1. 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. FW599Winter 2008

2. 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. FW599Winter 2008

3. 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). FW599Winter 2008

4. Stock Synthesis (continued) The maximum likelihood method is used for estimating the model parameters. log( Like. ) = j *  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. FW599Winter 2008

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6. FW599Winter 2008 Retention – small fish are often discarded

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