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

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In the area south of 5o·N, the same series
of biomass estimates was made except that with
the limited number of strata the combination of
small strata into depth levels was not used
(Table 8).
In considering the whole area of the
Esquiman Channel, the same series of estimates
was examined in the northern area except that
with the two larger areas a further estimate was
available by considering the whole area as made
up of two strata (Table 6).
Interpretation of the results is
complicated by the fact that different stratum
areas were used between the old and the new
strata, and this affects both the estimates of
the mean/unit area and the expansion of this
mean to biomass. However, within the various
combinations of the small strata a trend seems
to be clear in the northern and total area,
namely, that with the increased number of strata
the lower mean is obtained and this is
accompanied by an increase in precision as
indicated both by the variance and by the
confidence limits assigned to the biomass
estimates after areal expansion. In the
southern part of the area, the stratified
sampling was poor, with only three out of the
seven strata sampled enough to yield a variance
estimate and two strata not sampled at all.
Thus, it is not surprising that the best
precision was obtained by considering the area
as a single homogeneous one and sampling at
random within it.
With regard to the different estimates and
the possible bias that may exist in each, we are
in a quandary, as in each method of computing
the mean we have used different assumptions.
Intuitively, in this instance where we are
using a fairly large number of small strata
which we would expect to be reasonably
homogeneous, we might expect that the "best"
estimate might be obtained from using the
greatest number of strata; however, as we might
also expect from the lack of differences between
stratum means within the same depth zone, the
difference in estimates between this one and the
estimate derived from the strata combined by
depth is trivial. In this case, we think we
could conclude that the "best estimate" of
biomass for the total area might be 12,000 ±
2000 metric tons in 1975.
We can also conclude that combining strata
within depth zones is acceptable though clearly
when the larger strata of the new system of
stratification are used there is some widening
of the 95% confidence interval.
A comparison between the estimates of
mean/unit area obtained in different years is
also instructive (Table g), This table displays
the results obtained in each stratum when the
new stratification method was uniformly applied
through the series; thus, variation due to the
use of different areal weighting is removed, and
the estimates therefore represent sampling
variability and changes in biomass between
years. An examination of the means suggests
that we should not put too much emphasis on
sampling the deepest stratum (115) because of
the low shrimp abundance and the relatively
small area involved, while a perusal of the
coefficients of variation suggests that sampling
in Strata 113 and 114 is likely not too bad and
that any improvements are most likely
forthcoming from improving the sampling in
Stratum 112. From a knowledge of the
distribution of shrimp, it is likely that an
improvement would be obtained by refining the
depth stratification used and allocating sets in
proportion to observed variability in each
stratum.
PROBLEMS ASSOCIATED WITH DIURNAL VARIABILITY
In the biomass surveys in the Northeast
Gulf of St. Lawrence, sampling has been
restricted to the daylight hours; however, in
the offshore areas off Labrador and in the Davis
Strait area this has not been the case.
The vertical migration of shrimp has been
an accepted fact for many years. At various
times during the 24-hour cycle, especially at
night, shrimp move off the bottom and disperse
to considerable distances in the water column.
Sampling the stocks with "bottom" trawls other·
than at times of optimum density should lead to
an underestimation of stock size since those
animals occurring outside the range of the
trawl's vertical opening will not be accounted
for. Daylight sampling alone tends to be
inefficient, mainly because the migration often
extends well into the daylight hours and,
therefore, the problem is not resolved.
Intuitively, any adjustment for diel
variability should have at least two reasonably
profound effects: (a) the biomass should
increase; and (b) the variance associated with
the mean biomass should be reduced.
Attempts to achieve these effects have been
made but the data base is often rudimentary.
Catch data from a survey in the Davis Strait in
1979 (Parsons 1979) were adjusted to account for
diel variability: (a) observed from 24-hour
sampling during the survey (Figure 10); and (b)
according to patterns observed for the month of
September from more extensive commercial data
(Carlsson et al. 1978). In both cases, the
desired results were achieved (Table 10) with
higher biomass and finer confidence intervals
associated with the pattern observed at the time
of the survey. The exercise, however, did not
improve the data enough to give values of the
lower limits (95%) greater than zero.