14 THE LIFE OF THE SALMON 



deposit, be it natural sediment or polluting matter 

 of a more toxic sort, this transmission of gases 

 through the egg envelope is hindered or prevented, 

 with more or less serious results. 



The salmon's redd or spawning bed is therefore 

 selected in clean" and not too fine gravel, where a 

 current of water brings a constant supply of the 

 necessary oxygen and prevents deposit. By succes- 

 sive quick movements of the salmon's tail the gravel 

 of the selected spot is displaced and with the aid 

 of the current it is surprising how large the stones 

 overturned in this way frequently are. A shallow 

 trough is thus formed, and in this the eggs are de- 

 posited. In nature all the eggs are not extruded 

 at once, because all the eggs in the ovaries do not 

 become ripe simultaneously. The eggs at the 

 posterior extremity of the ovary become ripe first, 

 and becoming free for extrusion are first deposited. 

 On extrusion they are at once fertilised by the 

 attendant male fish, and are then covered by gravel 

 moved as before. The female then leaves the redd. 

 As the remaining eggs ripen she returns and repeats 

 the process, till after the expiry of several days all 

 the eggs are shed. 



It has repeatedly been noticed that the advent of 

 frosty weather induces the females to frequent the 

 spawning fords, and conversely a spell of mild 

 weather seems to prolong the period during which 

 spawning is carried on. Weather conditions there- 

 fore materially affect the length of time taken, but 

 I am inclined to estimate that on an average the 

 female salmon completes her reproductive functions 



