218 



POPULATIONS OF THE SEA 



09 



05 



03 



Fig. 9. Herring and plankton in the southern North Sea. The open line (scale 

 at left) shows the mean length of 3-year-old herrings in the East Anglian fishery 

 (data pro\ided b\- A. C. Burd of the Fishery Laboratory, Lowestoft). The solid line 

 (scale at right) show's the logarithmic mean per sample of Calaniis finmarchiciis, 

 stages V and M, in the Continuous Plankton Recorder collections in the southern 

 North Sea during the months Februar\- to July of each year (J. M. Colebrook, 

 unpublished). 



old herrings and the numbers of Calanus in the Plankton Recorder 

 collections in the southern North Sea in those months when the 

 herrings were feeding. It suggests that the supply of food during 

 their first three years of life influences the length of the herrings. 

 This kind of result is a most rewarding proof of the value of long- 

 term sampling. 



All the shapes of distribution and abundance which I have 

 shown so far have been based on a geographic scale or on a time 

 scale, but, in ecological studies, the position of a species may be 

 fixed by factors, other than geographic ones, which may influence 

 distribution and abundance. Dr. B. M. Bary, when he was in New 

 Zealand, plotted the occurrence of planktonic species on to a 

 temperature/salinity diagram. He was able to determine which 

 species would be likely to act as indicators of dififerent water 



