COMMUNITIES IN RUNNING WATERS 317 



not simply dependent upon elevation above sea level is proved by the 

 Aksai in the Tien Shan which flows clear and quiet at a height of 

 3000 m. above sea level and contains just as large fish here as farther 

 down. The trout, Sahno fario, reaches a length of only a few inches in 

 the mountain brooks of Switzerland and reaches a weight of only 0.5 

 kg., while in larger rivers, and especially in mountain lakes, it becomes 

 considerably larger. The average weight of all the trout caught in the 

 Aar basin during 1913-1914, for the Aar alone was 240 gm., and for the 

 tributaries alone only 132 gm. 22 An added reason, besides the sparse- 

 ness of food, may be found in the circumstance that these stronger 

 currents demand a much more strenuous use of the muscles of fish in 

 order that they may hold themselves in their chosen location, and that 

 because of this a much smaller margin of food supply is left for growth. 

 In regions where the conditions in headwater streams are less severe, 

 as in central New York or in Illinois, the numbers of fishes may be 

 two to three times as great per square meter of surface in the smaller 

 as compared with larger streams. The average weight per unit is 

 greater, however, in the larger streams. 4 



Fishes ascend to different heights in the mountains, in the Alps per- 

 haps to 2800 m. In the Asiatic highlands two species of fishes have been 

 taken in a spring with a temperature of 18°-20° at a height of 4780 m. 

 above sea level in the southern slopes of the Tan-la. 23 The fact that the 

 genus Nemachilus is represented by four species in these marginal 

 regions of fish distribution perhaps depends upon the circumstance that 

 in these regions of very low barometric pressure the breathing of at- 

 mospheric oxygen supplements the oxygen supply. This has been proved 

 to be true of the Cobitidae. 



Fish migrations.— The migratory fishes form a unique part of the 

 composition of the faunae of inland waters. These include fishes which 

 travel during the spawning season either from the ocean up the rivers, 

 like the salmon, or, more rarely, from the rivers into the ocean, like the 

 eel. Migratory fishes going upstream include the sturgeons, numerous 

 species of salmonoid fishes {Sahno salar in the north Atlantic, species 

 of Oncorhynchus in the north Pacific, species of Coregonus in the 

 Arctic), and several species of shad.* The migration often occurs in 

 great masses in such rivers where civilization has not too greatly 

 altered the environment. In the Rhine near Strasbourg, 143 salmon 

 were caught in one day in the year 1647. In the Columbia and Sacra- 

 mento rivers of the Pacific coast of North America, upstream migra- 

 tion of salmon (Oncorhynchus) lasts from spring until autumn; 24 the 



* Alosa alosa and A. finta in Europe, A. sapidissi77ia in North America. 



