MACKEKEL AND SUNSHINE. 395 



though even if a considerable proportion of it were found to consist of 

 minute animal organisms, these in their turn would feed upon the 

 phytoplankton. It is therefore to the conditions which favour the 

 production of phytoplankton, the fundamental food supply, that we 

 must turn. 



The three most obvious matters to be considered in connection with 

 tlie production of this vegetable plankton are: (1) the composition of 

 the sea-water itself, (2) the temperature, and (3) the amount of light 

 which is available for the production of plant life. 



With regard to the composition of the sea-water itself, the only 

 information available refers to its salinity, and up to the present it has 

 not been possible to show any simple relation between changes in 

 salinity and changes in the vegetable or animal production in the area 

 under consideration. The same is true of temperature, though this will 

 be considered in more detail below. 



It is the object of the present paper to call attention to what appears 

 to be evidence of the influence of the third factor, the intensity of light. 

 Experiments on the cultivation of marine plankton diatoms in the 

 laboratory, upon which I had been engaged, had drawn my attention to 

 the great importance to be attached to the intensity of the light ta 

 which the diatoms were exposed. It therefore occurred to me that a 

 special abundance of Copepods during the month of May in any year 

 might be due to a special amount of sunshine during the earlier months 

 of the year, which would increase the amount of phytoplankton, the 

 Copepod food. An attempt was therefore made to correlate the average 

 quantity of mackerel per boat taken in May with the number of hours 

 of bright sunshine recorded during the first quarter of the year. 



The ofticial statistics of mackerel landed are not very satisfactory for 

 such a purpose, since they give only the total quantities of fish and 

 give no information as to the number of vessels from which the fish 

 are obtained. In making use of them, therefore, one must bear in 

 mind that the number of vessels to which the figures relate varies from 

 year to year, although the amount of this variation over a small 

 number of consecutive years will not generally be very large. 



In order to get figures of a more definite character, I applied to 

 Messrs. Peacock & Co., of Lowestoft, who have had vessels engaged in 

 the western mackerel fishery for many years. Messrs. Peacock were 

 good enough to furnish me with a series of figures giving the number 

 of hundreds of mackerel landed each month from February to June, at 

 Newlyn and Milford,* by three of their steam drifters, for each of the 



* These vessels landed lish only at Newlyn and Milford, so that, by combining the 

 figures for the two ports, we get the total number of fish taken by each boat from the 

 western fishing grounds, 



NEW SERIES. — VOL. VIII. NO. 4. 2 E 



