The production of plankton ceases in autumn, but the consumption of it continues. This 

 causes a reduction or decrease in the number of the crustaceans . 



With the onset of winter, most organisms descend to a depth of more than 600 m. In this 

 layer of water, their number does not exceed 10 mg/mS, but at depths of 100-200 m., the biomass 

 of plankton hardly reaches 50-60 mg/m^- During the winter, the amount of plankton continues to 

 decrease, and towards spring it reaches the minimum. 



The life cycle of the Atlantic herring was evolved during the process of the historic develop- 

 ment of the species . It permits the herring to make the fullest use of the favorable periods in the 

 life of the sea for its own reproduction, (providing the larvae and the young with microscopic nutri- 

 tive substances at the time of the reproduction of Calanus ) and for the rapid growth or fattening of 

 adult specimens of herring at the cost of destroying the large forms of plankton. 



The feeding or foraging migrations of the herrings, their departure after spawning into the 

 western and north-western regions are brought about by the peculiar distribution and development 

 of plankton, especially during spring and smnmer. 



The development of plankton- -as mentioned previously- -can first be observed in the coast 

 waters. There also begins the fattening of herring that have approached the spawning grounds . At 

 first, the herring consume the spawning concentrations of Euphausiacea (February- March). 



Pre-spawning concentrations of Meganyctiphanes norvegica can be observed in February. 

 In the plankton net catches or hauls, these crustaceans are represented by only a few specimens. 

 But in the stomachs of individual herrings, as many as 65 specimens were found. The main body 

 of herring can not find sufficient food in early spring, seeing that the plankton of this period is 

 extra -ordinarily poor. 



In April, the stomachs of most fish were filled with Calanus (70-90%). But at this time in 

 the Faroe region herring could be found with stomachs containing up to 90% of Thysanoessa inermis , 

 because the spawning of Euphausi icea continues into April . 



Toward the beginning of April, the spawning of Calanus comes to an end near the shores. 

 The amount of plankton Is sharply reduced. The adult crustaceans die off. The main body of the 

 zooplankton is composed of the eggs and nauplii of the Copepoda, the larvae of bottom-dwelling 

 animals as well as the eggs and larvae of fish that spawned in shallow water . These microscopic 

 organisms are not suitable as food for herring of the older age groups . Large herring at first con- 

 sume the pre-spawning concentrations of mature or sexually mature Calanus . Mature Calanus 

 begin to concentrate in the upper layers, let us say, one to two months prior to the beginning of re- 

 production. This usually coincides with the flare-up of the development of phytoplankton (with the 

 "blooming" of the sea). Near the surface, Calanus feeds actively at first upon the sparse phyto- 

 plankton and stores up a large amount of fat. In March -April, the alimentary tract of these crusta- 

 ceans is usually stuffed with a green mass of algae. They themselves acquire a claret-red color 

 because of an abundance of fat. 



The fat pre -spawning Calanus is high-calorie food for the herring that were exhausted by 

 winter starvation . In their search for food, the herring leave the spawning grounds for the western 

 and north-western deep-water regions of the Norwegian Sea; for there the pre-spawning concen- 

 trations of Calanus appear later than in the coast waters. It must be said that the end of spawning 

 and the departure of the herring of the older age groups from shallow waters usually coincides with 

 the abundant development there of the "green blooming" (Phaeocystis) . 



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