© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Estimation of the age-specific rate of natural mortality for Shetland sandeels
FRS Marine Laboratory PO Box 101, 375 Victoria Road, Aberdeen AB11 8DB, UK
*tel: +44 1224 295393; fax: +44 1224 295511. e-mail: cookrm{at}marlab.ac.uk.
It is generally difficult to obtain reliable direct estimates of natural mortality, M, from conventional fisheries data and stock assessments. However, as a result of the closure of the Shetland sandeel (Ammodytes marinus) fishery from 1991 to 1994 and in the absence of any significant fishery in other years, research vessel survey data offer a rare opportunity to obtain estimates of M directly. A model is described that assumes that M can be decomposed into an age effect and year effects. Application of the model to the survey data produces values of M that decline from 2.1 for 0-group fish to 0.6 at age 2. There is some indication of an increase for ages 4 and older. Although there does not appear to be an overall trend in the mean value of M for the period 1985–1999, the annual values change by up to 50%. The values calculated from the model are in line with estimates obtained for the North Sea from multispecies virtual population analysis (MSVPA).
Keywords: natural mortality, research vessel survey, sandeel, separable model
Received 29 July 2003; accepted 24 November 2003.
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