An Investigation of Differences Amongst SCAA and ASAP Assessment (including Reference Point) Estimates for Gulf of Maine Cod
Various differences between SCAA and ASAP assessments of the Gulf of Maine cod, and their implications, are investigated further. Amongst the results are that neither all nor very few elements of the starting numbers-at-age vector for the assessment should be estimated, but rather an intermediate number which depends on the data available for years close to the starting year selected. This in turn leads to a demonstration that assessments commencing in 1970 incorporate reliable information about recruitment levels for the preceding 5 years, which therefore ought also to be taken into account in fitting stock-recruitment relationships and estimating MSY related reference points. A surprising result obtained in investigating such an ASAP surrogate is that fitting to abundance indices in terms of numbers rather than biomass radically changes perceptions for this particular scenario. The assessment then interprets the very high NEFSC spring survey estimates in 2007 and 2008, as well as high results for the Massachusetts survey in some recent years, as enhanced recruitment, with an associated strong increase in spawning stock abundance over the last few years. This points to the need for an improved understanding of the reasons underlying the occasional high indices of abundance forthcoming from the NEFSC surveys, so as to inform how these should best be treated in the assessment model fitting process.