SECT. 4] SOUND SCATTERING BY MARINE ORGANISMS 499 



measuring water depth and not until the widespread introduction of echo- 

 sounders that the study of sound-scattering by marine organisms properly 

 began. Although it is dangerous to ascribe beginnings such as these to single 

 events or persons, we choose the remarkable work of the British fisherman, 

 Ronald Balls, as the pioneering one here. In about 1930 Captain Balls installed 

 an echo-sounder in his herring drifter, Violet and Rose, and soon was setting his 

 nets on the basis of mid-water echoes he properly attributed to herring schools. 

 Balls's observations, through the encouragement of the British fishery biologist, 

 W. C. Hodgson, have been written down in two interesting papers (Balls, 

 1948, 1951). Early work was also done in Norway in apjDlying the echo-sounder 

 to the problems of fishery biology (e.g. Sund, 1935). 



In the last two decades this tool has been used in an ever-increasing diversity 

 of ways on the world's shoal-water fishing grounds in studying the life history 

 and distribution of commercially important fishes as well as by the practical 

 fisherman in better accomplishing his workaday chore. Much of the literature 

 of this part of the subject has been reviewed by Gushing, Devoid, Marr and 

 Kristjonsson (1952) and Hodgson and Fri6riksson (1955). 



We turn now from the shoal-water to the deep-water part of the problem, 

 which is the principal theme of this chapter. It has long been known that 

 species of marine animals are not uniformly distributed throughout the water 

 polumn but that each is limited to an often narrow part of it. Furthermore, it 

 has long been known that certain of these animals make a considerable vertical 

 diurnal migration, generally moving closer to the surface at sunset and away 

 from it at dawn. However, the drama with which the vertical distributions and 

 migrations of some of these animals would be revealed by underwater sound 

 tools was little, if at all, suspected. This part of the study began towards the 

 close of World War II when, during the course of sound propagation experi- 

 ments under conditions of strong downward refraction, workers in the Univer- 

 sity of California's Division of War Research consistently noted reverberation 

 which could only be attributed to a mid- water layer of scattering agents. 

 With the collaboration of Martin W. Johnson, Scripps Institution biologist 

 then with the UCDWR, a diurnal vertical migration and, therefore, the animal 

 character of the observed scatterers were demonstrated. These early investiga- 

 tions were written down in a series of reports by the UCDWR (1943, 1946, 

 1946a, 1946b) and later in a number of papers by the principal investigators 

 (Duvall and Christensen, 1946; Eyring, Christensen and Raitt, 1948; and 

 Raitt, 1948). The source of this reverberation was briefly called the "ECR 

 layer" after three of its discoverers but later called "deep scattering layer"; 

 the question became, "Exactly what causes it?" 



2. Occurrence and Description of Scattering Layers 



It became apparent through echo-sounder observations over broad reaches 

 of the world ocean, made in the late 1940's and early 1950's, that the term 

 "deep scattering layer" was a generic one — that many clearly separate 



