M. J. Tucker and A. R. Stubbs 313 
800 YDS. 
TATTR Oy EE Tee | 
i 
i. 307 DPSS be] SaO 
es 
. 
: 
aS ul 
z =I 
t: =! 
§ = 
4, 
; = 
7 
AS x 
a & 
as a 
Ld < 
ae 
Mb WA hy Ade eee 
Fig. 16.13. Asdic record showing large fish concentration in midwater. 
Surprisingly little is known about the composition of these layers. One can tow 
a plankton net through them and usually catch nothing which apparently could 
cause the level of reverberation actually observed. With the aid of acoustic 
techniques, it is sometimes possible to identify the organisms in one or two 
layers with a fair degree of certainty. In the record shown, for example, which 
was taken with a 36-kcps sounder, the diffuse layer between 250 and 350 fathoms 
did not appear on the record from a 16-kcps sounder. A net haul through this 
layer caught a number of small fish (Cyclothone sp.) 2 or 3 cm long, whose air 
bladders would resonate at a frequency in the region of 36 kcps; and though 
calculation showed that the number caught, compared with the volume swept out 
by the net, was hardly sufficient to cause the observed level of reverberation, 
it is known that active animalscanavoidthe net, and it seems reasonably certain 
that this layer was composed of these fish. We have occasionally managed to 
identify other layers by a similar technique, and Hersey [13] has managed to 
identify a scattering layer with some certainty using a more refined technique 
for measuring the frequency characteristics. 
From the acoustic results, it seems reasonably certain that the nets used 
at present by biologists are very inefficient for sampling certain types of or- 
ganisms in the sea, and we may well have a completely false idea of the relative 
abundance of the different species. 
