34
The Last Resource
regions while in the tropics and southern hemisphere the pelagic
sheries are more important. Much of the recent growth of the world
sh catch is explained by the expansion or creation of new sheries
south of the equator.
To catch sh one must rst nd them in sufcient numbers for
the protable deployment of expensive vessels, equipment and man—
power. In the past, success or failure rested almost entirely on the
skill and experience of the skipper. Over the years he gained an
intimate knowledge of the shing grounds and how conditions such
as weather and sea state might inuence shing on the grounds. A
mixture of shing lore, myth and sound commonsense provided the
ingredients for his decision but the nal test remained, as it does
today, the size of the catch. Since those times, science and technology
have intruded into the business. Most shing vessels carry some
kind of sh detection system, usually sonar, or echo sounder. Now,
instead of wrinkling his nose and watching for ‘signs’, the skipper
reveals his skills in the manner in which he interprets the informa—
tion arising from such instruments.
Sonar systems all work on the same basic principle: a sound, or
more accurately an acoustic, signal is transmitted towards the sea
bed or ahead of the boat and the echoes returning from obstacles in
its path are recorded and displayed. The time—lapse between the
initial signal and the echo gives the distance to the obstacle, in this
case
sh. The picture may be confused by echoes from larger
members of the zooplankton in the surface and midwater but it is in
the search for demersal sh on or near the bottom that the picture
becomes really complicated. To detect demersal sh, a very marrow
receiving beam is needed to prevent echoes returning from collec—
tions of sh from being obscured by a general mass of sea bed
echoes. On the other hand, it would take the skipper a very long
time to scan the shing ground using such a marrow beam. How
can such apparently contradictory demands be satised?
At the University of Birmingham, in England, a team of scientists
under Professor D. G. Tucker have been working on this kind of
problem for many years. They have found a solution which takes
advantage of the ability to switch electronically between a number
of marrow beam receptors—a product of the revolution in ‘solid—
state’ electronics. A wide beam ‘oods’ a sector with the acoustic
signal. This sector is then rapidly scanned by a narrow beam
receiver which collects the returning echoes and displays them on a
The Last Resource
regions while in the tropics and southern hemisphere the pelagic
sheries are more important. Much of the recent growth of the world
sh catch is explained by the expansion or creation of new sheries
south of the equator.
To catch sh one must rst nd them in sufcient numbers for
the protable deployment of expensive vessels, equipment and man—
power. In the past, success or failure rested almost entirely on the
skill and experience of the skipper. Over the years he gained an
intimate knowledge of the shing grounds and how conditions such
as weather and sea state might inuence shing on the grounds. A
mixture of shing lore, myth and sound commonsense provided the
ingredients for his decision but the nal test remained, as it does
today, the size of the catch. Since those times, science and technology
have intruded into the business. Most shing vessels carry some
kind of sh detection system, usually sonar, or echo sounder. Now,
instead of wrinkling his nose and watching for ‘signs’, the skipper
reveals his skills in the manner in which he interprets the informa—
tion arising from such instruments.
Sonar systems all work on the same basic principle: a sound, or
more accurately an acoustic, signal is transmitted towards the sea
bed or ahead of the boat and the echoes returning from obstacles in
its path are recorded and displayed. The time—lapse between the
initial signal and the echo gives the distance to the obstacle, in this
case
sh. The picture may be confused by echoes from larger
members of the zooplankton in the surface and midwater but it is in
the search for demersal sh on or near the bottom that the picture
becomes really complicated. To detect demersal sh, a very marrow
receiving beam is needed to prevent echoes returning from collec—
tions of sh from being obscured by a general mass of sea bed
echoes. On the other hand, it would take the skipper a very long
time to scan the shing ground using such a marrow beam. How
can such apparently contradictory demands be satised?
At the University of Birmingham, in England, a team of scientists
under Professor D. G. Tucker have been working on this kind of
problem for many years. They have found a solution which takes
advantage of the ability to switch electronically between a number
of marrow beam receptors—a product of the revolution in ‘solid—
state’ electronics. A wide beam ‘oods’ a sector with the acoustic
signal. This sector is then rapidly scanned by a narrow beam
receiver which collects the returning echoes and displays them on a