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The shrimp (Pandalus borealis) resource off West Greenland was assessed by fitting, to four fleets operating over different periods, separate CPUE models, which were then combined. A logistic model of population dynamics was then fitted to this data and a standard trawl survey series. Uncertainty was estimated by a jack-knife procedure— leaving out one year's data at a time—in several forms: omitting data from all steps in the process or only from the last step of fitting the production model; or omitting data either for one year at a time at each point in the series or for a sequence of years at the start of the series. Initial results from jack-knifing the entire procedure gave a small scatter (CV 8%) in optimal fishing mortality, but the sustainable catch had a 13% CV and ranged from 9% lower to 52% higher than the value based on all the data. Omitting the first year's data created an outlier with a 47% lower sustainable fishing mortality. When jack-knifing only the input to the logistic population model, the optimal fishing mortality retained a CV of near 8%, but the CV of sustainable catch fell to about 4% (ranging from 8.5% lower to only 9.2% higher). Much of the variation in estimated sustainable catch therefore appeared due to uncertainty in the standardized CPUE series. However, omitting the first year's data continued to give an outlying point, so the effect of leaving out the first year's data was investigated further. When the experiment was repeated with the first two data points permanently omitted, the CV of MSY remained near 4%, but the variation in sustainable fishing mortality and in standing stock became large. Also, there was little difference whether the jack-knife procedure included the GLM fitting of the CPUE series or whether points were simply omitted from a single series based on all the years' data. I.e. particular properties of the data points appeared to have significant effects. The jack-knife procedure appears useful for examining the reliability of this standard method of fishery assessment, and these results indicate that the uncertainty of such results may be unpredictable, large, variable between model parameters, and difficult to assign to any particular step in the procedure. 1 Author to whom correspondence should be addressed, at: Norwegian College of Fishery Science, Dept. of Marine and Freshwater Biology, University of Tromsø, Breivika, 9037 Tromsø, Norway. Tlf +47 77 64 60 22; Fax: +47 77 64 60 20; carstenh@nfh.uit.no.

The Uncertainty of an Assessment Procedure for the West Greenland Stock of Northern Shrimp, Pandalus borealis (Krøyer)