The Allt Cuaich will be familiar to many as the almost dry river bed under the A9 just to the north of Dalwhinnie. I use the term almost dry as over the last few years there has been a small residual flow, although I think that prior to that there were periods when with no visible flow.
The Allt Cuaich starts at Loch Cuaich, which lies on the hills above the A9. Loch Cuaich was utilised by the hydro electric companies in the 1930s/40s with the construction of a dam at the outlet exerting complete control over the flow from the loch and other surrounding catchments.
Below the dam there is a further weir across the Cuaich to ensure that water from two significant tributaries is not lost to the hydro system.
There are four sluices on the Cuaich weir each of which is leaking to a certain. Cumulatively the water from each of these leaks forms the entire flow down the Cuaich below the weir.
Last year SEPA completed two electrofishing surveys in the Cuaich, one above and one below the weir. The results from the survey below the weir (midway between A9 and the weir) revealed the presence of three year classes of salmon ; fry, one and two year old salmon parr. This shows that salmon were able to access the Cuaich and spawn in each of the last three years, and possibly earlier, although three year old parr are not encountered often enough to be classed as common anywhere in the upper Spey. These results caused great excitement within the agency and other organisations – obviously all that salmon needed to survive, and apparently thrive was a mere trickle of water.
The finding that salmon were present in the Cuaich was indeed a surprise as the system had been engineered to ensure that salmon couldn’t get into the burn. There is a “heck” across the lower Cuaich which is supposed to stop salmon running the burn during periods of high flow, e.g. when the dam is spilling, so that they don’t get stranded once the flow subsides.
As we were surveying the Truim catchment in detail this summer we included two sites on the Cuaich. We had never surveyed the Cuaich before so two new sites were established, one close to the area where SEPA had found salmon and the other just upstream of the A9.
We found salmon in both sites, although there were no fry in the upper site (same location where SEPA had found fry last year). The densities of parr in both sites were in the moderate categories so they were present in reasonable densities.
At the lower site (nice tranquil site just above the A9) we did find salmon fry as well as parr: maybe only a few spawning salmon made it past the heck last winter.
It is clear from these surveys that salmon are great survivors and colonisers. Conditions must have testing for these fish during the hot dry summer when water temperatures would have been high with all those exposed rocks; although being north west facing would have limited the exposure to the sun. The salmon density may be moderate but the wetted width of the Cuaich is currently a fraction of what it would be naturally. The old Spey spawning records show that up to 96 redds were counted in the Allt Cuaich before the hydro was built. It was probably an important spawning tributary for spring fish which may have spent the summer in the loch before dropping down to spawn.
SSE are proposing introducing a formal compensation flow as part of the Tummel hydro review but it is likely to amount to little more that the current leakage from the weir. The absence of fry in the upper site suggests that much more will need to be done to produce a truly sustainable and viable salmon population in the Allt Cuaich.