INTRODUCTORY 35 



to explain the presence of trout above the Falls of Clyde by assuming 

 that the stock ascended the river before subsequent elevation of the 

 land and denudation combined slowly formed the now impassable 

 barriers of Stonebyres, Corra Linn and Bonington Falls, and so isolated 

 the trout of the upper waters. The trout above these falls are 

 uniformly yellow, beautifully marked with brown spots and are, whether 

 old or young, as unlike any fresh-run, silvery sea-trout in general 

 superficial appearance as it is possible for any trout to be. On the other 

 hand, for thousands of years migratory trout must have had free access 

 to the Loch Lomond basin, which, in a comparatively recent geological 

 period, has been a sea-loch, and is now only 26.9 feet above mean 

 sea-level, being connected by the short even-flowing Leven with the 

 Clyde estuary. But in contrast with the trout of the upper reaches of 

 the Clyde, hardly any trout are so likely to be mistaken by the angler 

 for sea-trout as are the trout of Loch Lomond. Indeed, in the later 

 months, when sea-trout which have been in the loch for some weeks 

 have lost their silvery sheen, they approximate in appearance so closely 

 to the native non-migratory trout that it is difficult, even for experienced 

 anglers, to distinguish between them. In spring it is a practical 

 impossibility to distinguish young trout of two or three years old from 

 sea-trout parr of equal age. I have been able to discover no features, 

 beyond some small presumptive differences of colouration, by which 

 one kind of fish can be distinguished from the other, and boatmen with 

 life-long experience will seldom undertake to say with any assurance 

 which is which. 



The inference would seem to be legitimate that once the barrier on 

 the Clyde reached a certain height, and trout, though many might 

 descend, were barred from returning, the stock in the upper waters began 

 to revert to the original type of fresh-water trout, or at least to develop 

 along lines of colouration adapted to its isolated fresh-water environ- 

 ment, and has for centuries been so developing ; while, as regards the 



