212 POPULATIONS OF THE SEA 



this material a biogeographical series was compiled, rather like the 

 one shown (Fig. 2) for the Recorder Survey but much smaller. 

 It begins at the top with ubiquitous and typical North Sea neritic 

 species, passing through organisms characteristic of mixed North 

 Sea and Atlantic shelf conditions to species (at the bottom) from 

 the mixture of oceanic and shelf water. 



It is immediately apparent that there were three major periods 

 in the plankton. From 1949 to 1951, the North Sea neritic forms 

 were abundant but they then declined and the more oceanic 

 organisms increased in numbers during the years 1952-1954 and 

 1956. In the two years 1957 and 1958, there was a decrease in this 

 plankton and an increase of the species whose "average" distri- 

 bution pattern was intermediate between the North Sea types 

 and the oceanic shelf mixture. 



It would be outside the scope of this paper to describe the 

 fishery in detail, but I should mention that the importance of 

 these phases in the plankton is that they were contemporary with 

 three phases in the fortunes of the herring fishery. From this I 

 formed the hypothesis that the success or failure of the Scottish 

 summer herring fishery is partly dependent on the inflow of mixed 

 Atlantic oceanic and shelf water into the North Sea (Glover, 

 1957). 



To return to the shapes of distribution : In order to expand the 

 static background provided by the Recorder plankton atlas. Cole- 

 brook and Robinson haA'e charted the average monthly distribu- 

 tions during the same period of nine years; that is, all samples 

 taken during the nine Januaries or the nine Februaries were com- 

 bined to provide one picture for each month. Many organisms 

 showed remarkably static distributions throughout the year; 

 others showed a progressive advance and retreat of their distri- 

 bution patterns as the seasons passed. But some showed marked 

 seasonal trends and changes which were repeated in each year. 

 Figure 4 provides an example compiled from the nine-year a\'erage 

 distributions; it shows the two extremes of the seasonal distribu- 

 tion of Rhizosolenia stylijormis. As I suggested above, the most 

 likely explanation of patterns of this kind is that they result from 

 the existence of two biologically separate populations, or stocks, 

 or races of a species. 



