Separate Estimation of Fishing and Natural Mortality Coefficients

Abstract

The methods to be discussed in this section concern fundamentally the separation of the total mortality into its components, the fishing and natural mortality coefficients. As we have mentioned in the previous section, the detailed analysis of mortality requires certain information in addition to that obtained from age-composition data, and we have found it convenient to classify the methods of the present section according to the type of this additional data. In §14.1 is discussed the analysis of marking experiments to obtain, primarily, estimates of the absolute value of the fishing mortality coefficient, of its changes with age, and also of the constant c which relates it to the fishing intensity. Estimation of certain other parameters, such as transport coefficients, from marking data, is also mentioned. Another type of data we shall consider, in §14.2, is that of gear selection experiments, to provide estimates of the relative change of F with age. It will be appreciated that the natural mortality coefficient cannot be estimated directly, that is, we cannot count the numbers of fish that have died naturally, and M must be determined as the residue of the total mortality coefficient after F has been estimated independently and subtracted. In §14.3 we therefore return to the analysis of age-composition data and give various methods for the estimation of M from such data.