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

Yst = 1
A
where: Ah
V(Ystl = 1 .
1(2
area of the hth
A the total area
stratum
yh sample mean catch per tow
stratum
Nh
in the hth
number of tows in the h th stratum
2
Sh = sample variance in the hth stratum.
The minimum trawlable biomass B is calculated by
summing the biomass values obtained in each
individual stratum by the swept-area method:
B L Bh = L lh.:..!\11h
h b
where: Bh minimum trawlable biomass in the hth
stratum
b = mean area swept per tow
The variance VB of this estimate is given
by:
2 2
Afl. sh
VB = L
h b2.Nh
The precision of the abundance indices (Ystl can
be tested by deriving the coefficients of
variation C.V. of the weight and number means
(ratio of standard deviation to the mean).
For each species, a particular group of
strata corresponding to its distribution area
has been selected and is used for the
calculation of abundance indices.
SOME RESULTS: A DISCUSSION
The analysis of our results, in spite of
the lack of long series, indicates that the
stratum variance increases faster than the
mean. It appears also that the standard
deviation of the catch per tow is proportional
to the mean for the main commercial species
(Fig. ?a, ?b, ?c). As mentioned by Taylor
(1g53), this is due to the fact that fish are
not randomly distributed on the grounds but tend
to congregate in schools.
However, the fitting is not good for all
species and, for instance, in the case of
redfish (Fig. ?c), the scattering of the plots
indicates important fluctuations in the C.V. of
the mean weights calculated for each stratum.
The results also indicate that, on an
arithmetic scale, the C.V. of the stratified
mean for all main commercial species range from
0.11 to o.g6, with an average value around
0.30. The maximum value (0.96) is observed for
cod during the autumn of 1978 (cruise 782) and
is due to one exceptionally large catch.
The values of C.V. of the stratified mean
vary from one species to another. For the most
common commercial species (skates, cod, flatfish
... )the values are distributed around 0.20 but
for others (haddock, hake ... )they are higher
(0.59 and 0.49 respectively).
On a logarithmic scale, the C.V. of the
stratified mean range from 0.07 to 0.10 for the
main commercial species.
For a number of reasons, these calculations
provide only an approximation of the population
size. While some factors can be controlled and
results be corrected (for instance, selectivity
of gear), most factors remain uncontrolled, or
difficult to control. For instance, the
swept-area is assumed to be constant and equal
to 0.015 square nautical mile (trawled distance:
2 nautical miles, distance from wing to wing:
13.50 m). But the trawling speed has an effect
on the horizontal opening of the trawl and it is
known that the two parameters vary in the same
way, introducing large variations in the
swept-area. In the same way, physical factors
(depth-wire length relationship, type of bottom,
current strength and direction in relation to
towing direction ... ) may influence the behaviour
of the gear. Furthermore, due to the herding
effect of the sweepline and of the trawl boards,
the actual swept area is probably wider than the
distance from wing to wing.
The variability in the catchability
coefficient (q) is another important cause of
variation in the results. For our biomass
calculations the value of q is assumed to be
equal to 1.0, and these computations provide the
lowest limit of the trawlable biomass.
For the same trawl, the value of q varies
from one species to another due to differences
in distribution patterns, behaviour towards the
trawl and escapement. For instance,
echo-sounding records show that large segments
of some populations (redfish, cod, ... )may be
unavailable to the trawl because of their
distribution above the headline. The variations
in trawling speed which also induce variations
in the vertical opening, may also strongly
influence the available proportion of the
populations.
In some cases, seasonal variations in the
value of q are observed. So, during the autumn
season, the abundance of prey (mostly sand
launce) and the presence of a strong thermal
gradient induce cod concentrations near the
bottom and thus a larger availability to the
trawl.
All these variations in the value of q may
serve to explain'the greater tow-to-tow
variability observed for some species having a
77