230 FLY FISHING AND SPINNING 



while with a temperature of 36 F. the eggs will take about 

 115 days. It is supposed that after about 160 days the egg 

 may be considered dead, or incapable of producing living 

 alevin. This I am inclined to doubt, Nature under adverse 

 circumstances extending life and preserving the vitality of 

 the eggs in the moist atmosphere of a redd even when 

 exposed by the lack of water in a rainless season. Cold 

 retards the development, but does not deprive the ova of 

 their vitality, so long as the eggs are moist and in a moist 

 atmosphere. From ten to fifteen weeks, may, however, be 

 taken as covering the average period of incubation, provided 

 that no abnormal circumstances arise to check the process. 

 At the end of this period the walls of the egg break open, 

 probably owing to the growth and strength of its inhabitant, 

 and the alevin emerges. It is supposed by some that the 

 young fish nibbles its way through the shell. As, however, 

 the mouth of the alevin does not appear capable of a mus- 

 cular movement sufficiently great to effect this, I think 

 this must be a purely analogous supposition based on the 

 known action of young birds, etc. For a period of about 

 five weeks the alevin generally remains hidden amid the 

 crevices of the gravel and pebbles forming the redd. After it 

 has left the egg, its food is provided by the yolk or umbilical 

 sac attached to its body, which not only hinders its move- 

 ments and compels it to remain within the shelter of the redd, 

 but provides it with sufficient food to sustain life and add to 

 its strength and growth (see Plate XXVII). 



THE SALMON FRY 



Once again a higher or lower temperature plays an 

 important part in shortening or lengthening the alevin 

 stage of the salmon fry. At the end of this stage, the 

 length of which varies from five to eight weeks, it has 

 greatly increased in size, and measures from 1 to 1J 



