Bottom Trawl Surveys - Proceedings of a Workshop Held at Ottawa ...

the large angle of incidence and the open
(nearly square) mesh, resulting in escapement of
potential catch. Also, some species of fish
have been observed to be particularly sensitive
to holes in the netting. Even one broken mesh
bar provides enough discontinuity in a sheet of
netting that some fish head for it and escape.
This disruption of herding by discontinuities in
the netting is particularly important with large
mesh netting which otherwise successfully herds
fish which are small enough to escape through
normal meshes.
Small-mesh netting liners usually inserted
into the aft parts of groundfish survey trawls
have considerably greater hydrodynamic
resistance than does normal trawl netting. This
generates abnormal pressures in the trawl and
deflects some of the water flow from the lined
part of the net to unlined parts further
forward. Depending on the tailoring of the net,
this increased flow in the forward part of the
net could increase ballooning and escapement of
fish ahead of the liner.
In addition to this selective herding and
escapement of fish by size and species, there is
selective evasion of the trawl. There is
evidence that at least some species react to the
passage of the trawler overhead, long before the
trawl reaches their position. However, this
action and its effect on catch results have not
been studied. The hydrodynamic resistance of
the trawl generates an increased pressure ahead
of the trawl and it is likely that some
specimens sense this pressure and take evasive
action. Also, Schuijf (personal communication)
reports that fish are physiologically better
able than humans to identify the position of
sound sources. Certainly, fish have been
observed photographically to swim over the
headline of a trawl.
Many groundfish species are naturally
distributed vertically in the water column to a
certain extent, with some specimens further
above the sea bed than the headline of the
survey trawl. Obviously these higher specimens
are not candidates for the survey sample and are
usually ignored, even though they may be members
of the relevant population, with resulting error
in the inventory. The headline height above the
sea bed, in addition to being influenced by
structural parameters such as amount of
flotation and wing-bridle length, is very
sensitive to tow conditions. In particular,
headline height varies with the speed of the
trawl through the water, decreasing with
increasing speed. The trawl/water speed is the
controlling factor, with the result that, as a
consequence of different ocean currents, towing
at constant speed over the sea bed or at
constant vessel/water speed as is common
practice, results in different headline heights
and different sampling intensities between
tows. More important, the effect on fish
population sampling of the different headline
heights of different survey trawls in relation
to the vertical distribution of the fish must be
taken into account when comparing survey catch
results of different gears.
251
The footropes of commercial flatfish trawls
are usually wrapped with cordage or fitted
wholly with rubber discs so that the footrope
references well with the sea bed and minimizes
the escapement of fish under the trawl.
Unfortunately, this type of footrope becomes
very easily caught on sea bed obstructions and
the close proximity of the lower parts of the
net to the ocean floor results in severe damage
on rough sea bed. For more general application,
particularly in random surveys, the design of
the survey trawl must be compromised by
fitting bobbins to the footrope for gear
survival on rough sea bed. The fishing line,
rimming the lower mouth of the trawl, is usually
250-300 mm above the sea bed, and this
undoubtedly allows escapement of some fish under
the net, probably selectively as a result of
different behaviour characteristics of different
species. Associated with this effect, netting
made of polyethylene or polypropylene fibres is
buoyant in sea water (5-8 kg light per 100 kg in
air), whereas nylon netting sinks (12 kg heavy
per 100 kg in air). Consequently, the first two
types of netting tend to rise, clearing the sea
bed more than the last, and encouraging fish
escapement under the net. On the other hand,
the buoyant netting is somewhat less subject to
tear-ups in the lower wings and bellies and
there is therefore less escapement of fish
through the resulting holes in the net.
In some groundfish trawls, such as the
Engel and the Atlantic Western designs, the
netting in the lower wings is cut back (see
Figure 6) to reduce tear-ups on the sea bed in
this very vulnerable part of the trawl. In this
area, the footrope is depended upon to herd fish
into the net near the sea bed, as already
described for the bridles, and the door wakes
and netting in the upper wings are depended upon
to herd fish higher in the water column into the
net. This herding is also probably selective in
a species-specific way but, as with bobbin gear,
a compromise must be reached between this
escapement and unpredictable escapement through
holes caused by tear-ups.
These considerations of variations in trawl
behaviour, caused by the present, inadequately
controlled, fishing procedures, plus
considerations of variations in the reactions of
fish to the fishing operation, indicate that the
catchability of the gear is subject to
considerable fluctuation, not only in the short
term where data can be smoothed but also in the
long term where between-tow variations can
occur. Trawl selectivity depends on much more
than the size of the mesh in the cod-end, so
that results of comparative fishing and
selectivity experiments experience unnecessary
variance if the fishing conditions are not
closely controlled, or at least monitored,
during the fishing experiments.
TRAWL BEHAVIOUR HAS BEEN MEASURED
The relatively qualitative discussion so
far, concerning the behaviour of groundfish
otter trawls and the behaviour of fish in
relation to their capture by groundfish trawls,