Adaptation of the Simple Model to Give Other Characteristics of the Catch and Population

Abstract

In a study of an exploited fish population there are several quantities, besides the annual yield in weight, that will require assessment. From the economic aspect it is necessary to know the catch per unit effort, and, probably, the mean weight of fish in the catch, while the analysis of situations in which factors such as natural mortality and growth vary with the population density (discussed in Part II of this paper) requires expressions giving the numbers and biomass of the population. Finally, when the behaviour of population models comes to be tested by observation or experiment, or they are used as an adjunct to fishery management, it is of help to obtain from the models predicted values of certain quantities such as the mean length and mean age of fish in the catch, which are directly estimated from samples. To establish the equations giving the above characteristics we use the same definitions and assumptions as in the previous section, and it will be seen that the same general methods are applicable.