The distribution of the deep scattering layer during 

 Operation HIGHJUMP and other cruises is shown in figure 39; 

 its general appearance is shown in figure 40. With the ex- 

 ception of two days in the vicinity of the Marquesas Islands, 

 the layer was recorded the greater part of each day during 

 the daylight hours throughout the passage from San Diego to 

 the antarctic from 2 December to 23 December 1946. Con- 

 trolled by the length of daylight, the layer was detected at 

 least discontinuously for as short as a 10-hour period and as 

 long as a 19-hour period. Except in the early morning or in 

 the late evening at times when the layer was actively mi- 

 grating, it was developed at depths that ranged from 150 to 

 450 fathoms. Near San Diego a depth of about 150 to 200 fa- 

 thoms were characteristic whereas, in the tropical and 

 southern waters, a depth of 200 to 350 fathoms was more 

 typical. The ascent and the descent of the layer were closely 

 correlated with sunset and sunrise. With the exception of one 

 or two questionable sporadic displays of the deep scattering 

 layer, it was completely absent from the fathogram during 

 hours of darkness. The phenomenon of a double layer was 

 frequently displayed. 



The Antarctic Convergence was crossed at noon on 23 De- 

 cember in the vicinity of latitude 62°S. This water-mass 

 boundary was clearly indicated by the abrupt drop in tempera- 

 ture from 39 to 33 degrees F. It is noteworthy that the deep 

 scattering layer disappeared completely for three days after 

 the ship crossed this boundary and that it was seldom observed 

 to be well-developed in antarctic waters. 



During the period from 25 December to 2 March, the 

 HENDERSON navigated slowly westward along the western 

 Pacific and Indian Ocean sectors of Antarctica. Only a few 

 indications of layers of deep scatterers were recorded for 

 short intervals from 27 December to 21 January, a period of 

 permanent or of almost permanent daylight. From 21 to 29 

 January, the deep scattering layer was again well-developed 

 and during this interval displayed the typical diurnal cycle 

 (fig. 40). This display might be correlated with the return 

 of a day-and-night cycle because by this date there were a 

 few hours of complete darkness. However, after 2 February, 

 the deep scattering layer was seldom observed either during 

 the remainder of the antarctic cruise or en route to Australia. 



During the return passage from Australia to the United 

 States, the deep scattering layer was detected on the third 

 day out of Sidney. The typical diurnal cycle was recorded 

 each day until the echo sounder went out of operation (at 

 about latitude 20°S) because of mechanical failure. 



85 



