SECT. 4] SOUND SCATTERING BY MARINE ORGANISMS 507 



240-m layer begins its descent somewhat later and its ascent somewhat earlier 

 than the 500-m layer. 



The 500-m layer is not found in water depths of less than 1800 m, is present 

 in about half of our records made in depths between 1800 and 3600 m and is 

 more or less consistently present in records from depths greater than 3600 m. 

 The limiting depth of the shallower, 240-m layer is about equal to its own mid- 

 day depth. 



Although our observations are thinly distributed in the eastern North 

 Atlantic it appears that the regime described above applies equally there. 

 Observations made in the Mediterranean Sea indicate that the 500-m layer is 

 weak or absent there, the 240-m layer being the principal one. 



3. Identification of Sound Scatterers 



At the time of the discovery of deep scattering layers and the demonstration 

 of their widespread existence in the world ocean, it appeared that the identifica- 

 tion of the responsible organisms would be a relatively simple and straight- 

 forward task. This belief was largely due to the grossly exaggerated impression 

 of the density of animals in these layers that the echo-sounder record gives. 

 Consider a layer of 50 m thickness about a depth of 325 m viewed by an echo- 

 founder located at the surface whose principal lobe is 30° between three-dB or 

 half-power points. The volume of water within the layer that is intercepted by 

 the sound field of the echo-sounder under these conditions is equal to about 

 1.2 X 10^ m3. To pass a comparable volume through a net such as the Isaacs- 

 Kidd mid-water trawl, the area of whose mouth is about 6 m^, would require 

 about 40 h at a speed of 3 knots. It is only necessary to postulate, say, 100 

 scatterers in this volume, returning echoes above the marking threshold of the 

 echo-sounder recorder, to give the continuous, dark, diffuse stripe on the chart 

 which we call "deep scattering layer". Such a population would be equal to one 

 scatterer per 1.2 x 10^ m^ of water. Such a density of scatterers is probably 

 much lower than those found in nature although data on this point are almost 

 wholly lacking. Raitt (1948) determined that there was one scatterer of an 

 acoustical cross-section greater than 1 cm^ per 10^ m^ of water in that part of 

 the water column above a deep scattering layer in the eastern Pacific Ocean. 

 Kan wisher and Volkmann (1955) counted strong scatterers at a level above a 

 deep scattering layer off southern New England by means of a suspended 

 12 kc/s echo-sounder and found one scatterer per 8.5 x 10^ m^ of water. By the 

 same technique Johnson, Backus, Hersey and Owen (1956) found one scatterer 

 per 650 m^ of water in a migrating scattering layer north of Puerto Rico. The 

 desirability of making further such estimates is obvious, as they may then be 

 compared with what is known of the population densities of various mid-water 

 animals as shown by collection with nets. The hypothetical case drawn, and the 

 observation of Johnson et al., do, however, suggest the naivety of the early 

 suggestions that here for the harvesting was a vast, untapped food resource, 

 and also demonstrate the difficulty that may be encountered in identifying the 



