43 



trawl for bottom sampling. The instrument seems very promising, but as it 

 has not been thoroughly tested, it is as yet premature to discuss its merits or 

 defects. * 



A bottom sampler described by N. A. Holme (1949) has two cutting cylin- 

 ders which operate in opposite directions, the thrust of one working against the 

 other. 



Other instruments for observing benthic organisms will be discussed in 

 succeeding sections (see especially section on underwater photography and self- 

 contained diving units), 



MISCELLANEOUS INSTRUMENTS 



Sonic Devices - Several oceanographic instruments, designed primarily for oth- 

 er uses, are being successfully employed in studying the distribution and abun- 

 dance of animal populations. In this category are various sonic devices and 

 photographic instruments. 



As Johnson (1948) pointed out, there are two principal ways in which 

 sound, depending upon its origin, may serve in biological studies. The sounds 

 produced by marine animals, such as shrimp crackle and the drumnming of scia- 

 enids, may be picked up on hydrophones, recorded if a permanent record is de- 

 sired, and analyzed with appropriate equipment, 



Sound, projected as signals from echo-ranging or echo-sounding gear, 

 may be returned as reflected sounds by organisms. + The "deep scattering lay- 

 er", a sound-reflecting zone, has been so much discussed since it was first ob- 

 served in 1942, that it is familiar to anyone interested in oceanographic re- 

 search. It is generally assumed that the scattering is of biological origin, in- 

 asmuch as it is not stationary at any depth, but behaves "biologically" in per- 

 forming vertical diurnal migration in much the same manner as many planktonic 

 animals. The DSL, which occurs at depths of 150 to 450 fathoms during the 

 daytime, is thought to be caused by stratification of either planktonic animals 

 (such as euphausiids) or nektonic forms associated with the plankton (such as 

 squid or lantern fish). 



Lack of adequate instrumentation, particularly gear designed for quanti- 

 tative sampling of the animals within the sound scattering layer (or layers), has 

 delayed the solution of this problem. The simultaneous use of several deep 

 water trawls sampling different levels should aid materially in the identification 

 of the organisms responsible for the scattering, if the scattering is due to nek- 

 tonic forms. 



Sonic gear, especially echo-sounding gear, is often used in the location 

 of fish schools. In Norwegian fishery investigations, for example, echo-sound- 

 ing gear has been employed since 1935 in studying distribution of herring, with 

 outstandingly successful results. Hodgson (1950) and other European scientists 

 claim to have developed means of distinguishing the echoes of different species 



* - Editorial note: Since the symposium this dredge has been used with 

 success on the Trans-Pacific and other cruises to depths of 3200 fathoms. 



+ - In a recent paper. Gushing, Devoid, Marr and Kristjonsson (1952) have 

 summarized current research on the use of echo-sounding, echo-ranging and 

 aerial-scouting in fish detection. 



