190 BRITISH FRESH-WATER FISHES 
Another agent suggests itself as possibly prompting the 
return of a fresh-water fish to the river—namely, thirst. Salt 
water is not the native atmosphere of salmon; they endure it 
for a while without inconvenience, but the time may arrive 
when they can endure it no longer, and they may then 
dare every difficulty to reach the refreshing currents of their 
native stream. This can never be more than a hypothesis, 
and as such is worth but little ; nevertheless it is a question 
not to be dismissed as trivial, whether vertebrate animals 
born and reared in fresh water can inhale and swallow a 
strong solution of salt without experiencing some of the 
symptoms which are familiar to ourselves. 
There is a marked difference between the determination 
of a spring salmon and a gravid autumn fish in overcoming 
Salmon Obstacles to their ascent of a stream. In many 
leaps. rivers, a very moderate fall serves to deter fish 
in the early months ; whereas when the spawners are keen 
to get to the “redds” it is surprising what difficulties 
they will overcome. A good example of this is afforded 
by the Helmsdale, a river without nets or artificial impediments 
to the ascent of salmon, reserved exclusively for sport with 
the rod, and therefore an admirable field for studying the 
habits of these fish. Salmon run into this river continuously 
from New Year’s Day throughout the season, and the lower 
eight miles of water often contain a very heavy stock before 
the end of March. But at this distance above the sea there 
are the rocks of Kildonan, where the river tumbles over 
a very moderate fall. Small as it is, this fall serves to bar 
the passage of spring salmon. No fish is ever seen to 
attempt it until the middle of April ; after that, they ascend 
it in hundreds as easily as you or I go upstairs to bed. 
Far more formidable obstacles than Kildonan Fall are 
negotiated by fish in other rivers determined to reach their 
spawning-ground, and in some places many are killed by 
leaping out upon the rocks. Nevertheless the leaping powers 
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