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

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Much further work on fish swimming
performance and capability to detect stimuli
(including directional sensitivity) has been
carried out and is reported here in the
following section. Of particular importance are
the recent strides in direct observation of the
way in which fish react to trawls; these
observations are now possible with low-light
television operated by divers towed in wet
submersibles and by use of remotely-controlled
towed underwater vehicles. These video tapes
make it possible to examine what proportion of
the fish which accumulate in front of the
groundrope eventually fall back into the net;
this is accomplished by simply counting the
number escaping below the groundrope and those
rising above the groundrope and sv1imming in the
belly of the net. Crosschecks can also be made
by comparing the 'average' number of fish
accumulated with the total catch at the end of
the tov1. Computer- aided analysis of the tape is
planned. Preliminary results, mainly on
flatfish and using manual counting, suggest that
approximately two-thirds of the fish which
accumulate in front of the groundrope end up in
the codend.
Further information on the reaction of fish
to trav1l s comes from experiments conducted using
fish marked with transponding tags and observed
from sector scanning sonar equipment. This
work, reported by Harden-Jones, is discussed
further in the following section; unfortunately,
the enormous cost associated v1ith the technique
has not led to its widespread use in research.
An additional problem is the lurking suspicion
that tagged fish may show atypical behaviour.
The general principles of gear performance,
especially those confirmed by commercial fishing
operations (e.g., the effect of changing the
bridle length), together with direct and
indirect information, have been used in
providing coefficients or relationships for the
model presented later in "A Mathematical Model
of an Otter Trawl Catching Fish". Since there
is alv1ays the danger of entering into circular
arguments in such models, care has been taken to
keep the basic equations general with the view
towards further refinement of coefficients as
more information becomes available.
FISH BEHAVIOUR CONSIDERATIONS
The model discussed here attempts to
account for the sensitivity of fish to the
approach of various parts of the otter trawl
and, also, to account for some characteristics
of the induced sv1imming response.
The aim of this section is to examine all
knovm constraints on both swimming and sensory
performance, including observations from field
experience, in order to provide a critical
evaluation of the parameters and functions used
in the model. In pursuing this approach, it
becomes evident that some of the biological
constraints may be more important than others.
If the catching efficiency of an otter
trawl is influenced by these characteristics of
the fish, then it is evident that, during
fishing, there may be selection of those least
adapted to escape. This possibility, in
conjunction v1ith known characteristics of the
trawling operation, demands a discussion of the
suitability of the otter trawl as a sampling
tool.
The swimming performance criteria which
limit the chances of capture by an otter trawl
seem to be the maximum speed, the maneuver­
ability and the endurance of the fish.
Additional factors are the acceleration, the
time fish take to recover from exhaustion and
the nature of the avoidance reaction.
Analysis of the avoidance reaction
indicates that the first stage is one of
acceleration from rest or from the cruising or
browsing state.
Weihs (1973) and Webb (1976) have studied
the fast start response of fish. One remarkable
fact that has been noted is that the acceleration
rate of all species examined is relatively
uniform - from 6 to 16 m/sec/sec (Webb, 1978).
Webb (1976) showed that this acceleration rate
was independent of fish size. It must be
appreciated, however, that this acceleration
phase is generally of the order of one-tenth of
a second only; this suggests that all sizes of
fish can begin an avoidance reaction in much the
same way, irrespective of species or size. The
significance of this stage to a flatfish
partially covered with sand has not been studied
as far'as we know. Since the acceleration is
claimed to be largely due to the first full
stroke of the tail, it would seem that a
swimming motion with a "push-off" from the solid
substrate is hardly likely to be slower.
Maneuverability appears to be a critical
factor; if a fish makes a response to an
approaching gear, the probability of escape will
be lowered if the orientation it can achieve
relative to the gear is limited in any way.
Weihs (1972) drew attention to the fact that, in
a fast start in which the fish body took an
L-shape, the resultant acceleration was at a
slight angle; in its initial acceleration, it
was noted that the fish would achieve a slight
turn to one side or the other. This would
suggest that the need to make an avoidance turn
need not be compromised since the initial
acceleration could easily produce that required
turn. Indeed, Webb (1976) found that trout were
able to make a turn whilst accelerating; it was
observed that this turn had a radius equivalent
to less than 20 per cent of body length. In
instances where a turning response would be
undesirable, it seems that there is a fast start
response which Webb (1976) called S-shaped. The
resultant acceleration in these cases involves
no commitment to turn. Webb (1976) also noted
that for larger trout under observation, the
frequency of S-starts was greater; however, it
is not clear hov1 good the data are. Thus, it is
not clear whether or not this means that larger
fish are limited in their ability to make an
acceleration turn.