The effects of ageing biases on stock assessment and management advice: a case study on Namibian horse mackerel

 

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dc.contributor.author Wilhelm, M R
dc.contributor.author Durholtz, M D
dc.contributor.author Kirchner, C H
dc.date.accessioned 2018-01-19T09:27:23Z
dc.date.available 2018-01-19T09:27:23Z
dc.date.issued 2008
dc.identifier http://dx.doi.org/10.2989/AJMS.2008.30.2.6.556
dc.identifier.citation Wilhelm, M. R., Durholtz, M. D., & Kirchner, C. H. (2008). The effects of ageing biases on stock assessment and management advice: a case study on Namibian horse mackerel. African Journal of Marine Science, 30(2), 255-261.
dc.identifier.uri http://hdl.handle.net/11427/26843
dc.description.abstract We explore the influence of age-estimation errors on the results of the age-structured production model (ASPM) used for horse mackerel stock assessment in Namibia for the period 1961–2003. The analysis considered age data from eight readers collected during an otolith-reading workshop. Four scenarios of age-estimation errors were assumed: Case 1 — a reference age computed as the modal age of estimates obtained by the four most experienced readers; Case 2 — age readings from a precise and experienced (Namibian) reader of horse mackerel otoliths; Case 3 — age estimates from a reader that displayed positive bias compared with the reference ages; and Case 4 — age estimates from a reader that displayed negative bias compared with the reference ages. The age–length key of each case was applied to length distributions of survey, pelagic fleet and midwater fleet landings (1991–2003) to obtain catch-at-age data. These data were then used in the ASPM. Results obtained from Case 3 differed most significantly from the others and appeared to be unrealistic in terms of the state of the stock and negative log-likelihood estimates. The conclusion is that more resources need to be directed towards age determination, because management recommendations are highly sensitive to errors in ageing. Most effort should be placed into age estimation of age groups 3–5 (20–30 cm total length), but significant effort needs to be devoted to age estimation of midwater commercial samples. Finally, the extent of sampling and the raising strategy of length frequencies should be improved.
dc.source African Journal of Marine Science
dc.source.uri http://www.tandfonline.com/loi/tams20
dc.subject.other age-structured production model
dc.subject.other ageing error
dc.subject.other sampling effort
dc.subject.other Trachurus capensis
dc.subject.other ageing precision
dc.title The effects of ageing biases on stock assessment and management advice: a case study on Namibian horse mackerel
dc.type Journal Article
dc.date.updated 2016-01-22T10:04:55Z
dc.publisher.institution University of Cape Town
dc.publisher.faculty Faculty of Science en_ZA
dc.publisher.department Department of Biological Sciences en_ZA
uct.type.filetype Text
uct.type.filetype Image


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