Phenomena that are characteristic of the biological spring can be observed in the various 

 regions of the territory of the Norwegian and Greenland Seas . The gradual shift of the zone of the 

 "green blooming" to the north is depicted in Figure 4. Somewhat earlier, prior to the beginning 

 of the mass development of Phaeocystis , large herring pass through the deep-water regions of the 

 Norwegian Sea. Moving northward ahead of the "blooming", the herring of the older age groups 

 have a sufficient feeding base --for the entire migration route --in the form of large mature crusta- 

 ceans . 



At the very beginning of the biological spring, the pre -spawning Calanus , which is compar- 

 atively large, concentrates near the surface. It is the most accessible food for the herring that 

 were exhausted by winter starvation. This is the fact that compels the herring when moving north- 

 ward to comply with the conditions that are characteristic of the beginning of the biological spring. 

 In April-May in the southern regions of the Norwegian Sea, herring schools approach the boundaries 

 of the East Icelandic current. There they continue to feed in the mixed waters that are rich in 

 plankton. In these waters, we encounter comparatively large crustaceans such as Calanus finmar - 

 chicus in every stage of development or growth, Calanus hyperboreus , Themisto abyssorum , Th . 

 libellula, Metridia longa, Paraeuchaeta norvegica, Oikopleura labradoriensis , in the composition 

 of plankton. 



In the regions where the warm Atlantic current and the cold East Icelandic current touch 

 ufxjn each other, an intense intermixing takes place. This causes the cold deep waters to rise to 

 the surface, thereby enriching the upper layers of water with nutritive salts. 



The regular replenishment of the reserves of nutritive salts feicilitates the abundant and pro- 

 longed development of phytoplankton in the south-western regions of the Norwegian Sea. Not only 

 algae thrive in the intermixed waters; there is also much zooplankton there. It is possible to ob- 

 serve simultaneously several plankton complexes in these regions. They usually develop or grow 

 during various biological seasons (the "blooming", the pre -spawning accumulation of Calanus fin- 

 marchicus and Calanus hyperboreus , the young of Calanus , etc.). Successive generations of Cal- 

 anus provide an abundant and reliable food for herring during the spring months (March, April, 

 May). Somewhat later, in June (and at times even in July) the herring reach Mohn's Threshold 

 (boimdaries of the Norwegian and Greenland Seas) and penetrate ferther Into the warming polar 

 waters of the East Greenland current . 



The polar waters contain a large amount of nutritive salts . TTiese salts are not used for the 

 larger part of the year; for the ice hinders the development or growth of phytoplankton. In June- 

 July, when a large part of the waters of the Greenland Sea is free from ice and the edge of the ice 

 is retreating to the north-west, there is a tremendous plankton growth. The biomass of plankton 

 near the melting ice reaches 4000 mg/m3 in the 0-50 meter layer. The negative temperature (-0. 

 61 )renders this rich food base inaccessible for herring. 



Figure 5 shows how sharply the polar waters --during this time --differ from the Atlantic 

 waters with regard to the amount of phosi*orus, the oxygen content, and the amount of plankton. 



At the beginning of spring, in the waters of tfie East Greenland current, Calanus hyperboreus 

 rise to spawn. Many tiny young of Themisto libellula appear in the plankton. There is little phyto- 

 plankton; for it is consumed to a large extent by crustaceans (the alimentary duct of Calanus hyper - 

 boreus is stuffed with a green mass of algae; this is the case with regard to Calanus finmarchicus 

 at the beginning of spring). The polar waters abound in phosjAorus (25-30 mg/m3).2/ They are 



!_/ The hydro -chemical index (see Figure 5 and Table 1) obtained by M. A. Istoshina on the 

 5th voyage with the E/S "Professor Mesyatsev" in June 1954. 



115 



