20 FISHES AND FISHERY METHODS 



tions by growth and addition of new recruits, began in 1932. Manage- 

 ment of this resource has been strikingly successful, as shown by the 

 fact that since 1932 production has increased from 44 million pounds 

 annually to an average level of about 70 million pounds in recent years. 

 Present harvests of Pacific halibut are thought to be close to the maxi- 

 mum sustainable yield leveP. 



Pelagic Species. Until recently it was generally believed that abundance 

 of most pelagic (offshore free-swimming) species, such as the herring and 

 tuna-type fishes, was little affected by fishing. For these species natural 

 fluctuations were assumed to transcend the effects of man. Some inves- 

 tigators now believe, however, that increased catches of European 

 Atlantic herring in recent years may account for an observed reduction 

 in size of some of the stocks^ Similarly, fishing has been shown to signifi- 

 cantly affect the yield and stock size of yellowfin tuna in the tropical 

 Pacific Ocean^ On the other hand, many stocks of herring and tuna-type 

 fishes throughout the world yet are little affected by fishing. Major 

 changes observed in the size of tropical Pacific skipjack tuna populations 

 apparently have not been related to man's fishing activities; they can 

 only be attributed to changes that have occurred in the natural 

 environments 



Alteration of Environment. The effects of man's fishing activities often 

 are difficult to assess. However, his effects on anadromous fishes, such 

 as salmon, during upstream migration of adults or downstream move- 

 ment of young are all too apparent. Dams take their toll by interfering 

 with and sometimes preventing the passage of both adults and young 

 fish. Careless logging and farming practices have caused erosion which 

 has resulted in deposition of silt in stream beds. Improperly screened 

 irrigation diversions have allowed young salmon and trout to stray into 

 irrigation canals where they have perished. Removal of gravel from 

 stream beds has destroyed valuable salmon and trout-spawning areas. 



Pollution. By polluting streams and parts of the sea with sewage and 

 industrial wastes, man has destroyed fish and shellfish resources. Pollu- 

 tion is a particularly insidious problem since the quantities of introduced 

 waste materials are usually small at first and increase from year to year. 

 Thus, destruction of a resource often proceeds slowly, making it difficult 

 to determine whether a pollutant or some other environmental factor is 

 to blame. With the tremendous growth in human populations throughout 

 the world forecast to take place in the near future, pollution will become 

 an increasingly vital problem. 



With the coming of the atomic age man has created new problems in 

 conservation which may well be the most important he has yet faced. 

 More and more nuclear fission power plants will be used to produce 



