possibly for safety from their predators. However, many 

 marine zooplankton forms may be too small to scatter 18-kc. 

 sound effectively. For this reason it has been suggested 

 that the sound scatterers are nekton such as fish or squid 

 which follow and feed upon the zooplankton. In any case it 

 is probable that migrating zooplankton are at least indirectly 

 responsible for the deep scattering layer. 



There is comparatively little previous information re- 

 garding the geographical distribution of the deep scattering 

 layer. It is known to be frequently present off the California 

 and Baja California coasts, especially during the day. It has 

 been occasionally reported from various spots in the Pacific. 

 Recently, workers at Woods Hole Oceanographic Institution 

 have reported (personal communication) the presence of this 

 phenomenon in the Atlantic Ocean. Also it is probable that 

 some of the uncharted shoals reported by ships at sea are 

 false bottoms ascribable to the deep scattering layer. 



Deep Scattering Layer on the USS HENDERSON and 

 USS NEREUS Fathograms. Prior to a discussion of the 

 HENDERSON and NEREUS types, it should be pointed out 

 that the absence of a record of the deep scattering layer on 

 the fathogram does not necessarily preclude its presence, 

 because such absence may be due to mistuning or to other 

 causes. A low gain setting will fail to bring in the deep 

 scattering layer; on the other hand, a gain setting which is 

 too high may mask the layer. Such changes in sensitivity 

 undoubtedly account for its alternate disappearance and re- 

 appearance throughout certain days. The layer may also be 

 masked by background noise produced by the ship when 

 underway or by operation of ship's equipment of various 

 kinds. However when it does not show up throughout a long 

 period of time, it may be assumed that it is absent or at least 

 that it is very poorly developed. Throughout these cruises, 

 the echo sounders were primarily operated to obtain depth 

 information and the recording of the deep scattering layer 

 was incidental. The length of the outgoing sound signal 

 masked any echoes from the layer when it ascended to depths 

 of less than about 100 fathoms on the HENDERSON record 

 and less than about 60 fathoms on the NEREUS tape. The 

 time used throughout this paper is the standard time for 

 the zone in which the ship was located. The NMC fathometer 

 of the HENDERSON and the NMC-1 fathometer of the NEREUS 

 utilized 18-kc. sound pulses. 



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