A Method of Estimating Natural and Fishing Mortalities

A simple method of estimating instantaneous natural and fishing mortality rates is given. It uses the maximum of the available information. The method requires data on numbers caught by age-classes, and data on (effective) effort for a series of years. For each year-class is calculated the weighted sum of catches from all age-groups from a given age on, and these sums are used as estimates of year-class strengths. The weighting factors depend on the instantaneous fishing and natural mortality rates; the best values of mortality rates are arrived at by iteration. When statistics are not complete a correction factor can be applied. This method is compared with Fry's "virtual" population size estimates; in both cases the basic statistics are catches. We have, however, introduced correction factors to allow for varying fishing intensities and incomplete statistics at older ages.