xi CONTROL OF FISHING 295 



In practice it is much more economical to reduce the fishing mor- 

 tality by lowering the number of hours fished than by increasing the 

 mesh size. We require, therefore, to know the optimum mesh size for 

 each level of fishing effort. This we can obtain by plotting, as in Fig. 

 1 72 a, a series of yield/mesh curves for different sizes of mesh, that 

 is, ages of recruitment t p . From the maximum of these, plotted against 

 the corresponding fishing mortality, we obtain Fig. 172B, known as 

 the eumetric yield curve. This shows an increasing yield towards an 

 asymptote as fishing increases, but without a maximum. 



Evidently the greatest yield is obtained at the highest fishing inten- 

 sity, but this could be achieved only at a prohibitive cost. In order to 

 obtain the greatest possible yield, we have to consider the cost of unit 

 effort of fishing and the value of the yield. This has been done, making 

 certain assumptions, in Fig. 172c. Here the eumetric yield curve is 

 expressed by plotting value of catch against running costs. Another 

 line shows the profit (total value minus total cost) whose maximum 

 might be said in one sense to be the 'best' level of fishing. This curve 

 also crosses the x-axis, where there is no profit in fishing — -this being 

 the condition which an uncontrolled fishery tends to approach. 



Of course the actual 'best' level of fishing for any given situation 

 may be affected by many social and political factors. However, from 

 studies such as these it begins to be possible to understand the vari- 

 ables that are affecting a fishery and to express them precisely. The 

 value of such work is shown by the fact that international regulation 

 of some fisheries has been agreed. For example, since 1954 the white- 

 fish populations of the North Sea have been regulated by control of 

 the size of mesh used for fishing. 



