508 HERSEY AND BACKUS [CHAP. 13 



animals composing these layers. In spite of the fact that these layers are indeed 

 concentrations of organisms, their volume compared to the water volume 

 containing them is small. 



A. Capture 



A number of attempts have been made to correlate the vertical distributions 

 of sound-scatterers as shown by echo-sounder, and the vertical distribution of 

 marine organisms as determined by the hauling of nets. Although such studies 

 are often referred to as "direct evidence" as to the composition of certain 

 layers, this method like others has its limitations and, in fact, is an indirect one. 

 First, one is confronted by the selective effects of the net and the echo-sounder 

 which both necessarily neglect large portions of the animal and sound-scattering 

 spectra. Secondly, catches from hauls of practical duration made in scattering 

 layers of low population density may be so small as to prevent any conclusion 

 being drawn from them. (The difficulties of making highly controlled mid- 

 water collections are a lesser limitation and of another kind. With the 

 development of better opening- and closing-nets, with continuously available 

 depth-of-net information during the haul, and with the greater availability of 

 able vessels, better collections will be made. The frequently aired criticism that 

 many important sound-scatterers are fast-swimming and so escape present nets, 

 while undoubtedly true, has likely been greatly exaggerated. The many species 

 of bathypelagic fishes and larger crustaceans most commonly caught in the 

 past are of sufficient scattering cross-section and abundant enough to explain 

 much of the observed sound-scattering.) Thirdly, no matter how strong the 

 correlation between the observed scattering and the catch, one is not certain 

 that the captured animals are indeed the agents responsible for the observed 

 scattering. Many authors who have made such comparisons strongly express 

 their awareness of this fact. That this argument is a real one may be seen from 

 the fact that most net hauls contain a variety of animals. Furthermore it is 

 probable, as was early suggested (Dietz, 1948; Johnson, 1948; Tucker, 1951 ; 

 and Tchernia, 1952), that the depth of a principal scattering layer is likely to 

 be the focal plane of a complicated community of preying and preyed-upon 

 animals. A few or many of the species involved may be significant contributors 

 to the observed scattering. In spite of these limitations there is much to be 

 learned from net hauls, and they are desirable, if not essential, adjuncts to 

 other means of determining the animal composition of scattering layers. This 

 approach has been used in a number of studies of both shallow and deep 

 scattering layers. The most extensive of those of deep scattering layers will be 

 briefly noted. 



Boden (1950) studied the plankters taken in a series of net hauls made in the 

 northeastern part of the Pacific Ocean and off southern California. These 

 catches were compared with sound-scattering records made with a 17 kc/s echo- 

 sounder. In general the plankton volume was greatest at the level of the deep 

 scattering layer observed, and euphausids were accounted the most important 



