10 



Appendices to the Twenty -eighth Annual Report 



may be perpendicular. Fish ma) 7 be assisted up a sloping weir, but have 

 to be led round a perpendicular one. With natural waterfalls, however, a 

 very great variation exists, and an infinite number of expedients may be 

 resorted to. No hard and fast rule of any kind can be applied. Each 

 case must be judged separately. A tendency has appeared from time to 

 time to apply one type of pass, which has been successful in one or two 

 cases, to all classes of obstructions. This I believe to be fundamentally 

 unsound. A good type of pass at a comparatively low obstruction may be 

 useless when it has to be adapted to a high fall. 



Amongst preliminary considerations, it has to be borne in mind that no 

 pass, be it ever so good, is as efficacious for the ascent of salmon as the 

 unobstructed channel of the stream ; hence it follows that if conditions 

 permit, and the interests of the salmon alone can be considered, the best 

 possible course is to remove the obstruction altogether. Even a consider- 

 able waterfall can sometimes be entirely cleared away (as has been done 

 in the Tilt). Such a course is almost necessary if spring fish are hoped 

 for above, since an obstruction which would be quickly surmounted by 

 fish in summer or autumn forms a complete barrier when the water 

 temperature is low. There are very many small falls in Scotland up to 

 which spring fish ascend, but beyond which they will not attempt to go till 

 April or May, when the wintry conditions of temperature have left the 

 water. A spring fish does not leap at a fall ; a fish in summer does. The 

 particular time at which fish naturally congregate below an obstruction 

 has therefore a definite bearing upon the question as to the sort of pass 

 which is necessary. A pass with rough broken water is of no use for the 

 ascent of spring fish. 



Similarly, in the case of passes for summer and autumn fish, one has not 

 merely to ask what will the fish be able to surmount, but how can suitable 

 conditions be secured to enable gravid female fish to ascend without diffi- 

 culty. Man's knowledge of the salmon's energy and powers of leaping has 

 too often been a snare in trying to construct a salmon pass. A fairly 

 steep gradient may do well enough if a fish has to surmount only a few 

 feet, while a very easy gradient, when long drawn out, may be too difficult 

 for a running fish if no rest is provided. Undoubtedly, the gradient 

 should in all cases be as easy as possible ; but at the same time, in small 

 streams, if the obstruction is not far up from the sea so that it may be 

 reached by fish in vigorous condition, a very easy gradient is not so 

 essential. 



There are two points of special moment in all passes, apart from con- 

 siderations of gradient — (1) the position of the entrance or lower end, and 

 (2) the most suitable level of water at which to insert the pass. , 



(1) Entrance. — One may have the best possible pass, but if fish do not 

 readily find the entrance it will serve very little purpose. In this con- 

 nection the natural channel of the river below the obstruction must be 

 noted as a guide to the point at which fish will first encounter the obstruc- 

 tion, and the area in which they will naturally lie. In the case of weirs 

 there is usually little difficulty. Moreover, the water flowing over the sill 

 is usually fairly uniform, while the pass, adjusted as it should be at a 

 lower level than the sill of the weir, receives the chief flow. This con- 

 centration of water acts in itself as a useful guide to the entrance. Care 

 must be taken, however, that the entrance is not placed so far down 

 stream from the weir that ascending fish, by the time they reach the 

 obstruction, have already passed the entrance. There is a strong disposi- 

 tion frequently manifested, when it is proposed to make a straight chute- 

 like pass in the down-stream face of a weir which is of rather steep 

 gradient, to project the "pass down-stream into the pool below so as to 

 obtain an easier gradient. It is better to project the pass up-stream into 

 the pool above, cutting deeply into the sill of the weir to allow of the 



