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



Benthos biomass is highest in the summer ; in the autumn it is somewhat 

 higher than in the spring. 



The causes of this type of seasonal change lie mainly in the intensity of 

 the summer feeding of the fish. 



Fish 



Qualitative composition. The Aral Sea fish are represented by 1 1 families and 

 24 species; the family Cyprinidae alone comprises 12 species (50 per cent) 

 and the Percidae 3 species (13 per cent); the other 9 families (including Aci- 

 penseridae, Salmonidae, Siluridae, Esocidae and Gasterosteidae) are repre- 

 sented by only one species. Seven species were brought into the Sea by man 

 in recent years. 



The transplantation of the Caspian Caspialosa caspia aestuarina and two 

 species of Mugil into the Aral Sea was apparently not successful ; the herring 

 and mullet died out, because of the low winter temperature of the Aral Sea. 



G. V. Nikolsky (1940) notes that the fauna of the Aral Sea comprises three 

 genetic communities : (1) the remains of the upper-Tertiary fauna, (2) repre- 

 sentatives of Aral-Caspian fauna and (3) representatives of northern Siberian 

 fish. The Aral-Caspian forms constitute 45 per cent of the fish. They are 

 mainly members of the cyprinid family. The fish of the Aral Sea are much 

 poorer than those of the Caspian. Among the large lakes only Balkhash and 

 Issyk -КиГ are poorer in fish species. There are only nine endemic forms (38 

 per cent) among the fish of the Aral Sea ; moreover, the majority of them are 

 sub-species : there is only one endemic species (Aral barbel). 



The complete disappearance from the original Aral fauna of the members 

 of the families Clupeidae and Gobiidae, which are so characteristic of the 

 Caspian Sea, is most remarkable. Among the 24 species of Aral fish 14 are 

 common to it and to the Caspian Sea and 10 belong to other different sub- 

 species. Thus the Aral Sea fish are closely related to those of the Caspian Sea. 

 'It is well known that a gradual decrease in the number of fish species is 

 observed', wrote Nikolsky, 'as one moves from west to east — from the Black 

 Sea through the Caspian and Aral Seas to Lake Balkhash. Thus the number 

 of species in the Black Sea (without the basin, Slastenenko's data) is more 

 than 170. In the Caspian Sea the number of species falls below 100, in the 

 Aral Sea to 20 and in Lake Balkhash to 8.' Nikolsky thinks that the 'fish of the 

 Aral Sea were evolved from those of the Amu-Daria and its ancient tributary 

 Syr-Daria. 



Fish feeding. G. V. Nikolsky (1940) distinguishes two main biological group- 

 ings of Aral Sea fish — that of the open Sea and that of the coastal zones ; the 

 absence of small, benthos-feeding, comparatively immobile fish is highly 

 characteristic of the Aral Sea (also there are no fish which live permanently 

 away from the coast). The majority of Aral fish are good swimmers feeding on 

 pelagic and bottom fauna. 



The main commercial fish — golden shiner, vobla, bream, Abramis sapa, 

 Pelecus and Chalcalburnus — feed in the open parts of the Sea from the middle 

 of May to October at depths of 1 5 to 30 m and on the grey mud ; nevertheless 



