3. Diurnal Movement 



The fathometer traces usiially exhibited the typical diurnal move- 

 ment, descending at about sunrise and ascending at about sunset. 

 Apparent anomalies in this pattern can probably be attributed to time 

 lag within a time zone, heavily overcast mornings or evenings, or 

 faulty interpretation of poor fathograms, 



k» Depth of Deep Scattering Layer 



The prevailing daytime depth of the DSL on the majority of the 

 days was 200 fathoms. The DSL was recorded at depths of 225 and 2$0 

 fathoms east of New Zealand and at a depth of 275 fathoms south of 

 New Zealand, A double layer was evident on the fathogram of 23 February 

 at about 61*^ S, in the South Atlantic Ocean, the upper layer being re- 

 corded at 175 fathoms and the lower at 350 fathoms. Prevailing depths 

 of 150, 175, and 225 fathoms were recorded during several days, particu- 

 larly in the Atlantic Ocean, Variations in the depth of the daytime 

 level were evident during the majority of the days that the DSL was 

 observed. Usually the differences were of small magnitude, but 

 variations of 100 fathoms occvirred on 2 days and differences of 50 

 fathoms on several other days. 



The DSL was generally recorded over deep water. The minimtun bottom 

 depth over which it was found was about 500 fathoms, the maximum about 

 3,700. A possible reason for the minimum bottom depth limit is the 

 capability of the instrument itself. When recording on the 600-fathom 

 scale, the machine appears either to be unable to record the DSL or 

 records it very poorly. This is perhaps the result of the lower gain 

 setting used on this scale. On several daj'^s all evidence of a well- 

 developed layer, which was being recorded \n.th the fathometer set on 

 the 6,000-fathom scale, would disappear when the scale setting vas 

 shifted to the 600-fathora scale. One exception to the minimum depth 

 occurred on 9 January off the southeast coast of New Zealand, where a 

 trace was obtained while the fathometer was set at the 600-fathom scale. 

 The bottom depth varied between 250 and 700 fathoms. The trace began 

 at 1335 LST and consisted of intermittent groupings of traces usually 

 appearing at a depth of about 200 fathoms. Toward sun-Jot the traces 

 gradually rose; the last one occurred at about 90 fathoms. The traces 

 appeared similar to those that have been made of schools of fish. 



5, Discussion and Comparison With Other Observations 



During the daylight hours between the Panama Canal and New Zealand, 

 personnel operating the fathometer reported the DSL nearer the surface 

 on overcast days than on clear days (based on aural observations). 

 However, careful comparison between the daytime DSL depth, as recorded 

 on the fathogram, and the cloud cover observations for the same days 



59 



