376 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



increase in the Black and Azov Seas must be explained first by the changes 

 in the quantity of nutrient salts in the zone of photosynthesis. The subsequent 

 decrease in the Caspian and especially in the Aral Sea should be accounted 

 for by the qualitative changes in the flora and fauna composition and some 

 peculiarities of the hydrological conditions in these seas. 



As a result of the absence until very recently of any data on the quantitative 

 distribution of life in the Black Sea, owing to the hydrogen sulphide con- 

 tamination of its depths and the shortage of information on its very rich 

 pelagic life, and owing to the proximity of the Sea of Azov which is excep- 

 tionally abundant in life, a false picture of the poverty of life in the Black Sea 

 was gradually built up, beginning with Ratke and Nordmann. This was 

 furthered by the qualitative impoverishment of fauna as one passes from the 

 Mediterranean to the Black Sea, which had long been well known. 



In very recent years a quantitative investigation of phytobenthos (N. 

 Morozova-Wodjanitzkaja, 1936-41), and of zoobenthos (V. Nikitin, 1934, 

 1938 ; L. Arnoldi, 1941), of phytoplankton (S. Maljatzky, 1940 ; N. Morozova- 

 Wodjanitzkaja, 1940) and of zooplankton (E. Kosjakina, 1940; V. Nikitin, 

 1939; S. Maljatzky, 1940) and, finally, of the enormous wealth offish in the 

 pelagic life of the open seas led V. Wodjanitzky (1941) to carry out a thor- 

 ough revision of the data on the biological productivity of the Black Sea 

 (see below). 



In his estimate of the total resources of plant and animal organisms in the 

 Black Sea (not counting fish), Wodjanitzky calculates that there are on the 

 average about 150 g of organisms per 1 m 2 of sea surface, i.e. approximately 

 the same as in the Barents Sea. The productivity of the Black Sea, however, 

 must be several times higher than that of the Barents Sea owing to its much 

 higher temperature. 



Thus as regards its biological productivity the Black Sea should almost 

 occupy the second place in the system of the Mediterranean-Black-Azov- 

 Caspian and Aral Seas. 



Fish migrations 



This gradual increase of biological productivity from west to east in the 

 Mediterranean-Sea of Azov system has produced a peculiar pattern of 

 spawning and feeding migrations of the fish population; it seems, as it 

 were, to consist of three main links, besides a series of secondary ones. 

 This pattern of migration was brought into being largely through the effect 

 of the temperature-salinity range within the limits of the whole basin — in 

 summer time the temperature of the upper layers of water remains almost the 

 same throughout the whole basin, but in the winter the amplitude of its 

 fluctuations is more than 15° ; moreover the eastern part of the basin remains 

 covered by ice for a long time. 



The range of salinity, which is maintained naturally throughout the whole 

 year, is even more marked: from 37 to 38% in the Mediterranean Sea to 

 9 to 10% in the Sea of Azov. 



All these spawning-feeding migrations have a single general direction — 

 eastward for feeding, westward for spawning (Fig. 181). It is possible to 



