Irruptive Fluctuation 453 



certain plants and animals harvested by man. Among the latter the 

 most extensive records are those of trading posts for fur-bearing ani- 

 mals and those of fishery agencies for the landings of commercial fish. 

 We shall discuss some ecological aspects first of irruptive fluctuations 

 in which a population undergoes wide, irregular swings in abundance, 

 and then of oscillations that appear to be cyclic in nature. 



Irruptive Fluctuation 



Data from the commercial marine fisheries will serve admirably to 

 illustrate the circumstances of irregular fluctuation in a community. 

 It will be agreed that the general nature of the ocean has not changed 

 drastically during the last several hundred years or so; yet during that 

 time certain fish populations are known to have fluctuated in an ex- 

 treme manner. In the cod fishery off northern Norway, for example, 

 periods of scarcity severe enough to be recorded have occurred as far 

 back as the time of Leif Ericson. In certain intervening years, as in 

 the winters of 1714 and 1715, whole villages along the Norwegian 

 coast are reported to have starved because of the failure of the cod 

 fishery, whereas in the years immediately before and afterward codfish 

 were plentiful. In the same region similar fluctuations have con- 

 tinued to the present day, and equally extreme variations in the fish- 

 eries occurred on the American side of the Atlantic. The fluctuation 

 in the catch of mackeral by the American fishing fleet for a period of 

 150 years shown in Fig. 12.12 may be taken as representative of the 

 change in abundance of the mackerel population in the waters fished. 

 The most spectacular change during this period occurred between 



70 



60 



50 



40 



30 



20 



10 



Fig. 12.12. Fluctuations in the landings of mackerel {Scomber scomhrus) by the 

 fishing fleet from the east coast of North Ainerica. ( From data of the U. S. Fish 



& Wildlife Service.) 



