FLUCTUATIONS IN ABUNDANCE OF MARINE FISHES 189 



From a Study of Cycles. In fisheries where the catch records have been kept 

 for a long period (say 20 to 50 years) it is often possible to observe the occur- 

 rence of regular cycles of abundance. By application of certain methods of 

 harmonic analysis it is possible to make extrapolations of these cycles much as is 

 done by the Coast and Geodetic Survey in predicting tides. Because of short term 

 deviations from these cycles predictions for individual years based on the cyclic 

 treatment are usually not highly accurate. 



From Mortality Rates. In fisheries where the recruitment of the young is very 

 regular it may be possible to predict the yield of a fishery simply by a knowledge 

 of the fishing mortality rate in eflFect as related to the fishing effort applied. An 

 outstanding example of this is afforded by the halibut of the North Pacific area, 

 where such a method yielded very close results for a considerable period of years. 



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