Monitoring silver eel turbine passage mortalities on the Lower Shannon

We are currently assessing silver eel turbine passage mortality at Ardnacrusha hydroelectric station on the Lower River Shannon. Silver eels are adult maturing eels on their spawning migration to the Sargasso Sea after spending up to 20 years (or more) in freshwater. The European Eel Anguilla anguilla is currently listed by the IUCN as a critically endangered species. Current advice from ICES for 2016 is that “the status of eel remains critical and that all anthropogenic mortality affecting production and escapement of silver eels should be reduced to – or kept as close to – zero as possible.” The work that we are undertaking on the Lower River Shannon on silver eel turbine passage mortality is part of a European-wide effort to assess the main anthropogenic impacts on the European eel.  It is becoming increasingly clear that hydroelectric schemes are now the largest cause of mortality for this critically endangered species.

In 2014 the ESB admitted to killing almost 8 tonnes of silver eels at Ardnacrusha hydroelectric station alone, which probably amounts to something like 20,000 silver eels. On the River Erne they estimated that a total of 6 tonnes died at their two hydroelectric stations – which may represent 15,000 silver eels.  But these figures are likely to be significant underestimates – a dead or injured eel is swept down the tailrace as rapidly as a live one and are not easily discerned using any currently available tagging or sonar technologies. More worryingly there is increasing evidence to suggest that the impact of ESB hydro-power impacts on eels is not being accounted for accurately. But even the ESB’s own estimates (c.35,000 silver eel mortalities per year on the Shannon and Erne) do not consider the fact that many more eels die long after passage through turbines as a result of sub-lethal injuries. It’s a long way across the Atlantic ocean to the Sargasso Sea, and a fish has to be very fit and healthy to complete this journey and spawn successfully.

Ardnacrusha Hydroelectric Station, December 2015.
Ardnacrusha Hydroelectric Station, December 2015.
silver eels river shannon
Turbine killed eels, Lower River Shannon, November 2015.
More tubine killed eels from the Lower Shannon, November 2015.
More tubine killed eels from the Lower Shannon.

Passing through a hydroelectric station affects eels (and other fish) in all types of ways; both physically and as a result of stress. Silver eels migrating down a river like the Shannon (or Erne) have to cope with the often start-stop cycle of hydroelectric generation. Silver eels want to migrate when there is a flood in the river, but on rivers regulated for hydroelectricity production peak flows usually occur at peak times of electricity demand – even when there is a flood on the river. Flows on the Lower River Shannon can vary from 400+ cubic meters per second (cumecs) to one twentieth of this in around an hour. When there is very high rainfall in the Shannon catchment, the ESB keep Ardnacrusha hydroelectric station operating to pass water downstream and silver eels arriving into the Lower River Shannon keep going. They travel down the headrace and meet Ardnacrusha hydroelectric station, some try to avoid it – temporarily, some make the run but end up on the trash screens, the rest descend down a c.30m hydrological cliff before colliding with runner blades spinning at 150+ rpms. Many are chopped in half, more are bashed against shear edges, all are subjected to rapid pressure shocks and spins, before boiling into the tailrace and the awaiting predators. Eels approaching a turbine intake know there is something wrong and are reluctant to enter. On the River Shannon eels cannot avoid the turbines and have to go through. On the Erne silver eels are faced with two hydroelectric stations.

Map of the Lower River Shannon showing the location of Ardnacrusha hydroelectric station and Parteen Regulating Weir (click to enlarge).
Map of the Lower River Shannon showing the location of Ardnacrusha hydroelectric station and Parteen Regulating Weir (click to enlarge).

More photos from our November survey of silver eel turbine passage mortalities on the Lower River Shannon below Ardnacrusha hydroelectric station are provided here. Tens of thousand’s of silver eels are being killed at this and other Irish hydroelectric stations each year.

When asked to comment on some of the the photos of turbine killed silver eels we collected during November 2015, Andrew Kerr of the Sustainable Eel Group said “this is a ghastly and up until now hidden tragedy. There are 25,000 hydro power stations across Europe killing the brood stock as they seek to make their autumn migration to the sea. A typical female is carrying over one million eggs so every time you see a dead one think of the million it represents”.

Many of the eels that we found during our November 2015 survey had been severed into parts after passing though the turbines. Others had no visible external damage, but had broken spines and other internal damage. Some were still alive but dying from injuries. This is clearly no way to treat a ‘critically endangered’ species. But the majority of eels killed by turbine passage at Ardnacrusha are never seen due to the large size of the River Shannon, and the fact that the eels run at night and when the river is in flood. Large numbers of cormorants congregate in the tailrace of the power station (120+) and feed on the dead and dying eels. The silver eels are also eaten by other predators such as herons, gulls, otters, mink and pike. Many of the eels keep swimming but die at sea – and out of sight – a few days later as a result of their injuries. After turbine passage these eels may also be more susceptible to marine predators in the Shannon estuary.

The ESB operates a ‘trap and transport’ programme to move eels (silver eels and juveniles) around their dams, but this is neither a sustainable or effective solution. Juvenile eel (elver) traps on the River Shannon have not been operated during the peak elver migration period. Even when operational these traps are very inefficient with many of the elvers migrating upstream never finding the traps and dying from starvation or predation below the dam. It is also noted that over 300,000 juvenile eels died at the ESB elver traps at Cathaleen’s Fall dam on the River Erne in Ballyshannon in 2014. The elver run in 2014 was widely considered to be the best since the 1980s and an opportunity to restock the catchments above ESB hydroelectric stations with juvenile eels was sadly missed. Although monitoring eel recruitment has been identified as a critical part of Ireland’s eel management plan, there is no effective juvenile eel monitoring programme operated in Ireland. Incredibly when it was exposed in 2014 (with photographs, independent witnesses) that the ESB were not operating elver traps on the River Shannon by early May (the main elver run occurred in April), this was not acknowledged in the 2015 ‘Report of the standing scientific committee for eel‘ which risibly claimed that the traps were just down for maintenance for 2 hours! Unfortunately this undermines the credibility of all figures put forward by the ESB/IFI in relation to eel management and highlights the need for a truly independent and scientific committee for eels in Ireland. The data from Ireland’s eel recruitment sites is included in ICES datasets so this also undermines international recruitment indices.

eel turbine passage mortality
Turbine killed eels just below Ardnacrusha Hydroelectric Station, November 2015.

The ESB attempts to catch downstream migrating silver eels in nets and transport them around Ardnacrusha hydroelectric station. However, these eels are handled many times and a significant number are likely to die later due to physical impacts and stress. When they enter the nets they are held against the full flow of the river and then handled several times using nets and bins, stored in holding tanks, before being trucked and released. This handling almost certainly causes damage to the eels but this has not been investigated. The ESB claim that these are hardy fish adapted to migrating in rivers in flood; but there is no evidence to show that they can tolerate being handled seven times or more and held for extended periods as occurs during trap and transport. Eels are sensitive creatures and will suffer from stress, skin damage, and demucinisation during handling at very least. A significant proportion of the silver eels handled in this way will be stressed and injured, and may never spawn. Moreover, only a small proportion of these eels are actually moved downstream in this way, with the vast majority of eels (70%+) still having to pass though the turbines. We believe that the percentage of silver eels which are moved in this way has been overestimated, and that turbine passage mortality has been underestimated.  It is clear that the reporting of eel management in Ireland is not accurate. The large numbers of dead and dying silver eels we found during November 2015 in the Lower Shannon furthermore shows that silver eel turbine passage mortality at Ardnacrusha hydroelectric station is a major problem which has yet to be addressed.

What is immediately required here is the introduction of sustainable water management practices on the River Shannon, to include greater use of spillways and cessation of hydroelectric generation when eels are running.  The peak silver migration takes place over only around 20 nights and there should be no hydroelectric generation during this period.  Spilling water in the old river channel would also bring significant hydro-ecological benefits. When speaking to ‘The Times’ newspaper, Andrew Kerr of the Sustainable Eel Group agreed that “trap and transport” methods had proved largely ineffective. “The turning off of turbines during peak eel movement is clearly the solution. What we know is that their biggest movement is in a really short autumnal burst of rain and it is at night. During that time you could make a huge difference.” See this article here in an article entitled ‘Power plant ‘should be turned off’ to save eels‘. However, it is also clear that new eel passes and bypasses are needed. The latest European Commission report on the outcome of the eel management plans has specifically stated that more attention should be given to management measures related to non-fishing anthropogenic mortality factors, such as hydropower. To date there has been no change whatsoever in hydropower operation protocols in Ireland to facilitate the sustainable management of the European eel.

Turbine killed eels just below Ardnacrusha Hydroelectric Station, November 2015. Thousands of critically endangered silver eels are killed here each year.

Ireland banned traditional eel fishing, which can be sustainable and of significant benefit to rural communities, but has done little else to help the eel. There is no accurate monitoring of eel recruitment, which was identified as being a critical component of the national eel management plan. No opening up of migration pathways. No restrictions have been placed on hydroelectric generation. If we maximised eel recruitment and minimised turbine passage mortality on the River Shannon it would be possible to resume traditional sustainable eel fishing in the future. In the UK the Environmental Agency see addressing non-fishery sources of mortality such as hydropower as being their highest priority, while seeking to maintain a sustainable and economically viable eel fishery. In Ireland this seems to be the lowest priority – we put no restrictions on hydroelectric generation, closed all the traditional eel fisheries and only provided these ‘trap and transport’ schemes which are in no way a sustainable solution. Eel fishing has the potential to employ perhaps 200-300 traditional fishermen in the Shannon catchment. This would be of significant economic benefit to rural communities, unlike the appalling waste that the thousands of dead eels floating down the River Shannon at the moment represents.

Recommended actions

We need to introduce increased and varied flows to the old River Shannon (the original course of the river). The compensation flow for this waterbody was set in the 1920’s and urgently needs to be reviewed. The compensation flow provided is just 10 m3 sec-1 when the natural 95%ile flow would have been approximately twice this volume.  During the winter months the volume of water spilled though Parteen Regulating Weir needs to be significantly increased and varied to simulate natural flood events. Greater numbers of downstream migrating silver eels will follow this water and descend through the spillway. Passage through spillways is almost certainly less harmful to eels than passage through hydroelectric turbines. But there should be no night-time generation at the peak times when silver eels are running (e.g. the days around the new moon periods in November, December). During flood events the spillway at Adnacrusha Hydroelectric Station could be used at night-time in preference to the turbines. Water could also be spilled though the shipping lock.

Parteen Regulating Weir, December 2015.
Parteen Regulating Weir, December 2015.

Maximising the use of spillways is something that could be done immediately. However, a fish bypass system will also need to be installed. This would be used to allow all downstream migrants (e.g. silver eels, smolts, kelts) on the River Shannon to bypass the turbines. The main bypass will probably be located at Parteen Regulating Weir and could be associated with a new upstream fish pass for this location. Research completed by the University College Galway has estimated that the trap and transport programme for silver eels on the River Shannon “may presently be of the order of € 500,000 per year”.  As with other data coming from the River Shannon this is also almost certainty inaccurate. However, it is clear that the costs involved with trap and transport are significant and this money would be better spent on new fish passes and bypasses. New elver traps and passes also need to be built and operated effectively.

The alternative to the above will be increasing pressure to decommission this scheme altogether. What do we want here? A River Shannon with eel fisheries employing 200+ traditional eel fishermen (along with 50,000+ salmon* running upstream supporting an internationally significant recreational fishery) – or an archaic hydroelectric scheme providing <1% of national electricity output and supporting a few privileged jobs. There is a middle ground here. Removal of ecologically destructive dams like this is well under way in the USA, and – as extreme as this may sound now – serious pressure like this will eventually come in Ireland also if we don’t start reducing the serious ecological impacts of Irish hydropower schemes. There is a role for Irish hydroelectric schemes like Ardnacrusha in moving towards a future of reduced greenhouse gas emissions; but we need migratory fish and ecologically healthy rivers also.

*This is the conservation escapement target for the River Shannon above Parteen, the minimum number of salmon which should be ascending past these dams. In reality the average run of ‘wild’ salmon is less than 1,000 per year. 

Further reading

If you have any queries in relation to eels or eel management please do not hesitate to contact us.