454 Succession and Fluctuation 



1884 and 1886 when the catch of mackerel dropped from more than 60 

 milHon kg to about 10 milHon kg. Subsequently the catch fell to a 

 still lower figure but eventually rose again to a value of more than 30 

 million kg. The seriousness of the ecological, economic, and socio- 

 logical repercussions of these tremendous oscillations can easily be 

 imagined. 



Year-Class Analysis. The economic importance of the fluctuations 

 in the Norwegian herring fishery led the great marine biologist Johan 

 Hjort to examine the annual differences in the population of the 

 herring. By adopting an analytical approach to the problem he was 

 able to trace the source of the oscillations of the entire catch to the 

 special success of fish hatched in certain years. Hjort began by con- 

 sidering a hypothetical, ideal situation in which the same number of 

 young herring were spawned each year and in which mortality 

 caused a uniform reduction in the abundance of each "year-class." 

 Under these circumstances 1-year-old fish would always be the most 

 abundant, 2-year-old fish the next most abundant, and so on. Since 

 herring of 4 years of age and older are taken in the commercial catch, 

 an analysis of the catch under these ideal conditions would show that 

 the 4-year-old fish were always the most numerous, the 5-year-old 

 fish were the next most abundant, and so on. 



Applying this reasoning to the catch in 1910, for example, the fish 

 hatched in 1906 should be the most numerous, followed by the 1905 

 year-class, and then by earlier year-classes, provided always that the 

 same number were spawned each year and underwent the same mor- 

 tality. When the age composition of the actual catch in 1910 was 

 examined, however, a very different picture was presented, as shown 

 in Fig. 12.13. The 1906 year-class, which was 4 years old in 1910, 

 was by no means the most abundant, but occurred in extremely small 

 numbers, and the 1904 year-class was found to represent more than 

 80 per cent of the catch. Looking back in the diagram we see that the 

 1904 year-class dominated the catch in 1909 and in 1908; this same 

 year-class continued to dominate the catch for a good many years after 

 1910 and remained recognizable until 1921. No other dominating 

 year-class appeared until 1917 when the 1913 year-class entered the 

 scene, but in 1922 the 1918 year-class became prominent. We are 

 confronted then with the surprising discovery that during 20 years 

 only three year-classes were successful. A good year-class is obviously 

 the exception; in spite of the tremendous fecundity of the fish en- 

 vironmental resistance is overpowering in most years. The fluctua- 

 tion in the herring fishery, and in many others, is now known to be 



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