Figure 3. Distribution of the herring and seasonal variations in the qualitative composition 

 of its food. For key to symbols see Figure 2 and Table 6. 



In 1955, because the colder hydrological conditions were unfavorable for plankton develop- 

 ment in the surface layer, the herring fed at greater depths (compared with the preceding years), 

 in the layer inhabited by Calanus hyperboreus , during May and June . In contrast to the position 

 during past years, in June 1955 the principal food of the large herring east and north-east of Jan 

 Mayen was Calanus hyperboreus , (52%), Calanus finmarchicus (28%) and Oikopleura labradoriensis 

 (13%). In June 1955, the yoimg herring (1950 year class) concentrated in the eastern part of the 

 Norwegian Sea, not passing beyond the boundaries of the polar front. At this time, the older her- 

 ring occupied a much wider area and entered the cold polar waters . Owing to this territorial separ- 

 ation, the composition of the food eaten by the herring of the younger generation in June 1955 differ- 

 ed substantially from that of the food eaten by the older herring, whose principal food in the waters 

 of the polar front and beyond its boundaries, further to the north, was Calanus hyperboreus . The 

 principal diet of the herring of the younger generation (feeding in the eastern part of the Norwegian 

 Sea) consisted of Oikopleura labradoriensis (60%), Calanus finmarchicus (20%) and Themisto sp. 



151 



