Bottom Trawl Surveys - Proceedings of a Workshop Held at Ottawa ...
Bottom Trawl Surveys - Proceedings of a Workshop Held at Ottawa ...
Bottom Trawl Surveys - Proceedings of a Workshop Held at Ottawa ...
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170.<br />
CONCLUSIONS<br />
The survivor method efficiently<br />
estim<strong>at</strong>es stock abundance for c<strong>at</strong>ch projections<br />
when assumptions hold. Departures from<br />
assumptions can be tested st<strong>at</strong>istically provided<br />
residual vari<strong>at</strong>ion is sufficiently small.<br />
Subjective consider<strong>at</strong>ions in "tuning" cohort<br />
analysis are reduced to choice <strong>of</strong> a calibr<strong>at</strong>ion<br />
block <strong>of</strong> ages and years and an age <strong>of</strong> full<br />
recruitment to the survey.<br />
Since survivor estim<strong>at</strong>es have<br />
estim<strong>at</strong>ed variances, estim<strong>at</strong>es obtained with<br />
separ<strong>at</strong>e analyses using several research vessel<br />
or even commercial fishery abundance estim<strong>at</strong>es<br />
can be efficiently combined.<br />
Equ<strong>at</strong>ion (8) represents a<br />
generaliz<strong>at</strong>ion <strong>of</strong> sequential abundances<br />
estim<strong>at</strong>ion methods currently in use. Advantages<br />
over Gulland's and Pope's methods include<br />
explicit st<strong>at</strong>ement <strong>of</strong> dependence on "terminal<br />
'F"' (i.e. survivors) and ability to accommod<strong>at</strong>e<br />
an arbitrary distribution <strong>of</strong> c<strong>at</strong>ches within<br />
years and d<strong>at</strong>e-dependent n<strong>at</strong>ural mortality<br />
r<strong>at</strong>es.<br />
Applic<strong>at</strong>ion <strong>of</strong> the survivor method to<br />
examples <strong>of</strong> Canadian Atlantic fish stocks show<br />
th<strong>at</strong> the usefulness <strong>of</strong> the results depends<br />
heavily on the precision <strong>of</strong> the survey index.<br />
In the case <strong>of</strong> Southern Gulf <strong>of</strong> St. Lawrence<br />
cod, the estim<strong>at</strong>es <strong>of</strong> survivors were<br />
sufficiently precise for c<strong>at</strong>ch projections, but<br />
this was not true for Haddock in NAFO Div. 4X.<br />
REFERENCES<br />
Beacham, T.D. 1980. 1980 Assessment <strong>of</strong> Cod in<br />
Division 4T and 4Vn (Jan.-Apr.), CAFSAC<br />
Res. Doc. 80/22. 61 p.<br />
Boyce, W.E. and R.C. DiPrima. 1965.<br />
Elementary Differential Equ<strong>at</strong>ions and<br />
Boundary Value Problems. John Wiley & Sons<br />
Inc. , New York.<br />
Gulland, J.A. 1965. Estim<strong>at</strong>ion <strong>of</strong> Mortality<br />
R<strong>at</strong>es. Annex to Rep. Arctic Fish. Working<br />
Group, Int. Counc. Explor. Sea C.M. 1965<br />
(3). 9 p.<br />
O'Boyle, R.N. 1980. ICNAF Division 4X Haddock<br />
January 1980 St<strong>at</strong>us Report. CAFSAC<br />
Research Document 80/2.<br />
Pope, J.G. 1972. An Investig<strong>at</strong>ion <strong>of</strong> the<br />
Accuracy <strong>of</strong> Virtual Popul<strong>at</strong>ion Analysis<br />
Using Cohort Analysis. Int. Comm.<br />
Northwest Atl. Fish. Res. Bull. 9: 65-74.<br />
Rivard, D. 1980. APL Programs for Stock<br />
Assessment. Can. Tech. Rep. Fish. Aqu<strong>at</strong>.<br />
Sci. 953, 103 p.
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