Statistical catch-at-age analysis vs. ADAPT-VPA: the case of Gulf of Maine cod

• Published in: ICES Journal of Marine Science
• Main Authors: Butterworth, Doug S., Rademeyer, Rebecca A.
• Format: Text
• Language: English
• Published: Oxford University Press 2008
• Subjects: Articles; Butterworth; Gadus morhua
• Online Access: http://icesjms.oxfordjournals.org/cgi/content/short/65/9/1717; https://doi.org/10.1093/icesjms/fsn178

<qd> Butterworth, D. S., and Rademeyer, R. A. 2008. Statistical catch-at-age analysis vs. ADAPT-VPA: the case of Gulf of Maine cod. – ICES Journal of Marine Science, 65: 1717–1732. </qd>In 2003, given an estimate of a spawning-stock biomass ( B sp) of 27% of the maximum sustainable yield (MSY) level <f> </f> based on an adaptive framework-virtual population analysis (ADAPT-VPA) assessment using data only after 1981, the Gulf of Maine cod ( Gadus morhua ) stock was deemed “overfished” under the US Magnuson–Stevens Act. However, an alternative statistical catch-at-age assessment (SCAA) at the time, using survey data from 1964, indicated B sp above <f> </f>. This is investigated, together with other (sometimes conflicting) suggestions made during a number of recent assessment reviews of this stock. The primary reason for the different result is that the ADAPT-VPA assessment imposed asymptotically flat selectivity-at-age when there was strong statistical evidence for dome-shaped selectivity. Once adjusted for this, either assessment method robustly estimates B sp relatively close to <f> </f> rather than below the “overfished” threshold of 0.5 <f> </f>. SCAA allows the longer series of survey data available to be incorporated, providing a better basis to estimate MSY-related targets and doubling the related precision in some cases. As such targets are important when implementing the Magnuson–Stevens Act, SCAA seems preferable to ADAPT-VPA for assessing this stock. Some broader inferences to be drawn from this comparative process include the need for: (i) careful treatment of the plus-group, especially if selectivity may be dome-shaped; (ii) flexible parameterizations of selectivity-at-age in SCAA to avoid false perceptions of the precision of results; and (iii) care in the use of the Beverton–Holt stock–recruitment function, as it gives inappropriately low estimates of <f> </f> if there is an overall negative trend in the estimates of recruitment ...