CRUSTACEA IN RELATION TO MAN 251 



been shown by Mr. G. E. Bullen that the fluctuations 

 in the yield of the Mackerel fishery from year to year 

 follow very closely the fluctuations in the abundance 

 of the Copepod plankton on the fishing-grounds. 

 The investigation has been carried a step farther by 

 Dr. E. J. Allen, who points out that the abundance 

 of Copepods is determined by the abundance of the 

 Diatoms and other minute vegetable organisms of 

 the plankton. These organisms are very largely 

 influenced by the amount of sunshine during the 

 period of their development in the earlier months of 

 the year. Dr. Allen gives a diagram showing for 

 each of seven years (1902-1908) the average number 

 of hours of bright sunshine during the months of 

 February and March in the South- West of England. 

 With this he compares the number of fish caught 

 in the month of May in each of these years by 

 certain vessels engaged in the western Mackerel 

 fishery. The correspondence between the two is 

 very striking indeed, and justifies his conclusion 

 that the amount of sunshine in the early months of 

 the year determines the abundance of the vegetable 

 life of the plankton, and through it of the Copepods 

 and other animals which form the bulk of the 

 plankton a little later in the year ; and although 

 there are doubtless other influences at work deter- 

 mining the success or failure of the fishery, it is 

 largely a matter of the richness or poverty of the 

 plankton harvest. 



