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 
