272 PLANKTON STUDIES IN RELATION TO 



these early spring months mackerel may be present at the surface but 

 are dissemmatecl over a wide area. A careful comparison of the plankton 

 results for the February cruises, shown in the Bulletins Conseil Inter- 

 national ijour V creploration cle la Mer, shows that, at the western mid- 

 Channel and Bristol Channel stations (E. 5 and E. 6, the only two fall- 

 ing within the fishing area), plankton generally has reached a minimum 

 in comparison with other quarterly results. It is desirable at this- 

 point to state that, for the purpose of comparison in this and other 

 cases, an arbitrary numerical value was substituted for the comparative 

 signs employed in the tables, viz. + =10, c=100, cc=oOO, the signs. 

 " r " and " rr " being disregarded. In the present instance comparison 

 was made for all the years since the commencement of the international 

 investigations, and this condition appeared to be invariably the same. 

 This point is further supported by certain observations taken during 

 the present investigations (Nos. 2, 3, 4, 5, 6, Table I). 



These two main facts considered together cannot, however, be offered 

 as sufficient evidence that mackerel do not shoal closely at the 

 surface until there is a sufficiency of plankton to form food for their 

 support without much individual effort, especially in view of the fact 

 that mackerel are known to be shoaling densely at the bottom off 

 Start Point in the early part of the year, and, as far as our present 

 observations extend, are feeding there upon plankton. Possibly, how- 

 ever, it may be suggested reservedly that a lack of plankton may exercise 

 a retarding infiuence upon the shoaling of fish already at the surface^ 

 which are disseminated over a wide area, of the presence of which 

 evidence has already been given. Further, attention has already been 

 drawn to the fact that in the Plymouth to Lizard area in 1907 larger food 

 material was observed, in gradually increasing quantity as the season 

 advanced, from the end of April onwards. Throughout July, 1907, 

 from the writer's personal observations, the plankton in the Plymouth 

 area was observed to decrease very materially in quantity. In the 

 previous month, the tow-nettings brought to the Laboratory at Plymouth 

 from within and outside the Sound for the use of students, and for the 

 Plymouth plankton records taken weekly, were composed mainly of 

 Temora longicornis in extreme abundance. During the first few days of 

 July, within the first week, the Copepod in question, from being ex- 

 tremely abundant, rapidly disappeared from the tow-nettings, leaving 

 the samples almost clear, for the time being, of animal life. No 

 plankton observations were obtained from the western area during^ 

 July, 1907 ; but, confining our attention to the Plymouth to Lizard area, 

 it appeared that from all reports the shoals broke up at about this time 

 and the hand-lining season commenced. It may, therefore, be siig- 



