21. FISHERY DYNAMICS— THEIR ANALYSIS AND INTERPRETATION 



M. B. Schaefer and R. J. H. Beverton 



1. Introduction 



Most of the research on the dynamics of exploited fish populations is con- 

 cerned, directly or indirectly, with investigating and measuring the effects of 

 fishing on the population, with the objective of establishing, eventually, the 

 relation between the fishing activity on the one hand and the resulting sustain- 

 able catch on the other. Thus, the fishermen and the fish are treated as a 

 predator-prey system, with the centre of interest to fishery researchers lying 

 in the benefits to the predator. 



The basic dynamics of a fishery, that is of the predator-prey system, have 

 been simply outlined by Russell (1931) and may be illustrated, following 

 Ricker (1958), using the diagram in Fig. 1. The total weight, or biomass, offish 



Fig. 1. Diagram of the dynamics of a fish stock (fish of usable sizes), when there is no 

 fishing (above) and when there is a fishery (beloiv). (After Ricker, 1958.) 



in the exploitable phase consists of all fish above the average minimum size 

 at which they are capable of being retained in the fishery, i.e. the size above 

 which they are selected by the fishing gear, or by the selective efforts of the 

 fishermen's operations such as choice of fishing grounds or schools of fish of 

 certain sizes. This fished population, of biomass P, loses members by natural 

 deaths as well as by fishing ; it is replenished by recruitment of young fish as 

 they grow large enough to be caught and by the growth of the fish already in it. 

 In the virgin situation, with no fishery, all increments to the stock by recruit- 

 ment and growth are balanced by losses due to natural mortality. With the 

 imposition of a fishery there is an additional loss from the stock to the predator, 

 the fisherman. This added mortality results in a decrease in the abundance of 

 the fished part of the stock and a shift in its age composition towards the 

 younger fish. Eventually, when the amount of fishing has increased as much as 

 is permitted by the economic conditions of the fishery, a new equilibrium is 

 established in which the catch is balanced by the compensatory changes which 

 have occurred in one or more of the vital rates of natural mortality, growth 

 and recruitment. 



[MS received September, 1960] 



464 



