RETROSPECTIVE ANALYSIS OF VIRTUAL POPULATION ESTIMATES FOR ATLANTIC MENHADEN STOCK ASSESSMENT

Historical and retrospective comparisons of Atlantic menhaden virtual population analyses (VPA) from 1955 to 1995 revealed substantial incon sistency in estimates of management variables in the last year of stoc k assessments. Estimates of management variables from several historic al stock assessments were generally consistent throughout most of the time series. In the last two years, however, historical estimates have deviated from revised estimates. Relative performance of alternative ad hoc methods for estimating fully recruited fishing mortality (F) in terminal years showed that all methods were imprecise, but convention al catch-curve estimates were unbiased and had the least retrospective inconsistency. Retrospective differences in terminal estimates of age -1 F by separable VPA ranged widely for eight alternative settings but were clearly minimized by using seven years of catch data. The genera l magnitude of retrospective difference was +/- 1.2 billion recruits ( 46% relative difference), +/-9,000 metric tons of spawning stock bioma ss (33% relative difference), and +/-4.7 percent maximum spawning pote ntial (106% relative difference). Retrospective differences in recruit ment, spawning stock biomass, and spawning potential were positively s kewed but not biased, indicating that the frequency of positive and ne gative inconsistencies are equal but that the positive differences are much greater in magnitude. The skewed distribution of retrospective i nconsistency should be considered for managing the Atlantic menhaden f ishery.