Fish and Fishermen 22 S 



sustained yield" is the ideal sought. If a species is fished too 

 heavily, too many mature fish will be removed and too few 

 will be left to reproduce sufficient young to maintain the 

 population. The numbers will decrease until not enough re- 

 main to make fishing worth-while, and the species will no 

 longer contribute to the well-being of humanity. Eventually, 

 if left alone, nature will in most cases restore the numbers 

 to the point where it can again be utilized, but for a time it 

 will be out of the game. On the other hand, it is conceivable 

 that a species may be so lightly fished that a great many of 

 the individuals die of old age or other "natural" causes 

 (meaning almost any form of death not brought about by 

 man). This, from the human point of view, is undesirable 

 waste. 



The "maximum sustained yield" concept is complicated in 

 detail and in practice, but in theory it is simply that for each 

 kind of fish there is a surplus produced over and above 

 that needed to maintain the population at an optimum level, 

 and that that surplus, no more and no less, is the number 

 which should be taken by fishing. To determine the maxi- 

 mum sustained yield is a highly difficult problem, involving 

 the study of, among other things, life history and numbers 

 caught. Once an estimate of the maximum sustained yield 

 has been made, it is an equally difficult problem to evolve 

 methods whereby the catch may be held at the desired 

 number and to put those methods into practice. 



The halibut fishery of the Pacific Northwest is a classic 

 example of the realization of these objectives. Years of poor 

 catches had brought the fishermen to such a state of despair 

 that they asked for an investigation to see if any remedy 

 could be devised. The resulting program of research included 

 continuation of the life history studies which had already 

 been made of this large, long-lived fish, and analysis of the 

 log-records of their catches which many fishermen had for- 



