Tay District Salmon Fisheries Board

Protecting and Improving The Tay System


Threats to migratory species: Hydro Schemes

Many of the major hydro schemes were built in the 1950s before their full ecological impact was understood and at a time when methods to resolve these problems were poorly developed.
There a four major threats:
- Prevention of upstream migration of adults
- Prevention of downstream migration of smolts and kelts
- Loss of juvenile habitat through lack of flow
- Changes to water chemistry and temperature causing slow growth.

Prevention of upstream migration of adults:
In recent years two Borland fish lifts were thought to operate inefficiently due to poor flows. These have now been modified and are more efficient.
At Pitlochry Dam, grilse sometimes get trapped behind the downstream screens. When this happens the screens are opened and it has been found they leave and enter the pass.
On the River Gaur, the fish pass had insufficient flows to enable passage. Redesign and increased flows now allow salmon passage to many miles of potential spawning grounds. Salmon parr were found above Gaur in 2007 for the first time in many years.

Prevention of downstream migration of smolts and kelts:
Damage to smolts passing though turbines is a risk at some hydro stations although some turbines do little damage to smolts. However, most turbine intakes are screened to prevent smolts from entering. But since greater flow goes through turbine screens rather than down fish passes, there is potential for smolts to get confused and not find fish passes at all. At Clunie Dam, for example, it was found better to remove the screens and allow smolts to pass through the turbines.
However, all dams are screened for kelt sized fish. Their downstream passage is problematic.

Abstraction, loss and changes to water flows:
Without question the worst case of over abstraction was the River Garry. Here 13 km of a main river was virtually bone dry. However, since 2017 there is now a flow once again and salmon are being produced.
In addition to the Garry, there are many more instances of either reduced or regulated flows. Regulated flows are governed by compensation flow regimes which in some cases include artificial freshet regimes. In the past, compensation flows tended to be relatively constant and freshets released on set days, irrespective of other conditions. Nowadays there is recognition, where necessary, freshets might work better if timed to coincide with natural spates. Timing of freshets is now the responsibility of SEPA.
The old large hydro schemes are now being joined by a plethora of micro schemes. In each instance it is the Board's duty to advise on the environmental impact of such works.

Changes to water chemistry and temperature causing slow growth:
Work has been done on this on the River Lyon and Errochty Water. Colder water drawn from the base of dams can cause a reduction invertebrate abundance and slower growth of salmonids.