LOCH TAY 89 



considerable periods, and not infrequently descend again, 

 when met by exceptionally cold conditions of water. Tribu- 

 taries entering the lower portions of large rivers are as a rule 

 passed, by all early-running salmon, but upper tributaries if 

 supplied with good flows of water are entered when their 

 temperatures become approximate to the temperature of the 

 main river, or when their thermal conditions are already 

 relatively high. But it is clear that temperature must be 

 treated in close association with water-flow, and it may be 

 with certain peculiarities of water. Hence it follows that if 

 the cause prompting fish to enter any particular tributary at 

 a particular time is sought (and we are dealing exclusively with 

 fish in which the spawning instinct is still absent), we must 

 study the temperature plus the conditions of water-flow. We 

 find salmon in large tributaries when the temperature is that 

 generally associated with winter and spring ; we find fish in 

 smaller tributaries only when the water has lost its wintry 

 character. A certain ratio between temperature and volume 

 would therefore seem to exist, which must be determined in 

 every case ; and, given the infinite varieties of the infinite 

 number of tributaries in Scotland, we are not surprised that 

 different localities should present such different results in times 

 and seasons of successful fishing. 



This deduction has a most important application with regard 

 to the results which may be expected to follow the opening 

 up of natural and artificial obstructions, since such operations 

 influence neither temperature nor volume. On comparatively 

 small tributaries, while most valuable to enable fish to be more 

 widely distributed over suitable spawning grounds towards the 

 spawning season, such works would not influence the ascent of 

 early fish. Also, in large tributaries, such, for instance, as the 

 Tummel, it seems to follow that if any early fish are to ascend 

 to the upper waters, the pass on the falls must be of extremely 

 easy gradient, since even a comparatively slight obstruction, 

 when the water is cold, is sufficient to check early fish. 



LOCH TAY 



As every one knows, Loch Tay has ever been famous as a 

 place for spring fishing where the fish run heavy. The usual 



