GENERAL CHARACTERISTICS AND GEOLOGICAL HISTORY 377 



distinguish among them two large groups, connected with the thermopathy 

 and halopathy of the corresponding race of fish. Some fish move between 

 zones of small salinity range, keeping always within zones of similar salinities 

 (stenohaline) and during their whole existence living within the boundaries 

 of one body of water ; others can survive during their travels considerable 

 changes of salinity (euryhaline) and can pass from one body of water to 

 another. The same can be said about temperature conditions — some can 

 only survive limited changes of temperature during the year (stenothermic) ; 

 others can live through considerable temperature fluctuations (eurythermic). 

 This is illustrated by the diagram in Fig. 182. 



It is remarkable that Sarda, which populates the eastern part of the Mediter- 

 ranean, moves in the summer to the Black Sea for feeding and spawning. 



Fig. 181. General character of feeding migrations offish (3) in eastern part of Medi- 

 terranean, Black and Azov Seas, contrasted with abundance of plankton (7) (see 

 explanation in text) and with commercial productivity (2) (kg/ha) (Zenkevitch, 1947). 



This is possibly evidence that the past history of the eastern Mediterranean 

 shoal of Sarda was somewhat exceptional — maybe that its fate was linked 

 during some periods of the Quaternary Period with life in bodies of water 

 of low salinity. A series of most interesting regularities was established by 

 A. Svetovidov (1943, 1948, 1957) in his comparison of the taxonomic com- 

 position, distribution, biology and size of fish in the Azov-Black Sea and 

 Caspian basins. First of all, Caspian pelagic fish are larger than those in the 

 Black and Azov Seas. Caspiolosa brashnikovi, with a length of 20 to 35 cm 

 (C. br. brashnikovi) in the Caspian Sea and 16 to 20 cm (C. br. maeotica) in 

 the Black Sea, can be taken as an example. The longest specimens of these two 

 forms of herrings ever found were 49 and 31 cm ; C. caspia caspia is usually 

 18 to 22 cm long, its greatest length being 28 cm, while C. caspia tanaica 

 is 14 to 16 cm long, with a maximum length of 20 cm. This holds true for all 

 the members of the Caspialosa and Clupeonella genera. The same was 

 observed with grey mullet — the largest size of the Black Sea M. saliens is 

 34 cm, while that of the Caspian M. saliens is 39 cm ; M. auratus has corres- 

 ponding lengths of 42 and 54 cm; Atherina mochon pontica reaches a 



