible were the echo-sounder of sufficiently low operating 

 frequency. Planktonic animals including fish larvae with swim -bladders are 

 regarded as the probable source of these layers. They may or may not show a 

 diurnal vertical migration. Generally, but not always, such layers retain the 



100 



220 



300 



340 



380 



(min) 



Fig. 3. A shallow-water scattering layer over the continental shelf south of New England 

 recorded by a 12 kc/s echo-sounder. 



appearance of diffuse reverberation even when the range between them and the 

 echo-sounder transducer is made very short by lowering the transducer to a 

 level near them. This is consonant with the notion that such layers are com- 

 prised of large populations of relatively weak scatterers. 



The scattering layers in the near-surface portion of the deep ocean may 

 include some of the same phenomena already described for shallow water. 

 The class of layers which typifies deep water, however, and to which we restrict 

 the term "deep scattering layer", is one which is generally found at midday 

 depths of from 200 to 600 m but occasionally as deep as 1000 m. The lower 

 limit of the layers is approximately at the depth where, in daytime, the ambient 

 light level no longer decreases with depth but is maintained at a fairly constant 



