THE BALTIC SEA 349 



organisms', typical of a mesohaline zone. The 'euryhaline fresh-water 

 organisms', according to Wilier, are those which are met throughout Frish- 

 haff. In actual fact Willer's highest salinity, his ' polyhaline ' zone, corresponds 

 only to the lower part of Redek's mesohaline zone. The typical, widely dis- 

 tributed euryhaline Medusa, Aurelia aurita, becomes with Wilier a steno- 

 haline brackish-water form. 



F. Riech (1926), also for Frishhaff, and L. Szidat (1926) for Kurishhaff 

 followed practically in the path of Wilier. It is entirely understandable that 

 the whole classification is confused by the introduction of such schemes. The 

 problem becomes more controversial in the case of an independent and even 

 enclosed sea like the Caspian. In his quantitative survey of benthos in the 

 northern part of the Caspian Sea N. Tchougounov (1923) distinguishes the 

 'marine', the 'brackish- water' and other zones. The maximum salinity of the 

 Caspian Sea, except the highly saline inlets of the eastern shores, is no more 

 than 14% ; hence Tchougounov's marine zone has a salinity which is in prac- 

 tice never found in seas. There are sufficient reasons to regard the Caspian 

 Sea as a whole as a brackish-water basin, but according to Tchougounov 'the 

 brackish- water zone' is a narrow band close to the Volga delta. Thus, when 

 drawing separate local schemes of zonation according to salinity, the sub- 

 divisions used must be introduced as small units, after the determination 

 of the place of a given body of water in the general scheme for the seas. 



Fish 



Among the fish population of the Baltic Sea (it is poor in species), Myoxoce- 

 phalus quadricomis, salmon {Salmo salar) and representatives of coregonids 

 {Coredonus laveretus and C. albula) can be ascribed to the brackish-water 

 Arctic relicts. The most important from the commercial aspect are Baltic 

 herring {Clupea harengus membras), sprat {Sprattus sprattus balticus), and cod 

 (G. morrhua). Flatfish (Pleuronectes platessa and P. limanna in the southern 

 part of the Sea and P.flesus in the eastern) are of some importance in fisheries. 

 Fresh-water fish which are of significance for the industry are pike (Esox 

 lucius), golden shiner {Abramis brama), perch (Perca fluviatihs) and some 

 others ; river eel (Anguilla vulgaris) should be added to these fish. The fisheries 

 of the Baltic Sea outside the straits have a yield of about 3 million centners 

 offish, which gives about 80 kg/hectare, while the Gulf of Riga has an annual 

 yield of 500 to 600 thousand centners, or 30 to 36 kg/hectare (1 hectare= 

 1,000 m 2 ). 



The following are most important as food : among the molluscs : Macoma, 

 Mytilus and Lymnaea; among the crustaceans: Pontoporeia, Mesidothea, 

 Corophium, Gammarus, Idothea and Mysis, and among the insect larvae: 

 Chironomidae and Trichoptera. 



