Barriers to Dispersion of River Fishes 301 



Every year fishes are swept down the rivers by the winter's 

 floods; and in the spring, as the spawning season approaches, 

 almost every species is found working its way up the stream. 

 In some cases, notably the Ouinnat salmon * and the blueback 

 salmon,! the length of these migrations is surprisingly great. 

 To some species rapids and shallows have proved a sufficient 

 barrier, and other kinds have been kept back by unfavorable 

 conditions of various sorts. Streams whose waters are always 

 charged with silt or sediment, as the Missouri, Arkansas, or 

 Brazos, do not invite fishes; and even the occasional floods of 

 red mud such as disfigure otherwise clear streams, like the Red 

 River or the Colorado (of Texas), are unfavorable. Extremely 

 unfavorable also is the condition which obtains in many rivers 

 of the Southwest, as, for example, the Red River, the Sabine, 

 and the Trinity, which are full from bank to bank in winter 

 and spring, and which dwindle to mere rivulets in the autumn 

 droughts. 



Favorable Waters have Most Species. — In general, those streams 

 which have conditions most favorable to fish life will be found 

 to contain the greatest number of species. Such streams invite 

 immigration ; and in them the struggle for existence is indi- 

 vidual against individual, species against species, and not a 

 mere struggle with hard conditions of life. Some of the condi- 

 tions most favorable to the existence in any stream of a large 

 number of species of fishes are the following, the most important 

 of which is the one mentioned first: Connection with a large 

 hydrographic basin; a warm climate; clear water; a moderate 

 current; a bottom of gravel (preferably covered by a growth 

 of weeds) ; little fluctuation during the year in the volume of 

 the stream or in the character of the water. 



Limestone streams usually yield more species than streams 

 flowing over sandstone, and either more than the streams of 

 regions having metamorphic rocks. Sandy bottoms usually are 

 not favorable to fishes. In general, glacial drift makes a suit- 

 able river bottom, but the higher temperature usual in regions 

 beyond the limits of the drift gives to certain Southern streams 

 conditions still more favorable. These conditions are all well 

 * Oncorhynchus tschawytscha Walbaum. 

 t Oncorhynchus nerka Walbaum. 



