M. J. Tucker and A. R. Stubbs 311 
It is advantageous to be able to alter the relative amplitude of the side lobes 
so that they produce recordings of roughly equal density, and for this purpose 
the transducer is divided into three horizontal strips whose relative sensitivities 
can be altered. The most satisfactory arrangement for geological survey work 
is to have the relative sensitivities in the ratio 3 to 2 to 3. 
High resolution in the recorder and good correlation between adjacent traces 
is required, so a Mufax recorder as described above is used. The transducer 
is stabilized against the roll ofthe ship; otherwise the side-lobe patterns become 
confused in rough weather. The instrument has given useful results in depths of 
up to 100 fathoms. 
16.3. BIOLOGICAL APPLICATIONS 
16.3.1. Fish Detection 
Although this subject comes under the field of fishery research rather than 
oceanography, a brief account of it should be of interest here. 
An echo-sounder of adequate sensitivity will show echoes from midwater 
organisms. The most powerful echoes come from shoais of fish, and the vast 
majority of the larger fishing boats nowadays are equipped with echo-sounders 
specially designed for finding fish shoals. 
One of the most difficult problems for the fisherman is to decide what kind 
of fish he is seeing. At present, this is largely a matter of experience and local 
knowledge. He knows that at a particular time of year in a particular position, 
a particular type of pattern on the recorder usually means a certain kind of fish. 
Even an experienced fisherman often makes mistakes, however, and will catch 
valueless fish; thus, there is much interest in finding some means of identifying 
the fish. Present ideas suggest either using high-resolution echo-sounders with 
narrow acoustic beams [11], which will show more detail of the structure of the 
shoal, or measuring the frequency dependence of the echo strength. This latter 
idea is attractive in principle, but presents severe practical problems which 
have not yet been overcome. 
A second problem which has been solved with a fair measure of success is 
the detection near the sea bed of fish which can be caught by bottom trawls. The 
vertical height of the opening of such a trawl is about 12 ft, and it is fish within 
this distance of the sea bed which have to be detected. Most important is cod, 
a fairly large fish with an acoustic cross section of up to about 1000 cm? at 
typical echo-sounding frequencies. Echo-sounders can be made which will detect 
even individual cod up to the maximum trawling depth (say, 300 fathoms), the 
difficulty being to obtain sufficient resolution to separate these echoes from the 
bottom echo. Currently, the system most commonly used is to present the echo 
trace on a cathode-ray oscilloscope using an "A scan," but with the time base 
expanded and its initiation controlled so that only the last 50 or 60 ft above the 
sea bed is presented (Fig. 16.12). This is, however, rather a trying display to 
watch, and systems have been developed to separate fish and sea-bed echoes 
on a paper recorder [12]. 
The area examined by an echo-sounder is, of course, a comparatively small 
lane directly beneath the path of the ship; thus, horizontal echo-rangers (asdics) 
are beginning to be used to look for fish over a wider area. There are several 
