COMMUNITIES IN STANDING WATERS 341 



Lake fishes. — Fishes cannot be grouped into the three life regions 

 of lakes in the same manner as the less motile invertebrate animals, 

 because they may range from the littoral region to the abyssal, or 

 from the shore line to open water. For this reason they will be con- 

 sidered separately. 



Only minor differences exist in the composition of the fish fauna 

 of standing waters as compared with that of slowly flowing waters; 

 only the genus Coregonus in glaciated north and middle Europe is 

 entirely limited to lakes; in Siberia and North America a number of 

 species of this genus occur also in rivers and creeks. 



A varied fish fauna composes a part of the great wealth of life of 

 inland lakes. The largest yield of fishes from the rivers is furnished 

 primarily by the migratory fish coining up out of the sea or going 

 down to it, such as salmon, shad, and eels. The fishes of lakes, how- 

 ever, are autochthonic. The Lake of Fayum furnishes fish for all of 

 Egypt, and the Caspian Sea is the large fish basin of south Russia ; 

 the number of carp near the mouth of the Atrek defies estimation or 

 description. 43 



The fact that the smaller pedonic animals are more abundant 

 than those of the open water, a characteristic of the inland waters 

 as compared with the ocean, also determines the haunts of fishes; the 

 fishes of the bottom usually outnumber those of the open water. There 

 is, however, no sharp demarcation between the shore region and the 

 deep water. By far the majority of bottom fishes, of course, are found 

 in the region of plant growth; many, however, also visit the deeper 

 regions. The eelpout (Lota lota) is enticed into depths of 150 m. by 

 the spawn of the salmon, and the European catfish (Silurus glanis) 

 also goes to that depth. The masses of Tubificidae, pea mussels, and 

 gnat larvae in the pedonic mud furnish a rich feeding ground for fish. 

 Careful experiments have shown how the products of the fishing in- 

 dustry in fresh waters depend upon the bottom fauna. 5 In the lakes of 

 south Sweden the amount of bottom fauna fluctuates between wide 

 limits, between 1 kg. and 200 kg. per hectare. Lake Havgard with 

 93.3 kg. of bottom life per hectare produces 25.5 kg. of fish from the 

 same area annually, while Lake Borring with 2.8 kg. of bottom life 

 per hectare produces only 3.0 kg. of fish. The carp, for instance, be- 

 come dwarfed and reach less than a third of their usual length when 

 insufficient bottom life is present. Most predaceous fishes live in the 

 littoral regions. Fishes less often have the pelagic habit in the inland 

 waters than in the ocean; this fact is associated with the correspond- 

 ingly less-developed plankton in comparison with the pedonic fauna. 



