1 9 o 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 



