Estimation of Mortality Rates from Mean Length and Fishing Effort: a Modification of the Gedamke-Hoenig Length-Based Estimator

The Gedamke-Hoenig length-based estimator of total mortality rate was developed from the Beverton-Holt estimator to allow for non-equilibrium conditions. From a time series of mean length observations, the estimator yields period-specific estimates of total mortality rates and the corresponding year(s) of change in total mortality rates. We developed a new formulation of the Gedamke-Hoenig estimator that utilizes additional information from a time-series of fishing effort to estimate the catchability coefficient q and the natural mortality rate M and thus year-specific total mortality rates. It is assumed that catchability coefficient and natural mortality rate are constant across all fishable ages and years in the available time-series. Although estimates of catchability and natural mortality rate are highly correlated, we expect more accurate estimates of total mortality rate than of q and M.  The assumptions of constant q and M may be rather tenuous, especially in stocks that have been exploited for some time, but the primary utility of length-based estimators is in the realm of data-poor fisheries where our ability to estimate time-varying q and M is highly limited.