378 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



During and following the war, f athograms in deep water in both the 

 open Pacific and Atlantic have shown the presence of layers, of 

 dubious constitution, that scattered the outgoing signal to varying 

 degrees so that a false bottom appeared at levels down to several hun- 

 dred fathoms. The nature of this scattering layer has been the subject 

 of inquiry and controversy ever since it was first detected. (See the 

 discussion by R. S. Dietz in the Journal of Marine Research, November 

 1948.) At first it was believed that some physical discontinuity in 

 the water, such as a temperature change, might produce the effect, 

 but the intensity of the scattered sound was often so great as to rule 

 out a temperature change or other physical boundary. 



As the records became more abundant, and particularly after they 

 were made continuously over a 24-hour cycle, it became apparent that 

 the depth of the scattering layer differed during day and night. It 

 sank during the daytime and came nearer the surface at night. Such 

 a diurnal cycle immediately suggested that the cause of the scattering 

 layer might be migrating marine organisms. Biologists have long 

 known from laboriously collected net hauls that certain zooplanktonic 

 forms, notably the shrimp and prawnlike types, react negatively to 

 light ("exhibit negative phototropism"). Accordingly, these organ- 

 isms migrate toward the surface at night, presumably to feed on 

 phytoplankton in upper layers, and then descend to deeper and darker 

 water during the daytime. The extent of these daily vertical migra- 

 tions is of the order of many hundreds of feet, thus corresponding 

 well with the observed change in depth of the deep scattering layer. 

 Some of these zooplankton are almost microscopic in size, although 

 some, like the euphausid shrimps, are an inch or more in length. At 

 first it was suggested that the majority of zooplankton were too small 

 to scatter sound effectively ; hence, the actual scatterers might be large 

 schools of squid or fishes which follow and feed on the zooplankton, 

 and which the biologist with his clumsy and inefficient nets had not 

 been able to catch. If this were so, the use of sonar gear to detect such 

 schools in the open sea could open vast possibilities for the commer- 

 cial fisherman. 



Unfortunately, the bulk of evidence now favors the view that the 

 scatterers are mainly zooplankton. Recent experiments have shown 

 that minute particles do scatter high-frequency sound, and therefore 

 typical concentrations of even the small-sized zooplankton can account 

 for the deep scattering layer. Certainly more than one kind of ani- 

 mal is involved, and in some areas euphausid shrimps appear to be 

 the dominant element, but as yet there is no clear indication that 

 squid or fishes are the principal scatterers. At this stage it does not 

 seem that the deep scattering layer is destined to be a tool of great 

 direct significance to the commercial fisheries. Recent calculations 

 have shown that the living populations at depths where the scattering 



