The Stock Synthesis Approach

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The Stock Synthesis Approach
Based on many of the ideas proposed in Fournier
and Archibald (1982), Methot developed a stock
assessment approach and computer program called
Stock Synthesis. It has the following features

• Multiple fisheries and surveys, each with its own
  selectivity curve.
• Multinomial errors assumed for the observed age
  composition data (fisheries and surveys).
• The analysis is tuned using multiple biomass or
  abundance indices (surveys, fishery effort or
  CPUE), assumed to have log-normal errors.

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Stock Synthesis (continued)

• Catch biomass values are assumed to be known
  exactly and are removed mid-period. (There is no
  explicit fishing mortality coefficient.)
• Selection can be a function of length or age (or
  both) and can differ by sex.
• Mean weight-at-age (by sex) is derived from a
  growth model and a length-weight relationship.
• Most parameters can be configured to vary with
  time (e.g., changing selection coefficients).
• Unlike VPA or Cohort Analysis, Synthesis does not
  require complete catch-at-age data matrices.

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Stock Synthesis (continued)

• Seasons for seasonal fisheries or seasonal
  growth.
• Transition matrices can be used to create
  predicted distributions (e.g., age compositions
  with error).
• Synthesis can accommodate numerous kinds of data
• Observations of discarded amounts or percentages.
• Age or length composition data for retained,
  discarded, or total catch.
• Mean length-at-age data by fishery and survey.
• Age composition within specified length ranges.
• Mean body weight by fishery (retained or
  discarded).

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Stock Synthesis (continued)

• The maximum likelihood method is used for
  estimating the model parameters.
• log( Like. ) lj S log( Like.Component j )
• Parameters can be constrained by including
  penalty functions as log-likelihood components
  and mimic a Bayesian estimation approach.
• Synthesis II uses Auto-Diff Model Builder (ADMB)
  routines to find the parameter estimates.
• ADMB allows Synthesis II to produce variance
  estimates for all estimated parameters and for
  quantities derived from the estimated parameters.

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Retention small fish are often discarded
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