The vertical migrations were so clearly defined on the 

 NEREUS tape that it was possible to measure roughly the 

 rate of ascent and descent. In the morning the scatterers 

 appear to descend at a rate of about 10 to 15 feet per minute. 

 The evening ascent was slightly more rapid and was accom- 

 plished at a rate of about 15 to 18 feet per minute. 



In agreement with all other observations, the deep scat- 

 tering layer was almost entirely a daytime phenomenon and 

 the vertical migrations were closely correlated with sunrise 

 and sunset or, more exactly, with the beginning and end of a 

 certain, but unknown, amount of twilight. The descent, deeper 

 than 50 fathoms, began about one hour before sunrise and the 

 layer attained its maximum depth shortly after sunrise. 

 The layer varied only slightly from this depth throughout 

 the day. The evening ascent began shortly before sunset but 

 did not reach the 50-fathom depth until about one hour after 

 sunset. Just what became of the layer at depth shoaler than 

 50 fathoms is not known because any further rise of the 

 layer was masked by the outgoing signal. However, prominent 

 extension of the outgoing signal during hours of darkness 

 indicated the abundance of scatterers in the upper 50 fathoms 

 of water. 



During the period from 11 August to 1 September 1947, 

 the USS HENDERSON obtained a fathogram across the Pacific 

 Ocean with a recording NMC fathometer from San Diego, 

 California, to Yokosuka, Japan, via Pearl Harbor, Hawaii. 

 Although the fathometer was operating erratically most of 

 the time so that the fathogram is poor, the deep scattering 

 layer can be at least sporadically detected almost every day 

 at various times during daylight hours. From the fathogram 

 it appears probable that a properly working and sufficiently 

 sensitive echo sounder would have recorded a daytime de- 

 velopment of the deep scattering layer essentially continu- 

 ously across the Pacific. 



When the vertical migration of the layer was clearly 

 recorded, it invariably began to descend about one-half hour 

 or more before sunrise and to rise shortly before sunset; 

 however the upward migration was not completed until after 

 sunset. Multiple layers were occasionally present and the 

 scatterers were located at a depth between 150 and 250 fa- 

 thoms. 



During the first half of the San Diego to Pearl Harbor 

 passage, the layer was present during portions of each day 

 at a depth from 175 to 250 fathoms. Because of mechanical 

 failure, the echo sounder was not in operation during the last 

 half of this passage. 



