14 FISHES AND FISHERY METHODS 



was becoming over-fished. Some of our beaches have been denuded of 

 clam and oyster resources. 



Future needs for conservation will be even more pressing than in the 

 past. The population of the world is expected to increase by 1.2 to 1.6 

 billion people by 1980. Some of the food required to feed this increased 

 population must come from the sea. For the Ufiited States alone it is 

 estimated that by 1980 we will require an additional 2 to 3)^ billion 

 pounds of fish and shellfish annually. 



Our increase in human consumption of marine products during the 

 past 20 years has been satisfied by importations, but we cannot depend 

 on imports to take care of future increases in our needs. Other countries 

 may soon have to use all they can harvest from the sea to feed their 

 own peoples. Many of our present fisheries already are being exploited 

 to or beyond their maximum productivity. Consequently, we will have 

 to expand our harvest of presently underfished resources and begin har- 

 vesting unfamiliar species. This will create a multitude of conservation 

 problems. 



Theory of Fishing 



Four primary factors determine the size of any exploited marine popu- 

 lation. They are: recruitment of new individuals, growth, deaths due to 

 natural causes, and deaths due to fishing. By harvesting the resources, 

 man directly determines the last factor. His actions also may indirectly 

 affect the others since they are often interdependent. 



In the unfished state, population levels are in balance with their 

 environment, and growth and reproduction are balanced by deaths from 

 natural causes. Consequently, when fishing begins, most of the yield is 

 derived from the accumulated population. Sharp initial declines in popu- 

 lation size are inevitable since there is no immediate acceleration in the 

 compensating forces, namely, reproduction, growth, and recruitment of 

 new fish to the fishable stock. These initial declines in population size 

 sometimes are mistakenly attributed to over-fishing. 



After the fishery has operated for some time, lowered natural mortaUty, 

 accelerated growth, and increased recruitment will bring the population 

 into a new state of equilibrium. Increased growth rates result from the 

 fewer remaining fish having more food available per individual. Reduc- 

 tion in crowding of the population also may lower disease and parasitic 

 infestations and thus provide a higher survival rate for the remaining 

 individuals. 



The harvest will increase with increasing fishing intensity to the point 

 of maximum average catch. Beyond this point the sustainable catch 

 actually decreases with increasing fishing effort (Figure 2.1). By applying 



