Subsequent examination of a fathogram across the Pacific 

 and one from Hawaii to the arctic also revealed the frequent 

 display of this phenomenon. As these records greatly increase 

 the present knowledge of the geographic distribution and as 

 other new information was observed regarding this phenome- 

 non, the results of an examination of these three fathograms 

 is presented in this paper. 



Previous Investigations . Previous investigations'""'*" 

 made with sonar gear since 1942 by workers at the University 

 of California Division of War Research and later at the U.S. 

 Navy Electronics Laboratory, the Scripps Institution of Ocea- 

 nography, and the University of California Marine Physical 

 Laboratory revealed that sound scatterers are not uniformly 

 distributed in the ocean but that they exhibit a striking vari- 

 ation with depth. Frequently, at about 175 fathoms, an ex- 

 ceptionally well-defined layer of scatterers was found that 

 had at least ten times the scattering power of scatterers at 

 shoaler depths. 



It was noted that this deep scattering layer* is principally 

 a daytime phenomenon. In the morning, scatterers descended 

 from near the surface to form the deep scattering layer and in 

 the evening they ascended toward the surface. Such a diurnal 

 cycle strongly suggested that this phenomenon was caused by 

 migrating marine organisms rather than by a physical dis- 

 continuity in the water (e.g., a temperature change). Also, a 

 temperature-change boundary could not account for the in- 

 tensity of the scattered sound. It is well known from net 

 hauls that offshore zooplankton in general (such as the cope- 

 pods Calanus finmarchicus and Metridia lucens , pelagic 

 prawns, euphausiids, and many others) exhibit negative 

 phototropism and that they make daily vertical migrations 

 from the surface to depths as great as 2500 feet. Presumably, 

 many types of zooplankton migrate to the surface at night to 

 feed in the phytoplankton-rich surface layers and in the 

 morning they descend to regions of darkness at great depth, 



*It has also been suggested that this layer of deep scatterers 

 be called the ECR layer in recognition of the joint discovery 

 by C. F. Eyring, R. J. Christensen, and R. W. Raitt, in con- 

 nection with underwater sound work at the University of 

 California Division of War Research. Other workers who 

 have made important contributions to the study of this phe- 

 nomenon include C. Eckart, G. E. Duvall, R. Ely, and M. W. 

 Johnson. M. W. Johnson first showed the apparently biological 

 nature of the scattering layer. Most of the studies by these 

 workers are in anonymous wartime reports. 



