degree than the plankton of the North and Barents Seas . 



A study of the plankton of the Norwegian Sea was begun as early as the XIX century. But, 

 at that time, the study was of a descriptive nature. The monograph by Gran (33) is the most com- 

 plete summary of the investigations of the early period. He set aside groups of planktonic organ- 

 isms that characterize Atlantic and Arctic waters . Gran is of the opinion that the development 

 times of plankton during the spring- summer period can change depending on the temperature of 

 the Atlantic current. 



On the basis of oceanographic observations from 1900-1904, Helland- Hansen and Nansen 

 wrote also on the existence of annual fluctuations in the temperature cycle of the Norwegian Sea . 



These scientists came to the conclusion that in warm years cod would approach the shores 

 at a later date than in cold years . They are of the opinion that in cold years more of the polar 

 waters enter the Norwegian Sea . These waters abound in nutrient substances . This results In con- 

 ditions that are favorable for the production of plankton and, therefore, also for the fattening of 

 fish. Their hypothesis was confirmed also by our observations. 



At first, the PINRO investigations embraced only the region of the Lofoten Islands and the 

 south-eastern regions of the Greenland Sea (1947-1949). 



Since 1950, plankton has been collected also in the open part of the Norwegian Sea. The 

 region of the research increased each year and by 1954 it Included the large water basin of the 

 Norwegian and Greenland Seas . 



Our investigations made It possible to establish a few principles that govern the production 

 and distribution of plankton during the feeding period of herring. 



Material and Methods 



Material that had been gathered in the Norwegian and Greenland Seas during the past eig^t 

 years (1948-1955) formed the basis for our work. During this period, 4,700 samples were collected 

 at 2, 106 stations. 



Plankton was gathered with a Nansen closing net. It had a diameter of 50 cm. The material 

 used was gauze No. 3/23. The plankton was gathered at standard depths: 0-25 m,; 25-50 m.; 50- 

 100 m.; 100-200 m.; 200-500 m. 



The depths 0-10 m. and 500-1,000 m. were fished only on some occasions. In stormy 

 weather, as a rule, only a total catch was made (0-100 m., 0-50 m.). 



All the plankton samples were checked first aboard tlie vessel. The predominant mass- 

 groups of planktonic organisms were entered in a journal .— ' 



After finishing the routine survey, maps showing the distribution of plankton for this period 

 were compiled. These maps, together with the other material, were passed on to the administration 

 of the herring- fishing Industry. 



After returning to port, all the plankton samples were studied in the PINRO laboratory by 

 means of binoculars . The qualitative composition of plankton, the approximate number or quantity 



1/ This method was described in the work of B. P. Manteyfel' (17,19). 



106 



