ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on November 1, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(9):1641-1649; doi:10.1093/icesjms/fsm155
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Precisely wrong or vaguely right: simulations of noisy discard data and trends in fishing effort being included in the stock assessment of North Sea plaice
Wageningen-IMARES, PO Box 68, 1970 AB IJmuiden, The Netherlands
Correspondence to M. Dickey-Collas: tel: +31 255 564646; fax: +31 255 564644; e-mail: mark.dickeycollas{at}wur.nl.
Dickey-Collas, M., Pastoors, M. A, and van Keeken, O. A. 2007. Precisely wrong or vaguely right: simulations of noisy discard data and trends in fishing effort being included in the stock assessment of North Sea plaice. – ICES Journal of Marine Science, 64: 000–000.ICES stock assessments of North Sea plaice are routinely carried out with eXtended Survivors Analysis (XSA), based on landings and survey data. Recently, the assessments included data on discarded young fish, sampled with high variance. Fishing effort has been declining since the mid-1990s, so conditioning the estimated fishing mortality (F) on the recent past could introduce bias into the perceived stock size. Simulated populations with North Sea plaice-like characteristics are used to explore the dependence of the perceived stock dynamics on the inclusion of discards data at different sampling noise, using the same methods and XSA settings as ICES. The sensitivities of the results were tested against different trends in fishing effort and recruitment, and different scenarios for "shrinkage" (i.e. the way in which the past is used to estimate the most recent fishing mortality). Within the bounds of the simulation assumptions, the perception of population trends from an XSA stock assessment can be biased when there are trends in fishing effort: decreasing effort leads to underestimating SSB and overestimating F. When discards are not included, bias in SSB is greatest when effort decreases, and bias in F is greatest when effort increases. Bias in SSB and F were removed by including discard data, but at substantial loss of precision. If effort shows a clear trend and discards are substantial and estimated noisily, the recent trend in the target population may be hard to track with an XSA-type assessment methodology.
Keywords: bias, discarding, Pleuronectes platessa, simulated population, stock assessment, variance, XSA
Received 21 November 2006; accepted 1 October 2007; advance access publication 1 January 2007.
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