THE CASPIAN SEA 633 



their migration (except for that of herrings). In the literature there are but 

 few indications of the approach to the shores of some bullheads and marine 

 pike perch for spawning (N. Tchugunov and F. Egerman, 1932). Fish of 

 Mediterranean origin (Atherina, Pomatoschistus and Syngnathus) move into 

 the saline southeastern corner of the Caspian Sea for spawning, which they 

 do at a salinity of 30% o . 



The question of the causes which compel fish to accomplish long and com- 

 plicated migrations is an extraordinarily intricate one. To solve it we have to 

 turn to geological data. Some workers point to the extreme importance of the 

 post-glacial loss of salinity of the Caspian Sea and its effect on the working 

 out of the migrational rates of Caspian fish. Ya. Birstein writes (1935) 

 that ' the difference between a sea and a body of fresh water at that time had 

 probably become so negligible that for assimilated (formerly) marine fauna 

 the river was no longer an alien medium ; fresh-water fish also could readily 

 extend their habitat into the Sea, which was formerly closed to them owing to 

 us physicochemical conditions. The subsequent gradual increase in salinity 

 had apparently only slightly affected the habitats of fresh-water fish which 

 had mainly been formed in post-glacial time ; it may, however, have some- 

 what reduced their distribution in the Sea. It may have been this, in fact, 

 which assigned fish to the biological types — migratory, semi-migratory and 

 fresh-water non-migratory — which have already been established by Kessler.' 

 Schmidt thinks that ' the migratory routes of the herring now observed may 

 have begun to be developed at the end of the glacial period. When with the be- 

 ginning of the ice recession great torrents of fresh water began to flow towards 

 the Sea, some species of herring, probably already more adapted to fresh 

 water, used them for spawning, and the range of their migrations increased 

 more and more with the further withdrawal of ice and the lengthening of the 

 rivers.. Other herring species have remained marine or semi-migratory forms 

 up to our time.' 



Numerous species of the Clupeidae are the main consumers of plankton ; 

 not all, however, for some of them are predators (Fig. 296). Among the plank- 

 ton-eating Clupeidae three pelagic species of the genus Clupeonella (Clupeo- 

 nella and Sprattus phalericus), which form large colonies in the Caspian Sea, 

 are distinguished by their small size. Volga and Caspian herrings are also 

 plankton eaters, whereas the Brashnikov and Saposhnikov herrings are typical 

 predators. Some forms have a mixed diet as, for example, Caspialosa kessleri. 

 A. Behning (1938) showed that plankton Copepoda {Ewytemora grimmi) 

 feed mostly on Flagellata and unicellular algae. Mysids feed also on these 

 forms, as well as on small crustaceans. Sprats and herring-fry feed mostly on 

 copepods, while Caspialosa caspia feeds on copepods and pericardians. The 

 seal feeds on sprats and on Caspialosa caspia. A general diagram of the food 

 chain of Caspian plankton-eaters can be drawn from the data available 

 (Fig. 297). 



Feeding of benthos-eating fish. The problem of the nutrition and feeding correla- 

 tions of Northern Caspian benthos-eating fish has been carefully investigated by 

 Schorygin(1952). His research can be regarded as a model of this type of study. 



