July no closer than 600 miles to the Kurile 

 chain (on the western section) and only in Sep- 

 tember were they found at a distance of 300 

 miles to the south-southeast from the chain. 

 An abundance of Nannocalanus minor was dis- 

 covered when the temperature of the surface 

 layer rose to 22oC. (in July) and 25°C. (in 

 September). 



The cited examples of the distributions of 

 representatives of the three indicated groups 

 compel us to assume the presence of a well- 

 defined zonality (or banding) in the shifts of the 

 plankton in the area investigated. For clari- 

 fication of this problem let us look at the 

 changes in the plankton from the Kurile chain 

 to the south-southeast (roughly speaking, to 

 the south). 



Changes in the plankton from north to south . 

 In the immediate vicinity of the Kurile chain 

 the plankton in July and August, 1953, was 

 characterized by an abundance of microscopic 

 algae, which occupied a relatively narrow band 

 along the whole chain; the southern boundary of 

 this band was 40-50 miles distant from the 

 chain. In September this band was greatly 

 narrowed and in places was interrupted. From 

 investigations of the planlcton along the Kurile 

 chain in past years it was known that as a con- 

 sequence of strong mixing and low water tem- 

 peratures (about 2°-5°C.) the area of the 

 Kurile Islands is characterized by a prolonged 

 and intense plankton bloom. Seasonal changes 

 in the course of the spring, summer, and au- 

 tumn do, of course, show up in the decrease 

 in quantity and change in species of the algae, 

 but the bloom, although limited in extent, does 

 not disappear even in the summer. That was 

 the case, at least, in 1953. Analogous obser- 

 vations were made by us in 1950, too; the same 

 thing appears from the data of E. Fedorova 

 for 1952. 



The zooplankton in the zone of blooming is 

 poor in species, and in its composition it dif- 

 fers little from the zooplankton of the areas 

 slightly farther south; it may be only that its 

 biomass is somewhat smaller. 



Directly adjoining the zone of abundant 

 phytoplankton, there extends to the south a 

 broad strip, also parallel to the Kurile chain, 

 which is characterized by an abundance of a 

 few species (found in great quantities) of cala- 

 nids typical of the North Pacific Temperate 

 Region, which serve as forage for pelagic 

 plankton-eating fishes and baleen whales 

 (fig. 1). The width of this zone is determined 

 by the southern limit of the distribution of 



Calanus tonsus, which was discussed earlier. 

 There are present here, in all, 5 or 6 species 

 of calanids, 3 of which (Calanus tonsus , C. 

 cristatus, and Eucalanus bungii) are encountered 

 in mass quantities. Of the most abundant 

 species--Calanus tonsus, represented mostly 

 by the fifth copepodite stage--there were in July 

 about 200-800 individuals per m.^, or 240-960 

 mg. /m.^. 



The zone rich in large species of calanoids, 

 which constitute food for fishes and whales, ex- 

 tends along the whole Kurile chain from Shiko- 

 tan Island to Cape Lopatka. In the area of the 

 northern islands of the Kurile chain the zone of 

 abundance of Calanus tonsus is considerably 

 broader than at the southern islands. The phy- 

 toplankton in this zone was very poor, and no 

 tropical species of zooplankton were discovered 

 there in 1953. The temperature of the water 

 here is still relatively low; at the various sta- 

 tions in July and August, 1953, it varied from 

 5° to 15°C. at the surface. 



Judging by the volume of settled plankton 

 and the quantity of specimens of large copepods, 

 the zone with Calanus tonsus represents the 

 southernmost extent of the rich forage plankton 

 of the Far Eastern seas, that is, the southern 

 boundary of the plankton which is characterized 

 by high biomass. In September 1953 the bio- 

 mass became considerably less than in July 

 (fig. 2). The reasons for the decrease in the 

 biomass are not yet exactly known. It can only 

 be postulated that it reflects the migration of 

 the calanoids and the devouring by consumers; 

 it is hard to say now which of these causes has 

 the greater significance. 



South of the indicated zone, the quantity of 

 individuals of Calanus tonsus decreases, but the 

 quantity of Calanus pacificus increases notice- 

 ably. The latter, especially in September, 

 forms a continuous band lying approximately 

 between the 15°C. and 18°C. surface isotherms 

 in July. 



The transition from the zone of Calanus 

 tonsus to the zone of Calanus pacificus is grad- 

 ual in some places, but in others the composi- 

 tion of the plankton changes sharply. This is 

 connected mainly with low and high water tem- 

 perature gradients. Together with Calanus 

 pacificus in the plankton appear Eucalanus 

 hyalinus, Evadne spinifera, and other warm- 

 water species. Judging by the specific compo- 

 sition, the zone of Calanus pacificus evidently 

 represents a transitional region between the 

 faunas of the temperate and subtropical zones. 

 The boundary between these zones lies between 



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