which control the depth penetration of light. For example 

 previous observations^ i n coastal water off California 

 showed the characteristic depth to be about 150 to 200 fathoms. 

 A similar depth was observed off California on the HENDER- 

 SON records. However in the oceanic tropical and subtropical 

 Pacific under conditions of highly transparent water and an 

 overhead sun, the deep scattering layer was typically at a 

 depth from 250 to 350 fathoms. No good correlation was 

 detected between the amount of cloud cover and the depth 

 of the layer. 



An interesting question to consider is what effect the 

 continuous daylight of polar midsummer has upon the diurnal 

 cycle of the scatterers. From the HENDERSON fathogram it 

 appears that the layer generally does not form under condi- 

 tions of permanent or almost permanent daylight. Yet, 

 weakly developed and questionable layers of deep scatterers 

 were sporadically present. In this connection, net haul 

 studies of plankton by Bogorov^O are noteworthy. According 

 to his studies, zooplankton in the Barents Sea (latitude 75°N), 

 under conditions of permanent daylight, do not perform the 

 regular vertical migrations that are characteristic of zoo- 

 plankton of lower latitudes but rather they maintain an almost 

 unchanged vertical distribution throughout a 24-hour period. 

 Further south in the White Sea (latitude 65°S) Bogorov noted 

 the presence of a mixture of both the usual migrating types 

 and the polar nonmig rating types of zooplankton. 



Correlated with the return of a day-night cycle in the 

 antarctic toward the end of January, the diurnal migrating 

 of the scatterers and the development of a deep scattering 

 layer was once again detected. However, in this connection, 

 the almost complete absence of the layer during both February 

 and the first part of March is puzzling and without a reasonable 

 explanation. 



Investigation of the deep scattering layer is a fertile field 

 for research. This phenomenon is of importance in connection 

 with the transmission of underwater sound. If the scatterers 

 are, even in part, fish, a study of the layer is of obvious 

 direct commercial value. If the scatterers are zooplankton, 

 much can be learned about these organisms, their habits, 

 and distribution. Also, if measurements are made under 

 controlled conditions so a scattering coefficient can be ob- 

 tained, roughly quantitative data concerning the populations 

 of the scatterers might be obtained. A study of the variation 

 of the amount of scattering with sound pulses of various fre- 

 quency would yield information on the size of the scatterers. 



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