Management plan for the Lower River Shannon

We have been continuing our surveys of the Lower River Shannon. This study is being undertaken to assess the implications of current management regimes on the Lower River Shannon Special Area of Conservation, and in particular the impacts of the reduced post-Shannon hydroelectric scheme flows on this Natura 2000 river. The output of this and other surveys we are completing will be used to inform an urgently needed management plan for the Lower River Shannon SAC.

Our surveys are being undertaken along the river corridor of the old River Shannon between Parteen Regulating Weir and Limerick City. We have also surveyed the artificial headrace/tailrace system of Ardnacrusha hydroelectric station. We have collected extensive baseline video and photographs, and a selection of photos from the surveys we have completed during August/September 2015 are provided below. Click on any of the photos to activate the galley.

We have also provided some selected videos and discussions below.

Old River Shannon, Doonass

This is a video of the much diminished ‘Falls of Doonass’ on the old river Shannon downstream of Castleconnell village, Co Limerick. Most of the water in the Lower River Shannon is now diverted at Parteen Regulating Weir to Ardnacrusha hydroelectric station. The old river gets a compensation flow of just 10 m3 sec-1 when the 95%ile flow in this part of the river prior to the Shannon scheme was approximately 20 m3 sec-1. This extremely low and generally unvaried flow is inadequate to maintain Good Ecological Status in this waterbody. One of the impacts is the encroachment of alluvial woodland, which is very apparent in the video below.

We are currently comparing historical maps with the current day situation so we can quantity the amount of encroachment into the river channel. Although this woodland is an annex I habitat this encroachment is not sustainable.  It is generally relatively young woodland with associated reduced biodiversity and has also been invaded by a number of non-native invasive plant species, most notably Himalayan balsam, reducing its ecological status. It is possible that the reduced and unvaried compensation flow provided by the ESB has favoured the establishment of non-native plant species.

Old River Shannon at Castleconnell Village

This video shows the extensive modifications on the old River Shannon in the vicinity of Castleconnell Village. These fisheries “development” features were installed in the river by the ESB in the 1980’s and 1990’s in a misguided attempt to “restore” the river and cater for the reduced post-Shannon scheme flows. However, these works caused major ecological damage during their construction, and continue to impact on the natural processes on the river today. Rather than enhancing the river’s ecology, many of these intrusive modifications are working against the rivers natural hydro geomorphological and ecological processes, increasing encroachment of vegetation, and therefore reducing the channel’s conveyance capacity.

These features will have to be removed, and a sustainable compensation flow will also have to be provided to comply with the requirements of the EC Habitats and Water Framework Directives.

Ardnacrusha Hydroelectric Station, Lower River Shannon

This hydroelectric station has been in operation since 1929. No fish passes were included when this dam was constructed. A Borland type vertical fish-lift was installed in the 1960’s but this has only been of limited use to migrating fish. Indeed, it is currently used by just a few hundred salmon each year – on a river that has a conservations escapement target of 45,000 per annum. There are no bypasses for downstream migrating fish, including the critically endangered European eel.

In Ireland we banned traditional eel fishing – which can be sustainable and bring significant benefit to rural communities – but placed no restrictions whatsoever on hydroelectric generation. The ESB operate a trap and transport scheme for eels to move them around their dams. However, there are a number of problems with this scheme both for helping elvers move upstream, and allowing adult silver eels pass downstream unharmed on their migration to spawn in the Sargasso sea.  The elver traps are often not operated, and even at their best are inefficient. Most silver eels (and salmon smolts) migrating down the River Shannon still have to pass through the turbines.

The run of elvers last year was widely considered to be the best since the 1980’s, however the elver trap at Ardnacrusha was not operated in time for the run. Therefore a key opportunity to restock the River Shannon with juvenile eels was missed.  However, even when the elver trap at Ardnacrusha is operated it is inefficient, with poor attraction and predator protection. Most elvers that end up in the tailrace are attracted to the turbines and a leaking spillway, never find the trap, and die from starvation or predation. The results speak for the themselves and the low elver catches on the Lower River Shannon are an anomaly when compared to European rivers. Things will have to be improved here.

The ESB traps and transports less than 30% of the adult silver eels migrating down the Shannon, with the rest going through the turbines and suffering a high mortality rate. When there is an exceptional flood on the river water in excess of hydroelectric generation requirements is spilled though Parteen weir, and this potentially increases escapement. However, the question is why do we have to have a major flood to spill water to protect a critically endangered species? The peak runs of silver eels on the Lower River Shannon occurs over only around 20 nights and protecting eels during this period should surely take precedent over hydroelectric generation. The truck and transport programme also involves repeated handling, which can damage and stress the eels resulting in delayed mortality. There is no mitigation for salmon smolts – a species at unfavourable conservation status and listed under Annex II of the Habitats Directive.

For further information on the issues raised in this post, also see the following article.

It is becoming increasing clear that the there needs to be a major change in the way Ireland’s hydroelectric rivers are managed. The output of our current surveys will be used to inform a sustainable management plan for Lower River Shannon and SAC.

For further information please contact us and we would be delighted to hear your views.

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