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

Abstract

Abstract 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. In 2003, given an estimate of a spawning-stock biomass (Bsp) of 27% of the maximum sustainable yield (MSY) level 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 Bsp above . 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 Bsp relatively close to rather than below the “overfished” threshold of 0.5 . 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 if there is an overall negative trend in the estimates of recruitment plotted against Bsp.