xi FISH CYCLES 285 



stages of life varies most mysteriously from year to year. By study of 

 the markings on the scales, bones, and otoliths it is possible to show 

 the age composition of a fish population and it is found that the 

 hatches of certain years predominate. Thus the herring hatched in 

 1904 dominated the population in some parts of the North Sea for 

 years afterwards (Fig. 169). Subsequent good herring years occurred 

 in 1913 and 1918. Similarly there were good cod years in 1904 and 

 191 2. Haddock broods in the North Sea follow a rather regular 





r < 



1919 



12-2 



1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 



Year class 



Fig. 170. Relative strengths of haddock broods in the North Sea, showing cyclical 

 fluctuations. (After Raitt, from Russell, The Overfishing Problem.) 



rhythm with a period of about three years (Fig. 170). These cyclical 

 changes in hatching do not appear to produce fluctuations in the total 

 population of that species, though we have inadequate data on the 

 point. It seems likely that there is a limit set to the total of fish of the 

 species, irrespective of age-grouping. 



In addition to such cyclical changes there are also larger ones, such 

 as the increase of cod in the far north around Bear Island in recent 

 years, apparently due to a northerly extension of warm water, the 

 reason for which is not known. Since so many changes are going on 

 in the eternal sea is it to be wondered at that the animals in it are 

 changing? We might be surprised rather at the slowness of their 

 evolution. 



All the fish-life in the sea depends ultimately on the diatoms, 

 flagellates, and other green organisms of the plankton, reaching the 

 fish usually after passage through one or more animals, especially 

 copepods. Bottom-living fishes also depend on plankton, in this case 



