The NMFS-AFSC Technical Memorandum series is now being used by the Alaska Fisheries Science Center. Technical description of the stock synthesis assessment program. Commer., NOAA Tech. It includes a population simulation model to calculate the abundance and mortality of a harvested population, an observation model to relate this population model to observable data from the population, and a statistical model to adjust parameters of the population model and observation model to achieve the best fit to all the data. Both are capable of simultaneously examining data from several fisheries and several surveys, each with its own pattern of selectivity.
Synthesis calculates selectivity with modified logistic functions.
Parameters of these functions can take on time-specific values, thus allowing flexibility to track changes in fishery selectivity. The goodness of fit is quantified in terms of a log-likelihood function composed of independent terms for each kind of observation from each type of fishery or survey. Synthesis estimates the best parameter values through numerical calculation of parameter derivatives and application of a modified Newton method. This provides estimates of parameter variances and covariances, and provides for calculation of the variance of spawning biomass through application of the delta method. Both versions of synthesis allow inclusion and estimation of spawner-recruitment functions.