Applications of Statistical Catch-at-Age Assessment Methodology to Gulf of Maine cod, October 2012
The Statistical Catch-at-Age assessment conducted by the authors earlier in 2012 is updated to take account of more recent data, and refined by introducing two new features: fitting to length distribution data for the NEFSC surveys in the 1960s for which age information is not available, and adjusting the externally provided estimates
of the Bigelow-Albatross calibration function through adding the calibration information contained in cohorts present both before and after the survey vessel change to the model fitting process. The options selected for the Base Case assessment are those motivated in the assessment conducted earlier in the year. The resultant estimate of the 2011 spawning biomass is 12.0 thousand tons with a CV of 13%. The survey calibration function is slightly modified, resulting in an increase of about 3% in the 2011 spawning biomass. The survey catch-at-length data are consistent with previous estimates of poor recruitments from relatively large spawning biomasses in the 1960s. This last result is robust under a range of sensitivity tests, and is suggestive of a Ricker-like stock-recruitment relationship for the stock. These sensitivity tests also suggest that the 2011 spawning biomass estimate of 12.0 thousand tons is robustly determined. The range of this estimate across these sensitivities is 9.9 to 16.6 thousand tons, with lower values arising from the sqrt(p) weighting approach for proportions data and from forcing selectivities above age 6 to be flat, and the higher values coming from inclusion of the stock-recruitment function in the assessment and increasing the value of M. The evidence for commercial selectivities to be domed relative to the NEFSC surveys appears reasonably strong, but less so that for the selectivities for these surveys themselves to be domed.