I052 FRESH-WATER BIOLOGY 



America a southward migration has probably not taken place since 

 that time. 



Fishes of cold waters are primarily members of the families of 

 salmon and trout, whitefish, miller's- thumb, and blackfishes. The 

 check by cold has not been placed on any individual migration or 

 limits set to the adult. Rhinichthys dulcis and the many species 

 adapted to the great range of variation in the temperature in any 

 of our temperate lakes shows this. The temperature factor de- 

 termining distribution is set rather by the adaptation of the eggs 

 to warm or cold water. Our trout, salmon, and whitefishes breed 

 largely in winter when the temxperature is low. The rate of de- 

 velopment of their eggs, like that of all cold-water eggs, is slow. 

 The warm-water species are warm-water species not because their 

 individuals are incapable of entering cold water, for many of them 

 do, but because their eggs will not develop in anything but water 

 much warmer than that in which the eggs of cold-water species 

 develop. Their eggs are of rapid development. They are ad- 

 justed to fluctuations in temperature and they respond to such 

 fluctuations in temperature by hastening or slowing their rate of 

 development.^ The point of attack of temperatures is on the eggs 

 and young, not on the adult, and temperature controls distribu- 

 tion through its influence on the eggs. 



In all cold waters of the United States accessible to them, trout, 

 salmon, and whitefish are found. Some of them, the brook trout, 

 Rocky mountain whitefish, Coulter's whitefish and salmon, are 

 adjusted to swift currents; others, the lake trout and many white 

 fishes, to the stagnant waters of lakes. Some of the latter are 

 littoral or abysmal or pelagic, depending on the nature of their 

 food. The elevation of a stream has probably primarily nothing 

 to do with the distribution of its inhabitants, but because elevated 

 waters are usually cold (and frequently swift) ah accessible moun- 

 tain waters are inhabited by cold-water species. The number of 

 species adjusted to cold waters is not as great and their afiinities 



^ The cod eggs which hatch in thirty days at a temperature of from 0.0-2° C. 

 hatch in thirteen days in a temperature of 6-7.9°. Herring eggs which require forty 

 days at a temperature of 2-3.9° C hatch in eleven at a temperature of 10-11.9°; the 

 shad which hatches in eleven days at a temperature of 13.5° hatches in three to five 

 days in a temperature of from 20-23°. 



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