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Mackerel and Sunshine. 



By 

 E. J. AUen, D.Sc, 



Director of the Marine Biological Association. 

 With Figs. 1-5 in the Text and Tables I. -VII. at the end. 



In his paper on " Plankton Studies in Eelation to the Western 

 Mackerel Fishery," in the last number of this Journal (Vol. VIII., p. 

 269), G. E. BuUen shows that for the years 1903-1907 there appears 

 to be a correlation between the number of mackerel taken during 

 May and the amount of Copepod plankton, upon which the mackerel 

 feed, taken in the neighbourhood of the mackerel fishing grounds 

 during the same month. 



It was clearly worth while, therefore, to consider what conditions 

 favour the production of an abundant supply of Copepods in the 

 fishing area, since it appears to be this supply of food which attracts 

 the mackerel into that area, or at any rate into its surface waters. 



The hydrographical investigations carried out at the mouth of the 

 English Channel have rendered it probable that the movement of the 

 water there is comparatively slow. It may therefore be assumed that 

 on the mackerel grounds to the westward of the Cornish coast the 

 water which is present at any particular time has not recently moved 

 into the district from any very remote region, and, treating the matter 

 broadly, has been subjected for some time to the general climatic 

 conditions of the neighbourhood. 



The question then suggests itself, can the differences which occur from 

 year to year in the abundance of the Copepods be referred in any way 

 to such climatic conditions? If such a connection exists it will probably 

 be not direct, but indirect, through the action of the climatic conditions 

 on the food of the Copepods. The food of Copepods seems to be largely 

 the vegetable organisms of the plankton, chiefly diatoms and Peridinida3,* 



* This has long been recognised in a general way, but useful direct evidence of it has 

 recently been brought forward by W. J. Dakin. Notes on the Alimentary Canal and Food 

 of the Gopepoda. Internat. Jievve der gesam. Hydrobiologie u. Hydrographie, I., 1908. 



