IV. biological observations 



DEEP SCATTERING LAYER 



In recent years, since the development of the more power- 

 ful echo sounders which make a continuous tape recording of 

 depth versus time, layers which scatter sound have been fre- 

 quently noted in the ocean at depths of from 100 to 450 fathoms. 

 The trace on the fathogram caused by scattering layers has the 

 appearance of a false bottom. Typically developed only during 

 the day, these deep scattering layers descend from the surface 

 in the morning and rise in the evening. The scatterers are 

 presumably certain types, or perhaps many types, of marine 

 zooplankton which exhibit a marked negative phototropism 

 dominating a negative geotropism. These organisms swim 

 to the surface at night to feed in the diatom-rich surface 

 water and descend during the day to regions of darkness in 

 order to avoid destruction by their predators. Although prob- 

 ably zooplankton, the scatterers may possibly be nekton (e.g., 

 fish or squid), which follow and feed upon the migrating 

 zooplankton. Although comparatively few in number, nekton 

 are generally much larger than zooplankton forms and there- 

 fore more efficient scatterers of sound. 



The deep scattering layer was recorded on the fathogram 

 of the USS NEREUS during the passage from Hawaii to Adak, 

 but this particular layer has been reported on separately by 

 Dietz. Within the limits of the Bering and Chukchi Seas the 

 deep scattering layer was noted only on 26 July when the 

 USS NEREUS, during her passage from Adak to the Pribilof 

 Islands, crossed the deep oceanic basin which comprises the 

 southern portion of the Bering Sea. 



This record of the layer can be seen in figure 3A. As is 

 usually the case, the layer is absent during the night and is 

 developed only during the day. It can first be seen at 75 fath- 

 oms as a distinct layer resolved from the outgoing ping at 

 0700 LCT (1800 Z). Echo extension of the outgoing signal for 

 about two hours prior to this time suggests that, although the 

 record of the layer is partially masked by the outgoing ping, 

 it actually began to form and descend at about sunrise (0500 

 LCT on this date). 



The scattering layer is continuously developed all during 

 the morning and early afternoon hours at a depth of from 75 

 to 100 fathoms. This is an unusually shoal depth for the 

 development of the layer which, in other parts of the Pacific, 

 is more commonly at a depth of from 175 to 250 fathoms and 

 occasionally at a depth as great as 450 fathoms. Since the 



