One of the main problems facing salmon in the River Shannon after the Shannon scheme was opened was that the main flow of water was now coming out of the tailrace. Most of the salmon ascending the River Shannon then ended up at the power house at Ardnacrusha where there was no fish pass. Many of these fish stayed here and died, and this was a major impact of the Shannon scheme. Genuine efforts were made to keep fish out of the tailrace, including the installation of an electric barrier. This however was unsuccessful and this system also resulted in many salmon being killed, before it was removed.
Salmon did continue to enter the Old River Shannon, and did access the fish pass at Parteen and continue to the upper Shannon. However, with the loss of habitat due to reduced flows in the Old River Shannon, the absence of a fish pass at Ardnacrusha, and downstream mortality of smolts due to delays and turbine mortality, salmon numbers and their individual size declined drastically on the River Shannon. There were also on-going changes in the middle and upper River Shannon catchment at this time that would also have affected salmon stocks. These would have included changes in land use due to intensification of agriculture, water pollution, and other factors such as arterial drainage schemes. However, the most significant influence on the River Shannon salmon populations was undoubtedly the Shannon scheme.
At the end of the 1950’s the Borland Fish Lock at Ardnacrusha was installed and opened, and this helped matters relative to the previous situation. At the same time Parteen Hatchery was opened, and the mass stocking of fingerlings and smolts shored up salmon stocks to some degree. In the 1960’s this was the best management approach known, and the methods being rolled out on the Shannon were being widely implemented on the hydro-electrified rivers of North America. However, with the increase of drift netting and disease problems (UDN) in the 1960’s and 1970’s, stocks continued to free-fall. By the early 1990’s there were less than 4,000 salmon passing upstream through the Ardnacusha and Parteen; less than 10% of what should be there based on the habitats present upstream.
But could this get worse? It did and salmon numbers through Ardnacrusha have continued to decline and one would have to conclude at this stage that the Ardnacrusha Borland Fish Lock, along with current salmon management programmes, have been absolute failures. In 2013 less than 500 dalmon passed though Ardnacrusha on a river with a conservation target of 45,000 per annum.
The Ardnacrusha Fish Lock – Consider that fish have to enter one of two 0.68m diameter pipes to access the fish lock. They have to find this entrance in an immense tailrace with up to 400 m3 sec-1 (cumecs) discharging though the turbines at any time. The flow through the fish lock is only 0.5 cumecs and it operated just 6 times in a 24 hour period. Although a cooling water discharge was diverted to the base of the fish-lift in the 1970’s, this provides only limited additional attraction in such a huge river. The reason ESB diverted the cooling water here in the 1970’s was that they knew there was a problem with attraction. This did not resolve it. It is also more than likely that salmon try to enter the draft tubes of the turbines, and get exhausted or killed. There is a major fish passage problem at this facility and this is why salmon counts here have fallen to the low hundreds.
So what could be done?
- The technology for electric fish barriers has improved significantly since the 1940’s as you would expect, and graduated electric field barriers are now widely used in North America with significant success. It is clear that it is time to look at this technology again for deployment in the tailrace. This should be looked into immediately. Fish would then be confined to the Old River Shannon and pass upstream through the relatively suitable fish pass at Parteen weir (applying measures from the previous post). There are also other fish diversion technologies available now that could be looked at comparatively to pick the best solution for the site.
- The above measures would have to be associated with new water management protocols on the Old River Shannon, and reduced hydro generation at Ardnacusha at key times. For example, increase and variable flows (freshets) will need to be initiated through water releases from Parteen Weir. Generation would need to be curtailed at Ardnacrusha during times when peak upstream salmon migration from the estuary occurs (i.e. incoming spring tides in key months). This will cost ESB some money with lost water for generation; however it is clear that the the externalised cost of not having salmon in the River Shannon is no longer acceptable. It is not the 1920’s anymore, salmon are important!
- Alternatively, build a new fish pass. This would be the most expensive option, and would not work as well as keeping fish on the old river directing them to an open pass at Parteen.
It is a national disgrace that salmon escapement through the Shannon dams has fallen so low, when measured by successes on other European Rivers with many more problems than the River Shannon has (i.e. Rhine, Thames, and Seine). There should and can be salmon in the upper Shannon.
Also see the previous posts on this topic:-