Ardnacrusha; water management, silver eels and cormorants

These are some photos of Ardnacrusha hydroelectric station taken on the morning of the 24th November 2014. In addition to the four turbines being on full load, water was also being released from the spillway at the time of taking these photographs. Overall it is estimated that 500 cubic meters of water per second (cumecs) were being released at Ardnacrusha. Meanwhile flows in the Old River Shannon were being reduced back to the compensation flow of 10 cumecs.

Instead of spilling water though the spillway here we would argue that any ‘excess’ water like this should be released though Parteen weir, as this would help maintain the old river channel. A higher and more variable compensation flow is required for the old River Shannon to restore its good physical and ecological status. At the time of taking the photos below there was approximately 10 times more water going though the Ardnacrusha spillway alone than is normally released down the old River Shannon. For more about compensation flows on the Old River Shannon please see this post.

Ardnacrusha hydroelectric station on the Lower River Shannon, November 2014.
Ardnacrusha hydroelectric station. At the time of taking this photo all 4 turbines were on full load, and and “excess” water was also being released though the spillway.
Ardnacrusha spillway (1)
View of the spillway at Ardnacrusha, November 2014
Ardnacrusha spillway (2)
Spillway at Ardnacrusha with an estimated 100 cumecs of water being released. This water should have been released into the Old River Shannon to help maintain this channel and also assist silver eels.

It is noted that it would also probably be better for silver eel escapement to spill water through Parteen weir as the pressure changes etc. would be less due to the lower head at Parteen weir. It is clear that any silver eels using the spillway at Ardnacrusha could also be damaged or killed.

We are now at the peak of the silver eel run on the River Shannon and most of the eels migrating downstream have to pass though the turbines, and suffer high rates of damage and mortality. We tried to collect some dead silver eels but the flows in the tailrace were too high at the time of our visit.

There were over 120 cormorants foraging in the tailrace of Ardnacrusha during our visit this week, and they were apparently feeding on silver eels killed or damaged after passing downstream. The cormorants make the most of this situation, but the abundance of dead and dying eels probably keeps the local cormorant population artificially high.

Cormorants River Shannon (3)
Cormorants feeding on turbine damaged silver eels at the junction between the boat canal and the tailrace. Water flows in the main tailrace were very turbulent and cormorants were observed feeding in backwater areas like this.
Cormorants River Shannon (2)
Cormorants resting on Caslaunnacorran Tower, near the junction of the tailrace and the Old River Shannon.
Cormorants River Shannon (1)
Cormorants feeding at the confluence of the tailrace and the longshore. A Grey Heron is also apparent in the image.

The ESB operate a ‘trap and transport’ programme for silver eels. The silver eels are captured in coghill nets at Killaloe and transported downstream around the turbines. However, this programme is not operated every day, and is thought to capture only about 30% of the downstream migrating eels when it is being operated. The nets only cover approximately half the width of the river, and any eels captured in the nets are held overnight against the full flow of the river and handled a number of times before their ultimate release. We believe that this ‘trap and transport’ programme is not a suitable mitigation response for the endangered European eel, and only serves to allow the ESB to continue hydroelectric generation operations without compromise. We believe that water should be spilled though Parteen weir and hydroelectric generation suspended during the peak of the silver eel runs, to allow silver eels to safely migrate down the Old River Shannon channel. An increased and variable flow in the old river would also bring significant hydro-ecological benefits.

This is the site of the former Clonlara eel weir on the ‘new’ Shannon (i.e. the headrace to Ardnacrusha hydroelectric station) earlier today. When the ESB were catching silver eels commercially they set massive nets here each night during November to catch eels to sell for huge profits all over Europe. However, once they had to catch silver eels for their ‘trap and transport’ programme they decommissioned this fishery. Most of the silver eels passing downstream now pass through the turbines at Ardnacrusha.
River Shannon, Castleconnell
Meanwhile on the Old River Shannon at Castleconnell, the ESB were tapering back the flows to the base compensation flow of 10 cumecs. A higher and more variable compensation is needed for this waterbody.

For further information please see the following posts:

Also, if you have any queries or comments contact us at

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s