40
The Last Resource
through as many stages as a new aeroplane or boat. After the rst
design came tests with models in tanks where conditions met by the
working trawl could be simulated. After that, it was back to the
drawing board and further tests until eventually the design ‘wrinkles’
were all ‘ironed out’. Now came tests with larger prototypes, and
nally the rst trawl was ready for its trials. Apart from avoiding
the need for excessively expensive modications, the detailed
preliminary work does much to compensate for the uncertainties
inherent in observing the action of large trawls moving at ve knots
through murky waters at a depth of 500 feet or so.
Advances in net design coupled with progress in electronics have
brought renewed prospects for increasing the exploitation of mid—
water sh. Apart from long—line sheries which have expanded
considerably in recent years, the mid—water region has been largely
neglected; shermen have concentrated on pelagic and demersal
sh. This apparently myopie view stems mainly from the lack of
suitable trawls for mid—water shing and the difculties of operating
in a region unbounded by either sea oor or surface. How can one be
sure of the position of the net in the mid—water region? Are sh
escaping by ducking underneath or rising above the approaching
net? Can the net be kept ‘on station’ or will it undulate out of
control? These are typical problems facing the prospective mid—
water trawl man. Because they operate in ‘three dimensions’, mid—
water trawls usually have a square mouth. ln some, the bottom of
the net protrudes like a jutting lip to prevent sh from diving
beneath it, others have elaborate oats and hydrofoils to keep them
in position, and the majority have specially—designed otter boards to
hold the mouth of the net wide open. Their depth is controlled by a
combination of speed and the length of towing warp let out from the
trawler.
Since the performance of mid—water trawls is still largely a matter
of chance, various acoustic devices have been tested to feed back
information on the net’s progress through the water. The N etzsonde,
invented in 1958, is typical of these and works on the echo—sounder
principle. It is tted on to the headline of the trawl and relays
information to the boat by a cable connection. By the signals
returning from this device the sherman can learn how wide the net
is opened and how deep the trawl is lying. It even gives some
indication of sh entering the trawl. However, many vital questions
still remain to be answered. Take as an example the speed at which
The Last Resource
through as many stages as a new aeroplane or boat. After the rst
design came tests with models in tanks where conditions met by the
working trawl could be simulated. After that, it was back to the
drawing board and further tests until eventually the design ‘wrinkles’
were all ‘ironed out’. Now came tests with larger prototypes, and
nally the rst trawl was ready for its trials. Apart from avoiding
the need for excessively expensive modications, the detailed
preliminary work does much to compensate for the uncertainties
inherent in observing the action of large trawls moving at ve knots
through murky waters at a depth of 500 feet or so.
Advances in net design coupled with progress in electronics have
brought renewed prospects for increasing the exploitation of mid—
water sh. Apart from long—line sheries which have expanded
considerably in recent years, the mid—water region has been largely
neglected; shermen have concentrated on pelagic and demersal
sh. This apparently myopie view stems mainly from the lack of
suitable trawls for mid—water shing and the difculties of operating
in a region unbounded by either sea oor or surface. How can one be
sure of the position of the net in the mid—water region? Are sh
escaping by ducking underneath or rising above the approaching
net? Can the net be kept ‘on station’ or will it undulate out of
control? These are typical problems facing the prospective mid—
water trawl man. Because they operate in ‘three dimensions’, mid—
water trawls usually have a square mouth. ln some, the bottom of
the net protrudes like a jutting lip to prevent sh from diving
beneath it, others have elaborate oats and hydrofoils to keep them
in position, and the majority have specially—designed otter boards to
hold the mouth of the net wide open. Their depth is controlled by a
combination of speed and the length of towing warp let out from the
trawler.
Since the performance of mid—water trawls is still largely a matter
of chance, various acoustic devices have been tested to feed back
information on the net’s progress through the water. The N etzsonde,
invented in 1958, is typical of these and works on the echo—sounder
principle. It is tted on to the headline of the trawl and relays
information to the boat by a cable connection. By the signals
returning from this device the sherman can learn how wide the net
is opened and how deep the trawl is lying. It even gives some
indication of sh entering the trawl. However, many vital questions
still remain to be answered. Take as an example the speed at which