282 PLANKTON STUDIES IN RELATION TO 



a diametrically opposite fluctuation of phytoplankton, but we shall not 

 here attempt to discuss the causes which promote paucity or abundance 

 of phytoplankton. 



But little is known at present of the food of Copepods, and 

 the inversion of the phytoplankton curve in comparison with that of 

 zooplankton cannot be offered in any way as evidence that Diatoms, 

 Peridiniales, and other comparatively large protophyta taken in 

 tow-nettings are the food of Copepods. Therefore the paucity of phyto- 

 plankton correlating to a large extent with the abundance of mackerel 

 (Fig. 2) need not at present be considered as other than additional 

 evidence to show that where zooplankton is in excess of phyto- 

 plankton mackerel are more numerous — the subject of a former 

 paragraph. 



Briefly now to summarize the results of the present investigations. 

 The following considerations are submitted as forming the principal 

 points of correlation between the plankton and mackerel during 1906 

 and 1907. 



(i) That during the three months forming the more important part of 

 the spring mackerel fishing in the west part of the English Channel 

 and Bristol Channel, viz. April, May, and June, mackerel appear to 

 feed for the greater part on plankton. 



(ii) That the plankton organisms observed in the stomach contents of 

 mackerel are also to be found in tow-nettings taken on the same 

 position from whence the fish are derived. Further, that in a majority 

 of cases, the relative quantities or proportions of such species are also 

 common to both tow-nettings and stomach samples. 



(iii) That in April of the two years under present consideration, 

 where zooplankton was in excess of phytoplankton mackerel were more 

 numerous. 



(iv) That the abundance or paucity of zooplankton during a certain 

 number of years (1903-7) appears to be correlated with the greater or 

 less abundance of mackerel. 



III. THE " STAET " MACKEKEL FISHERY. 



I. GENERAL CONDITIONS IN 1907. 



From the reports of fishermen at Plymouth and Newlyn, together with 

 information derived from a fish salesman agent at Boulogne, it 

 appeared that from the end of December, 1906, and throughout 

 January, February, and the greater part of March, a regular and 

 profitable trawl fishery for mackerel was carried on in an area of 

 20 to 40 miles S.S.W. to S.E. of Start Point. 



