GULLAND and BOEREMA: SCIENTIFIC ADVICE ON CATCH LEVELS 



as a function of fishing effort will have a clear 

 maximum, for which the corresponding fishing 

 mortality can be reasonably easily determined. 

 This fishing mortality might then be considered 

 as being one possible value of the optimum fish- 

 ing mortality. However, in practice the yield 

 curve will be quite flat in the neighborhood of 

 the maximum. This may make determination of 

 the position of the maximum difficult and will 

 certainly mean that obtaining the absolute 

 maximum (e.g., moving from the effort giving 

 99% of the catch to that giving 100% ) will require 

 a disproportionate increase in effort and hence 

 in costs (Gulland, 1968a). Given particular 

 values of the price of fish and costs of exerting 

 unit fishing mortality, a mortality which will 

 maximize the net economic yield can be cal- 

 culated, and might be considered the optimum 

 for the given economic situation. Different 

 economic conditions will result in somewhat 

 different optima, but for most conditions the 

 optimum will lie within a fairly narrow range. 

 It is, therefore, possible to define a target of the 

 desirable fishing mortality in the middle of this 

 range which will receive general acceptance, or 

 at least, especially for a heavily fished stock, a 

 minimum target at the upper end of this range. 

 Whatever the precise objectives and economic 

 conditions of any individual country participat- 

 ing in a fishery, the reduction of the amount of 



fishing to this target level should be desirable. 



A more objective method of calculating a 

 limiting value to desirable fishing mortality 

 may be derived from considering the marginal 

 yield, i.e., the increase in total yield achieved 

 by adding one extra unit of effort (Gulland, 

 1968b). The marginal yield will be equal to the 

 slope of the tangent to the curve of catch against 

 fishing effort. It will always be less than the 

 catch per unit effort, which is the slope of the 

 line joining the point in the curve to the origin. 

 The economic optimum, i.e., the greatest net 

 economic return, occurs when the value of the 

 marginal yield is equal to the marginal costs 

 of a unit of effort. At the point giving the 

 maximum physical yield the marginal yield will 

 be zero. Clearly from any practical viewpoint, it 

 would be undesirable to increase the amount of 

 fishing beyond the level at which the value of 

 the marginal yield is small compared with the 

 costs of the extra unit of effort required to 

 produce that yield. The question then arises as 

 to what might be considered as small. An arbi- 

 trary figure, which has in fact been used in 

 connection with the herring on Georges Bank 

 (ICNAF, 1972) is a marginal yield equal to 

 one-tenth of the original catch per unit effort 

 in the very lightly exploited fishery. This is 

 illustrated in Figure 3. The two straight lines 

 through the origin show catches per unit effort 



Morginol yield = 10% of initial cpue 



Figure 3. — Determination of 

 an economic optimum position 

 at which the marginal yield is 

 10% of the initial catch per 

 unit effort. 



Fishing mortality 



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