salinity increase. A sound channel, which starts out at 32° S. latitude in the 

 tongue of Antarctic Intermediate water at a depth of 1200 meters, ascends to 

 1000 meters at station 1 1 (20° S . latitude) as it follows the tip of the tongue 

 toward the surface. North of the divergence, the sound channel again drops 

 to 1200 meters and continues at 1200 meters as far as 6° S . At the equator, the 

 sound channel has descended to 1500 meters and with slight variation, maintains 

 approximately this level to the end of the profile. 



Sound velocity between stations 27 and 30 (Fig. 19) shows almost no change 

 at or near the surface. Isopleths are nearly parallel with the surface until the 

 4925 line,which dips sharply downward west of station 29. This dip is reflected 

 In the location of the sound channel which rises from 1500 to 1100 meters at 

 station 28 and then drops to 1400 meters at station 27. 



H , Transparency 



Secchi disc transparency was determined whenever light conditions permitted; 

 18 out of the 30 stations include such observations. On the southern 32° S. sec- 

 tion three transparency readings averaged 29.5 meters and ranged from 25 to 38.7 

 meters. On the south-north section 12 transparencys averaged 25 meters with a 

 range between 22 and 30 meters. Three transparencys taken at stations 28, 29, 

 and 30, averaged 30 meters with a range of between 27 and 38 meters. The high- 

 est or best transparency observed was at station 4 (38.7 meters) and second highest 

 or best was at station 28 (38 meters). Thus, an average of all stations measured 

 in the Indian Ocean comes to about 26 meters transparency. 



I . Deep Scattering Layer 



The deep scattering layer was followed by observing the fathometer trace 

 three times per day, and it remained between depths of between 100 and 300 

 fathoms until the evening of 1 April at about 104° E. longitude. That evening 

 it was weak at 250 fathoms and was not observed again until 11 April at latitude 

 8° S .,when it reappeared on the trace at between 200 and 400 fathoms. It was 

 evident also at that time that at least part of the DSL had come to the surface 

 because of the abundance of lumenescent ctenophores, fish, and squid that were 

 dashing around under the powerful winch light, when stations were taken at 

 night. The DSL continued on into the waters off Ceylon, and it was followed a- 

 cross the northern Indian Ocean but disappeared in the Red Sea. 



The disappearance of the Deep Scattering Layer in mid-Indian Ocean and 

 its reappearance near the Indian coast duplicate its performance in the Pacific 

 Ocean where this phenomenon has been observed several times en route to New 

 Zealand from Panama. It is the author's belief that no DSL exists In mid-ocean 

 because of the scarcity of plankton, hence scarcity of plankton feeders, squid, 

 and fish . 



41 



