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

contribute significantly to the estimated TAG.
For cod in 4T -4Vn, short term projections are
not influenced by this type of error (see Table
9.C.1). In fact, a 100% increase in partial
recruitment at age 3 and a 40% increase at age 4
for the years of projection will change the
estimated 1981 catch biomass by less than 2% and
the 1981 stock size estimate by less than 0.1%.
This type of non-random error is thus
negligeable in comparison to the relative error
due to the variance of catch biomass estimates.
We also analyzed the effect of some other
sources of systematic errors on projection
estimates (see Table 9.C). For instance, a 50%
discard at sea for age 3, together with a 25%
discard at sea for age 4 (unreported) does not
change significatively the 1981 stock size
estimate and the 1981 catch biomass (TAG)
estimate. Similarly, a 80% discard at sea for
age 3 (unreported) from 1970 through 1979, does
not have a significant effect on projection
estimates. In a fourth case, we observe that a
20% under-reporting (or mis-reporting) for all
age groups in 1979 will change the 1981 stock
size estimate by less than 0.7% and the 1981
catch biomass estimate by 2.5%. In conclusion,
systematic errors of that nature become a minor
source of concern when using initial stock size
estimates derived from Doubleday's method.
Ironically, systematic errors appear as the
major source of concern (and source of
discussions) when standard analytical methods
are applied.
Variance estimates are quite important -
may be as important as parameter estimates
themselves - since they are indicative of the
corrective measures to be undertaken for
improving the data base. Analytical assessments
do not provide information on such corrective
measures. As we know, it is relatively easy to
manipulate analytical solutions, within the
range of biologically-acceptable parameter
values, so as to reflect or to mimic any desired
trend. In absence of an objective measure of
the 'quality' of the estimates obtained with
analytical methods, analytical assessments may
become a vehicle for subjective evaluations.
From the management viewpoint, the assessment of
the limitations of the data base and the
identification of corrective actions are as
important as the provision of TAG estimates per
se.
The preceding analysis is limited to the
study of one case, i.e. cod in 4T-4Vn; our
conclusion that uncertainties of mean
weight-at-age are a major source of
uncertainties for TAG estimates is valid for
this stock only. For other groundfish stocks,
the uncertainties associated with research
survey estimates may become the major source of
u ncerta inti es for catch projections. In fact,
Table 4 indicates that, for a given precision of
mean weight-at-age estimates, the uncertainties
associated with initial stock size estimate.s are
a key factor for controlling the uncertainties
of catch biomass estimates in the years of
projection. When an assessment is amenable to
109
Doubleday's method, an important benefit is that
catch projections become practically independent
of systematic errors of input parameters
(misreporting, discards at sea, etc.).
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Doubleday, W G. 1979 A Sensitivity Analysis of
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Fish Committee, C.M 1979/G 53. 7 p. +
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Doubleday, W.G. 1981. A method of estimating
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26 p.
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