Fishing power, fishing effort, density, fishing intensity and fishing mortality

This paper derives relationships between fishing power, fishing effort, density, fishing intensity and fishing mortality. Fishing power is defined as the product of the area of influence of the gear during a unit operation and the efficiency of the gear during that operation. As a consequence of its definition it is shown that fishing power is the effective area covered by the gear during a unit operation. Fishing effort is defined as the product of fishing power and the number of unit operations and is therefore the total effective area covered by the gear during a number of unit operations. Consequently catch per unit effort is a direct measure of the density of the fish. Fishing intensity is defined as the fishing effort per unit area per unit time and hence is equal to the effective area covered by the gear per unit time as a fraction of the area occupied by the stock. It also shows that fishing intensity is the fraction of the fish which are caught per unit time provided certain requirements underlying the theory are well met. Fishing mortality is defined as the fraction of the fish caught per unit time from which it follows that when the requirements referred to above are satisfied fishing mortality is also equal to fishing intensity. Fishing power is difficult to measure and therefore the concept of relative fishing power as a measurable quantity is introduced. This leads to a discussion of relative fishing effort, relative density and relative fishing intensity. Relative fishing intensity is proportional to fishing mortality and as such can be used to record changes in fishing mortality. Furthermore, if the constant of proportionality, the catchability coefficient, is measured, the relative fishing intensity can be used for direct estimation of fishing mortality.