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mputerge-
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enced very
The Euro-
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tivation of
| areas in-
sh such as
a period of
ausing low
prices. European producers have traditionally specialised
in the production of premium quality produce. Conse-
quently, damaging though this price slump is, the pro-
ducers are still able to export to the many world-wide
markets which recognise their high quality. Due to the
continual and catastrophic decline in wild fish stocks, and
the corresponding chaos in the hunter fishing industry, it
is clear that the future of European fish farming looks
promising.
In addition to the basic need to maintain structural in-
tegrity under extreme weather conditions the following
problems with existing pen systems can be identified.
e From an economic perspective, it is clear that for a
pen collar of a particular size, it is desirable to
maximise the available volume within the pen net. It
is important therefore that the volume achieved does
not decrease significantly under the effect of tidal
current. The problem of volume loss due to current
also occurs when stocked pens are towed from one
site to another. At present such manoeuvres are only
performed when absolutely essential due to the ex-
treme problems involved and the stress caused to
the fish. Towing must be performed at speeds of less
than two knots and requires perfect weather condi-
tions. Currently towing is mostly performed to avoid
problems with algal blooms, though if increased cur-
rent resistance is achieved, it would be possible to
move farms away from oil pollution incidents such
as the recent disasters of the Braer in Shetland or
the Sea Empress in Wales.
e The problem of wild animal predation on farmed
salmon has been endemic throughout the industry.
Stock loss to birds has been largely solved by cover-
ing the pens with either large mesh netting, or else a
similar system of string. Predation from seals has,
however, proved significantly more troublesome.
e Under particularly violent sea conditions it is possible
that stock damage can occur due to the pen sides
impacting with the fish.
By gaining a better understanding of the most effective
ways to stiffen net systems, each of these problems can
be reduced. It is believed that through the exploitation of
state-of-the art computational modelling, existing tension-
ing practices can be greatly improved. In particular, by
increasing the stiffness due to geometric rather than
purely elastic factors, significant economic benefits
should be gained. Pen volume loss will be reduced giving
farmers greater flexibility in their stocking density. This
will in turn allow them to keep fish from the market during
periods of overproduction. If, as expected, the tensioning
improvements result in a reduction in fish scale damage
due to dynamic wave action, the products premium
value will be preserved. By reducing the effectiveness of
seal attacks, the economic losses due to both stock de-
struction and net damage will be lessened. Additionally,
since farmers will resort to shooting seals much less, the
negative environmental image associated with this prac-
tice will be disassociated from the industry.
1.3 State of the Art and Innovation
The original response to seal predation was the installa-
tion of secondary defensive nets outside the main stock
525
nets. Despite this measure, it was found that predation
was still a serious problem, resulting in the shooting of
large numbers of seals. Since the use of anti-predator
nets involved considerable cost, both capital and opera-
tional, a more effective alternative strategy was sought.
It was found that if tension could be maintained through-
out pen nets, predation was mitigated. Accordingly, a
variety of techniques were developed to apply prestress
to the nets. With small to medium sized square pens, it
was found that the introduction of a semi-rigid frame at
the pen base proved effective. Such a strategy was found
to be too unwieldy for the larger square pens, however,
due to the very much larger steel tubes required. The
tensioning system developed to cater for these larger
pens has since been applied to many smaller systems
due to its simplicity and cost effectiveness. Several im-
plementations exist, but the general principal behind
them all is to suspend heavy weights from the pen col-
lars, and then tension the nets from these weights. This
system, like the semi-rigid frame system can not be
effectively applied to round pens. Despite the pen ten-
sioning approaches already taken, existing systems are
still susceptible to predator and other loading induced
problems.
The expected design improvement proposals are based
on an extension of the existing tensioning technology, as
well as completely new strategies. The key element
among the proposed technological developments is the
underwater net geometry measurement system. At pres-
ent no data exists on the accurate shape adopted by
fishfarm nets, and no system is available to address this
need. Without such data it is impossible to calibrate the
computational structural models required to evaluate
design improvements. With respect to the completely
novel design proposals, two specific areas which look
particularly promising are the use of curvilinear net cut-
ting patterns and stiffening from pneumatic pressure.
In order to determine underwater net geometry, 3D pho-
togrammetric reconstruction techniques are applied to
networks extracted from digitised video data.
2 PHOTOGRAMMETRIC DATA ACQUISTION
The main goal of this first on-site test was the evaluation
of the functionality and suitability of the components.
There are different topics and special problems of photo-
grammetric data acquisition in the underwater environ-
ment. In the following we outline the problems and de-
scribe the chosen solutions.
2.1 Object Preparation
The net must be marked to highlight specific key points.
Contol points must be attached to the pen structure,
which is difficult in the unstable underwater environment.
It is not possible to use natural points on the nets due to
the weak visibility conditions. Retroreflective targets
cannot be used because the plankton will drift to the light
source and reflect the light so that the visibility will be
even worse. Also, retroreflective targets exhibit strong
directional bias. It is therefore difficult to illuminate all
targets satisfactorily. Active lights must be used to
identify the net points and control points. We used strings
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
