456 



Succession and Fluctuation 



due, in part at least, to the occasional production of a year-class in 

 which many more fish reach commercial size than in other years. 



The cause of the varying success of different year-classes is difficult 

 to ascertain. Usually plenty of eggs are spawned every year; rarely 

 does unusual mortaHty occur after commercial size has been reached. 

 The critical point comes somewhere in very early life. Since the gen- 

 eral nature of the ocean does not change, some subtle variation in the 

 environment must arise during a sensitive stage of development. Per- 

 haps an abnormal temperature in the spawning area is to be blamed; 

 possibly a serious change in the abundance of planktonic members of 

 the community— either enemies or food organisms for the young fish- 

 is responsible. Quantitative measurements of plankton in various 



500 



Feb. Mar. Apr. May June July 

 Fig. 12.14. Annual variation in average volume per standard net haul of a com- 

 mon species of planktonic copepod in the waters between Cape Cod and Cape 

 Hatteras. (Sears and Clarke, 1940, Biological Bull.) 



