CHAPTER 11 



Fluctuations in Abundance of Marine Fishes: Their 

 Measurement, Causes, and Prediction 



Ralph P. Silliman 



Chief, Section of Anadromous Fisheries, 

 U. S. Fish and Wildlife Service 



Introduction 



Of all the characteristics of marine fisheries one of the most outstanding is the 

 violent and usually unpredictable fluctuation of the catch. This aspect is of in- 

 terest not only to the scientist and conservationist, but also to the practical 

 fisherman because of its economic impact. Since some communities, as, for 

 instance, certain towns in Norway, depend on successful fishing for their entire 

 economy, extreme distress is experienced during periods of poor fishing. There- 

 fore, attention has been focused on the fluctuations in marine fisheries almost from 

 the beginning of the study of fishery biology. Interest in this subject was greatly 

 intensified after the publication in 1914 of a paper, "Fluctuations in the Great 

 Fisheries of Northern Europe," by the Norwegian marine scientist, Johan Hjort. 

 Since that time the matter of fluctuations has continued to be of paramovmt im- 

 portance to practically all marine fishery biologists, and a great deal of work has 

 been done to ascertain the causes of the fluctuations and possible means of 

 lessening their impact on the fishermen. 



A very few illustrations of catch records from marine fisheries will serve to 

 illustrate the type of fluctuations encountered. One of the most famous examples 

 and the one which caused the economic upheavals in Norway was the Norwegian 

 herring, the recent history of whose catch is shown in Fig. 11-1. Although the catch 

 averaged about 700,000 metric tons from 1850 to 1870, it had fallen to 75,000 

 by 1904, as indicated in the graph. Low catch levels continued until 1925 when it 

 surged up to about 600,000 metric tons, with fluctuations of greater or lesser 

 degree since then. 



Turning from the pelagic Norwegian herring to an anadromous fish we find 

 the matter of fluctuations again exemplified in the catch of the Hudson River 

 shad (Fig. 11-1). Here the earliest avaflable data show the fishery already well 

 developed. There was a catastrophic decline around 1904 followed by continued 

 low catch levels for about 30 years. After 1935 very high levels were attained, 

 until a peak of 5,000,000 pounds was reached in 1944. The years subsequent to 

 1944 seem to indicate the start of another decline. 



Another anadromous fish of great commercial value is the Columbia River 

 chinook salmon, the course of whose catch is depicted in Fig. 11-1. The story given 

 here includes the early history of the fishery; it is somewhat similar to that of the 



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