510 HERSEY AND BACKUS [CHAP. 13 



principal scattering layer extended from 330 to 390 m in the daytime and rose 

 at night to some level centered near 50 to 60 m. In a sejjarate set of observations 

 made at the same time, two one and one-half hour hauls at 70 m were made 

 under identical conditions save that one was made near sunset when the layer 

 had begun its ascent but was terminated before the layer reached the level of 

 the tow. The second was made after the layer had completed its upward 

 migration. The first showed a large number of euphausids but no fishes ; the 

 second contained a large number of fishes (98 myctophids per hour) and an 

 increased number of euphausids. 



Methods of capture other than towed nets have been little used but gear of 

 suitable design for use at great depth, such as gill nets, should be effective in 

 catching the larger mid-water sound-scatterers. A number of imaginatively 

 designed devices for fishing at depth are under development at the Scripps 

 Institution of Oceanography by John Isaacs and associates. 



B. Photography 



Early in the history of scattering-layer research a number of attempts were 

 made at the Woods Hole Oceanographic Institution and other places to identify 

 by photography the animals composing scattering layers. Cameras making 

 exposures at regular set intervals were lowered into the layers from slowly 

 drifting vessels, and lengthy series of photographs were made. Pictures made in 

 this way generally show little. i Such a procedure has been likened in its pros- 

 pects to the photography of birds by pointing an unattended camera into the 

 sky and making exposures with it at some regular interval. It appears that mid- 

 water photography, like other kinds, requires for success the aid of some sort 

 of " viewfinder" . Four sorts of viewfinders or triggering devices have recently 

 been used for this purpose. In one, used in the photography of bioluminescent 

 animals, a photomultiplier tube, on receipt of a bioluminescent flash, triggers 

 the camera with the assurance that the responsible animal is within the pur- 

 view of the optical system (Breslau and Edgerton, 1958). In a second, the 

 interruption by an object of a beam of light impinging upon a photocel 

 triggers the camera with like assurance (Breslau and Edgerton, 1959). In a 

 third, baits have been attached to a mechanical trigger for the purpose of 

 attracting and photographing mid-water animals (Baker, 1957). In a fourth, 

 for the photography of mid-water sound-scatterers, an echo-sounder is used 

 that is capable of resolving individual scatterers at short enough distances to 

 be within camera range. In such observations the echo-sounder transducer and 

 camera are arranged so as to examine as nearly as possible the same volume of 

 water. An electrical cable connects the echo-sounder transducer to its ship- 

 borne amplifier and recorder, and the camera to a trigger located at the 



1 Craig (1955) reports some success in photographing schools of fish in shallow water by 

 this means ; yet he notes, "it is surprising how many blanks are obtained even in a trace 

 of some density". Occasional successful pictures of mid-water objects have also been 

 made by Edgerton and Cousteau (see, for instance, Cousteau, 1954). Their rarity is partly 

 compensated for by their highly interesting nature. 



