CONSERVATION OF MARINE RESOURCES 



15 



the appropriate amount of fishing effort, a population may be stabihzed 

 at any level of density between its environmental maximum and near- 

 extinction. At each population level it can provide a particular level of 

 yield. Populations about midway between near-extinction and virgin 



> INTENSITY OF FISHING 



Figure 2.1. Relations between fishing intensity, population size, 

 and average sustainable total catch from a fish population. After 

 Schaefer from Encyclopedia of Science and Technology, 1960. 

 (McGraw-Hill Book Co.) 



levels generally are believed to be those capable of providing the maxi- 

 mum sustainable yield ^ Therefore, maximum sustainable yields may be 

 obtained by regulating the amount of fishing to bring the populations 

 to these "mid-levels." 



If stable environmental conditions exist, once the optimum percentage 

 of the population available for harvesting is exceeded, the sustainable 

 catch not only decreases but the costs of production increase. With pro- 



