On the implication of tagging analyses for the shape of selectivity at age for Gulf of Maine Cod

Working Paper


Permanent link to this Item
Journal Title
Link to Journal
Journal ISSN
Volume Title

University of Cape Town

This paper provides some comments in response to Hart and Miller’s (2008) interpretation of the implications of their analyses of tagging data regarding the dome shape estimated for selectivity for Gulf of Maine cod in the ASPM Reference Case assessment of Butterworth and Rademeyer (2008a). It is first informative to consider the sensitivity of the 2007 Reference Case ASPM assessment of Butterworth and Rademeyer (2008a) to alternative values of age-independent natural mortality M. The results are shown in Table 1 and Fig. 1 below. From Fig. 1 it is clear that the inference or otherwise of a dome in selectivity depends strongly on the value assumed for M. If M=0.2 yr-1 as assumed for the Butterworth and Rademeyer (2008a) Reference Case, then selectivity for the NEFSC surveys is appreciably dome shaped with a rapid decline with age above age 7. However for M ~ 0.4 – 0.5, the selectivity for these ages becomes effectively flat. Interestingly, the tagging analyses of Hart and Miller (2008), from which they infer such flat selectivity, estimate (approximately) M=0.5 for length60 cm.