of the Fishery Board for Scotland. 



11 



easy gradient in the pass. A rectangular trough excavated in the weir 

 below the floor of the pass is also, under certain conditions, of great 

 advantage. As " breaks " to descending water the alternating projections, 

 which give a zig-zag direction to the water, adopted by Foster, Forsyth, &l 

 Smith, of Deanston, are now generally discarded, and rows of upright 

 stones are much used. This chute type of pass is, however, only suitable 

 for low weirs. If possible, a pass of easy gradient may be carried round 

 one end of the weir, or even, first of all, in a direction away from it, and 

 then returning to the pool above, the arrangement being adopted as the 

 river bank and bed most readily permit, so long as an easy gradient be 

 secured and the entrance kept close to the foot of the obstruction. It 

 should be understood, also, that an easy and uniform gradient is quite 

 compatable with a pool pass, the various sills of pools being points in the 

 uniform gradient. 



A recent example of a pass of uniform gradient, but with the mouth 

 placed too far from the obstruction, is at the Airthrey Dam Dyke, in Allan 

 Water, just above Bridge- of- Allan. Here the new pass has been carried up 

 the right bank, beyond some flat rock-shelves, the entrance being at the side 

 of the river, but so far from the dyke that ascending fish pass it as they swim 

 up the main flow. The greater part of the pass has a gradient of about 1 in 

 17, but the lowest section has a considerably steeper gradient, so that the 

 water becomes very rapid and broken, and additionally unsuitable for the 

 entrance of fish. The rock ledges which now lie between the pass and the 

 weir are at a considerably higher level tha,n the bed of the river, and, 

 moreover, have sheltered pools some four feet or so in depth between the 

 ledges. Ascending fish naturally run up to those pools, and in the 

 autumn of 1909 they accumulated here in great numbers. Unfortunately, 

 the District Board have agreed, when arranging with the owner of the 

 weir and with the tenant of the adjoining paper mill, as to water flow in the 

 lade, to the placing of a beam all along the sill of the weir so as to heighten 

 it by six inches. This now gives an abrupt copeing at the crest of the 

 dyke/ so that fish ascending from the rocky ledges are quite unable to sur- 

 mount the crest. It seems to me that in allowing this beam to be placed 

 on the sill the District Board have omitted to notice that Clause 7 of 

 Schedule G of the 1868 Salmon Act reads: — "No dam shall be so altered 

 "as to create a greater obstruction to the free passage of fish than at 

 " present exists." In any case, the presence of the beam is now most 

 injurious, for fish do not enter the pass, and are now prevented ascending 

 the weir. On the 3rd and 6th November last, 293 fish were lifted out in 

 landing nets below the weir and carried to the water above. 



With waterfalls the selection of the best place for the entrance is more 

 difficult. The pool or pools below a fall may be extremely deep, and fish 

 may have several "lies," and may move from one lie to another under 

 different conditions of level, or even at different times of the day. Some 

 falls which are partial obstructions are ascended by fish in dead low water ; 

 others can only be negotiated in flood. As in the case of artificial ob- 

 structions, it is important that the entrance be not placed too far down- 

 stream from the fall. The position must be selected in relation to the 

 most suitable level of the river for fish running, coupled with the adapt- 

 ability of the neighbouring rocks for constructing the lower end of the 

 pass. For instance, if " running " fish freely enter a large pool below a fall, 

 but, on account of heavy white water, are unable then to ascend further, 

 the best position for the entrance is not close up to the fall, but close to 

 the " lie " where the running fish congregate below the white water. 

 When the river shrinks to a lower level these fish will probably move up 

 nearer the fall and leap at the obstruction ; but the river is not now in its 

 best running order, though it is reduced so that fish can try the leap. If 

 they can be assisted in their leap good and well — an additional chance of 



B 



