After leaving Pearl Harbor, the layer was fairly well- 

 developed the first day out of port. During the next two days, 

 it was not detected. Yet from the fourth day out of port 

 until arriving in the shallow water off Japan, five days later, 

 the layer was recorded every day and it appeared to become 

 more strongly developed with westward penetration. 



Discussion and Conclusions. Examination of three fa- 

 thograms, which cross the Pacific in both a North-South 

 and an East-West direction and cover a sector of the Antarctic 

 Ocean, shows a wide development of the deep scattering layer. 

 It appears to be an oceanic phenomenon which is Pacific-wide 

 and probably world-wide. 



All of these records further substantiate the diurnal 

 cycle by which the scatterers descend in the morning and 

 ascend to near the surface at night. On the NEREUS fatho- 

 gram, the scatterers begin their descent about one hour 

 before sunrise and they do not complete their ascent until 

 about one hour after sunset, so the scatterers are in the 

 surface waters only during rather complete darkness. Ap- 

 parently the scatterers are motivated by a small amount of 

 light, so the migrations are actually correlated with a certain 

 amount of twilight rather than sunrise and sunset. No infor- 

 mation-was obtained on the distribution of scatterers in the 

 surface water at night because of masking by the outgoing ping. 



Because of the diurnal cycle, it is evident that the deep 

 scattering layer is a biological phenomenon, so the scattering 

 agents must be zooplankton, nekton, or bubbles associated 

 with some organisms. Yet, from these records, no definite 

 conclusion can be drawn as to whether zooplankton or fish 

 cause the scattering. It will be necessary to make numerous 

 laboratory experiments on the scattering characteristics of 

 the various forms and to correlate underwater sound data 

 with net hauls before the nature of the scatterers can be 

 established. However, the extensive distribution of the deep 

 scattering layer is suggestive of zooplankton. The compara- 

 tively slow speed and the migration of the scatterers as a 

 layer is, perhaps, more suggestive of the general zooplankton 

 than of faster swimming fish. The occasional development 

 of night layers, of double descending layers, and of multiple 

 deep layers of scatterers show that the phenomenon is com- 

 plicated and probably involves many kinds of organisms and 

 different stages of development of a single species. 



The amount of scattering is dependent in part on the sum 

 of the cross-sections of all of the objects present. The total 



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