THE CASPIAN SEA 539 



hydrological and biological foundation on which wider and profounder 

 researches were to be based in Soviet times. 



Third period 



Little was added to our knowledge of the Caspian Sea during the sixteen 

 years following Knipovitch's expedition. In this period the following should 

 be noted : N. Tchougounov's work on the census of the North Caspian ben- 

 thos (1923), on the feeding of the young of commercial fish (1918), and on the 

 North Caspian plankton (1921) ; and A. Derzhavin's thorough examination of 

 starred sturgeon, vobla and bream (1915, 1918 and 1922), and certain others. 

 The herring expedition (1930) and the All Caspian Fisheries Expedition 

 (1931-34) concentrated their attention almost exclusively on scientific-trade 

 problems. 



Fourth period 



In 1932 large-scale biological investigations were begun in the Caspian Sea 

 by the Oceanographic Institute and its branches which have eventually de- 

 veloped into a comprehensive study of all sections of oceanography within the 

 system of work of the All Union Institute of Marine Fisheries and Oceano- 

 graphy. During this fourth period the study of the hydrochemical conditions, 

 of the quantitative distribution of life and the phenomena of biological pro- 

 ductivity and of means of acclimatization have become particularly important 

 and widely developed. 



The Astrakhan (1904) and Baku (1912) Scientific Fisheries Stations have 

 played an important part in the study of the Caspian Sea. 



HI. PHYSICAL GEOGRAPHY, HYDROLOGY, HYDRO- 

 CHEMISTRY AND GEOLOGY 



Situation and size 



The Caspian Sea (see Fig. 256) extends in a north-south direction and is about 

 1,204 km long, with a width of from 204 km (opposite the Apsheron penin- 

 sula) to 566 km (in its widest part). 



It lies between 47° 13' and 36° 34' 35" N latitude and between 46° 38' 39" 

 and 54° 44' 19" E longitude. The area of the Sea is 436,000 km 2 . Its volume is 

 about 77,000 km 3 , with an average depth of 180 m. The northern part of the 

 Caspian (north of a line Chechen Island to Tyub-Karagan Point) has an 

 average depth of only 6-2 m and its volume is less than 1/100 of that of the 

 whole Sea (0-94 per cent), whereas in area the Northern Caspian constitutes 

 about 27-73 per cent of the whole. The Central Caspian, if it is bounded on the 

 south by a line from Zhiloy Island to Kuuli Cape, forms a little more than one- 

 third of the volume (35-39 per cent), and about 36-63 per cent of the area of 

 the whole Sea, its average depth being 175-6 m and its greatest about 770 m. 



The Southern Caspian, which is the deepest part of the Sea, has a greatest 

 depth of about 1,000 m and an average depth of 325 m. In volume this part 

 is a little less than two-thirds of the whole body of water (63-67 per cent); 

 and its surface area is 35-64 per cent. The depths of the Central and Southern 



