82 Appendices to Twenty -iirst Annual Report 



give other than conjectural answers. Sport in the autumn has lately been 

 almost phenomenal on the lower Earn. It remains to be seen whether, 

 with greater possibilities of distribution and breeding, fish will ascend the 

 river at an earlier date. When, however, we think of rivers such as the 

 South and North Esks, or of the Helmsdale and Brora, as early rivers, 

 no very sufficient reason occurs to explain why the Earn should not also 

 be an early river, unless it be that, first, the greater volume of the river 

 Tay has a powerful attraction upon early fish and prompts the great 

 majority of them to pass the sluggish mouth of the Earn, and secondly, 

 that the Earn itself is beset with many and serious obstructions ; and we 

 have already seen that, unless in very large rivers, a slight obstruction is 

 sufficient to stop fish in the cold months of the year. 



In seeking to elucidate the bearing which temperature has upon the 

 salmon's ascent of tributaries, it seems clear that we are dealing with quite 

 different conditions from those which govern the ascent of salmon from the 

 sea to fresh water. The papers on this latter subject indicate the negative 

 result that temperature is not the causal factor, but exercises merely a 

 secondary influence. Once the salmon has entered fresh water, however, 

 the question of temperature seems to play a more important part. The 

 steady ascent of the main river is continued, though early fish certainly 

 pause for considerable periods, and not infrequently descend again, when 

 met by exceptionally cold conditions of water. Tributaries 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 associa- 

 tion 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 volumn. 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 earlj'^ fish. 



