planktonic organisms (of which Calanus finmarchicus forms a considerable portion) remains in the 

 benthic layers or at considerable depth. The biological spring is marked by mass production of 

 phytoplankton (blooming of the sea) and migration of Calanus finmarchicus from great depths to the 

 surface layers for reproductive purposes . 



Large herring of older age groups, usually spawning in March, begin feeding on the concen- 

 trations of reproducing Calanus finmarchicus and Euphausiacea. With the emergence of nauplii 

 and early stages of young Calanus , the biomass of plankton becomes usually greatly reduced, since 

 adult Calanus disappear from plankton after the reproduction has been completed . 



During the season of biological summer, young Calanus finmarchicus become transformed 

 into fat red Calanus , as a result of which the biomass of plankton greatly increases, reaching its 

 maximum. The herring which have spawned for the first time feed on red Calanus . 



With the approach of autumn, Calanus finmarchicus begin diurnal vertical migrations. At 

 night, the amount of plankton in surface layers increases relatively, although its overall biomass 

 decreases considerably. During the autumn the main bulk of plankton consists of small Copepoda, 

 such as Pseudocalanus elongatus, Oithona atlantica, and Oithona similis . 



At the beginning of the winter, surviving Calanus finmarchicus specimens sink to depths of 

 over 300 meters. 



The dates marking the beginning of the biological seasons and the duration of these seasons 

 in the southern and northern regions of the Norwegian and Greenland seas are variable . TTie 

 spring-summer period continues in the littoral waters of Norway and in the southern zone of the 

 Norwegian sea for half a year, whereas, in the central deep water regions, it does not exceed four 

 months and in the Greenland Sea is occasionally shorter than three months . As a result of these 

 fluctuations in the periods of plankton production, the biological seasons seem to shift gradually 

 from the southern coast regions to the north and north-west, to the borders of the east-Iceland 

 and east-Greenland cold currents . The feeding herring migrate to the north and north-west follow- 

 ing the displacement of the biological spring. 



By July, the herring of older age groups arrive in polar waters at the very beginning of bio- 

 logical spring in that region (phytoplankton develops, Calanus hyperboreus reproduce). 



Young sexually immature herring and the herring that have spawned for the first time re- 

 main for most of the feeding season in the waters of the Norwegian current, where they feed on red 

 Calanus . Their migration paths are shorter than those of large herring. In July, they arrive in 

 the region of mixed waters, near Mohn's Threshold, however, they do not enter the polar waters. 



The dates at which the herring arrive in different regions may vary considerably during 

 different years . Observations between 1947 and 1955 showed that one of the factors accounting for 

 fluctuations in the dates of arrival of herring on the feeding grounds, i.e. at the border of polar 

 waters, lies in the annual fluctuations in the period of plankton production. The development of 

 plankton is affected in its turn by the general thermal state of waters and meteorological conditions . 



The above observations led us to assume that during the early summer, as a result of the 

 intensive feeding, the herring grow mainly in length, whereas during the latter half of the season 

 they increase in weight as a result of the accumulation of fat deposits . 



The annual increments (in length) of adult herring during the feeding migrations are on an 

 average as follows: 6th year - about 2 cm . , 7th year - 1 . 5 to 2 cm . , 8 - 9th year - about 1 cm . , 



35 



