SECT. 4] 



SOUND SCATTERING BY MARINE ORGANISMS 



505 



lowering the transducer to a position just above the layer. This is so because 

 the scatterers have a relatively high scattering cross-section and are sufficiently 

 few within the near part of the water volume insonified by the echo-sounder. 

 The result of such an observation, just prior to sunset (Johnson, Backus, 

 Hersey and Owen, 1956) are shown in Fig. 5 in which a transducer was lowered 

 to a depth midway between the surface and a deep scattering layer. The echo- 

 sounder was held in this position and the scattering layer migrated past on its 

 way towards the surface. Examples of deep scattering layers recorded by surface 

 echo-sounders are shown in Figs. 6, 7 and 8. 



We may describe the deep scattering layers of the western North Atlantic, 

 as recorded by 12 kc/s echo-sounders, as representative of observations in open, 

 deep ocean areas. About 150 records have been examined for making these 



DEPTH 

 (fathoms) 





4^00 



5 minutes 



Fig. 6. The sunrise descent of deep scattering layers recorded by a 12 kc/s echo-sounder in 

 the eastern Pacific off northern Chile. A layer, which appears to have remained at 

 depth throughout the night, is shown near 300 fathoms. 



generalities. Here one finds two principal deep scattering layers. The shallower 

 of these has an average midday depth (midway through the layer) of 240 m, 

 the extremes of the variation being 185 and 405 m. The average thickness of 

 this layer is about 75 m. The deeper of the two layers has an average midday 

 depth of about 500 m, the extreme values being 405 and 590 m. The average 

 thickness of this layer is about 130 m. 



Both of these layers show the familiar diurnal vertical migration although 

 there are non-migratory elements in both, especially in the deeper layer. Like- 

 wise both layers often show a splintering during the vertical migration such 

 that as many as four separate elements may be formed from what appeared to 

 be one layer prior to migration. The record of ascending and descending layers 

 may also be complicated by the sudden appearance or disappearance on the 



