that each predator requires a certain 'economic' return from its 

 searching efforts to maintain its metobolic 'economy' and to grow. 



"It is from this point of view that analysis of the natural preda- 

 tor-prey system has special relevance to systems in which fishing 

 vessels are the predators. The effect of heavy fishing on natural 

 populations is to decrease the average size of the individual prey 

 organisms, leading to an increase in the average production/average prey 

 biomass (P/B) ratios of the exploited stocks. Under these conditions, 

 increasing the yield by maintaining or increasing the ecological effi- 

 ciency requires an increase in the grazing coefficient, F. Initially 

 this may be accomplished by a simple increase in the numbers of fishing 

 vessels. However, as the P/B ratios increase, the economic return to 



these large vessels drops sharply, limiting the possible increase in 

 F...(15) 



2. Identification of the yield-effort curve : 



"When fishing is initiated an additional unit of effort produces a 

 relatively large increase in yield. When fishing develops beyond its 

 initial stages, larger increases in effort are required to produce the 

 same increase in yield. Ultimately, more effort may even result in a 

 decrease in yield... this diagram (below) is... the most fundamental element 

 of the theory of conservation."^ ' 



yield 



effort 



