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A report of the 21st Northeast Regional Stock Assessment Workshop. Predicting spawning stock biomass for Georges Bank and Gulf of Maine Atlantic cod stocks with research vessel survey data
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  • Alternative Title:
    Predicting spawning stock biomass for Georges Bank and Gulf of Maine Atlantic cod stocks with research vessel survey data
  • Description:
    Indices of mature biomass, derived from spring and autumn NEFSC research vessel bottom trawl surveys, were used to derive long-term trends in spawning stock biomass (SSB) for stocks of Gulf of Maine and Georges Bank cod for years before VPA-based estimates were available. The relationship between survey mature biomass indices and VPA-derived estimates of SSB were determined by linear least squares regression with the survey index as the dependent variable. Both variables were transformed to logarithms and the survey indices were smoothed by an integrated moving average procedure before performing the regressions. To obtain estimates of SSB in years prior to VPA estimates, the linear equations were rearranged to predict the independent variable (SSB) from the smoothed survey indices. Regressions were jackknifed to estimate extrinsic prediction limits. For Georges Bank cod, predicted survey values from the time series smoothing exhibited stronger relationships with SSB than unfitted indices. The log transformation linearized the relationships, homogenized residual variance, and improved overall fit, suggesting a lognormal error structure. Both surveys predicted SSB with an extrinsic error of 18-21%. Overall, SSB for Georges Bank cod was estimated to have declined from 100,000-140,000 mt during the 1970s to 40,000-50,000 mt in 1993 and 1994. Results for Gulf of Maine cod were less conclusive as the relationship between SSB and the survey indices of mature biomass was not well defined.

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