Lies, damn lies, and eel T&T models

Lies, damn lies, and the ESB’s silver eel Trap and Transport models

Trap and Transport (T&T) is where eels are captured and moved around dams and other obstacles. The ESB runs a T&T programme for both upstream migrating juvenile eels (elvers), and downstream migrating silver eels on the River Shannon. This is done as part of the mitigation for the Shannon Hydroelectric Scheme.

The ESB has recently published a model to summarise the results of the 2020/21 silver eel T&T programme. This model is included in the Shannon Fisheries Partnership Report (ESB, 2021). The model presented in Figure 7 of this report is given below. It makes a claim that 91% of the silver eels migrating down the River Shannon during the 2020/21 season made it out to sea unharmed. This claim has been signed off by both Inland Fisheries Ireland and the Shannon Fisheries Partnership. However, this model is totally flawed and contains a number of serious errors which totally undermine its credibility. There is no scientific basis for this claim – it overestimates the benefits of T&T for silver eels and underestimates the impact of turbine passage mortality.

The summary of silver eel production and escapement in the River Shannon during the 2020/2021 eel migrating season (from ESB, 2021)

The ESB has estimated silver eel production in the Shannon catchment by assuming a fishing efficiency rate of 29.2% at Killaloe eel weir. It is claimed that “this fishing efficiency rate is based on Mark-Recapture experiments (n = 14) conducted by NUIG from 2016/17 – 2019/20”. However, the problem with this is that fishing efficiency relates to the average estimated catch efficiency at this site in one night when all the nets are down. It does not relate to the catch efficiency across the full silver eel migration season – as this of course varies depending on the actual fishing effort. The ESB has estimated silver eel production in the catchment based on the catches made at Killaloe during November and December 2020. However, this is not the full silver eel migration season – the peak of which extends from October to March (and indeed some eels will be running all year). Also, the ESB was not operating the nets on all nights during November/December 2020. The private contract fishing crews operating on the Shannon work to a quota and stop fishing when this quota is reached – not when the eels stop running. Moreover, on many nights even during the fishing period not all of the nets were put down – reducing the catch efficiency even further. So there is no scientific basis whatsoever to use the eel catch at Killaloe over a few weeks to estimate the overall annual production in the catchment. They have significantly underestimated silver eel production in the catchment – and significantly overestimated the benefits of the T&T scheme. The model they have produced is not credible and is totally misleading.

Killaloe bridge on the Lower River Shannon, January 2021; ESB did not operate their silver eel Trap and Transport nets on this night (which was ideal on a new moon night and high flow rates), and several checks during this month showed that the nets were not being operated.
High & dry; silver eel Trap and Transport nets at Killaloe, Co Clare, November 2021. The nets were not being operated despite a c.300 cumec flow in the river.

The model also overestimates the numbers of eels that escaped down the old River Shannon through the spillway at Parteen Regulating weir. In the model, they say that 2,645 kg (13.0%) migrated via the Old River Channel. It is claimed that “this is determined by the amount of spillage to the Old River Channel, using a regression model based on historical telemetry studies of route selection”. However, there is no evidence that a regression model for the 2020/21 season was actually prepared and this also seems to be a general guesstimate from a previous NUIG commercial survey (and probably undertaken during a very wet winter). The number of eels that use the spillway at Parteen Regulating Weir is related to the amount of water that is actually spilled at this point. During most of the silver eel migration season in 2020/2021, the ESB was abstracting up to 97% of the flow down the headrace, leaving just 3% of the flow in the river below Parteen Regulating Weir. The eels follow the water and it is highly unlikely that the percentage of eels traveling down the old river channel is more than the percentage of water released at this location. There is again no scientific basis for taking a generic guesstimate figure of 13% and then using a 29.2% catch efficiently figure for the Killaloe eel weir to come up with a figure for eel escapement down the old river channel. The figure of 2,645Kg that they have come up with for eel escapement down the old channel is completely erroneous and fabricated.

The ESB report then goes on to grossly underestimate both the numbers of eels arriving at the intakes of Ardnacrusha hydroelectric station and the turbine passage mortality. They claim that eels passing through the turbines have an estimated mortality of 21.5%. This figure has also come from historical telemetry studies undertaken under commercial contract to ESB by the National Univerity of Ireland, Galway (NUI,G). However, this study used a limited number of silver eels and underestimated turbine passage mortality. It has now been shown that telemetry results for turbine passage studies are unreliable as dead fish will be washed down a river in a similar way to live ones (Havn et al, 2017). The NUIG study also did not assess sub-lethal impacts and indirect mortality such as predation. All of the eels passing through the turbines will be either killed, injured or disorientated. They will all be impacted in one way or another. The large female eels carrying millions of ova are the most severely impacted by turbine passage. They will suffer pressure shocks, hit the running blades, and will often get severed. There is a 30m drop through the turbines here with up to 100 tonnes of water per second passing through blades spinning at 150 RPM (or more for the Kaplan turbine). Many eels will have suffered injuries and will die later – often out at sea days later. Up to 120 Cormorants and many other predators are present in the tailrace when eels are running and the injured and disoriented eels make for easy pickings. None of these impacts were accounted for in the limited NUI,G ESB-funded commercial study which will have erroneously also counted dead and dying eels floating past in the 400 cumec flow in the tailrace. They also only used a very small number of eels in this study. The turbine passage and associated mortality factor at Ardnacrusha is significantly higher than 21.15% and has never been estimated accurately. It is likely that the vast majority of the large female silver eels are killed or injured. Moreover, the figure of 17,674Kg that they have come up with for the numbers of eels migrating down the headrace has no scientific basis and is again completely erroneous and/or deliberately misleading.

Turbine injured eel found dying below Ardnacrusha hydroelectric station, November 2021. See the video here.

There are many other problems with silver eel T&T. Eels are handled excessively. Captured eels will sit the coggle nets all night with the full flow of the river pressing on them. In the morning they will be emptied into a container on a boat. They are then poured into a holding tank and can be held here for several days. They are then netted out, placed into another container, and again poured into a transport tank before being driven to the release point. The eels are caught by private crews working to a quota – the eels are weighed after they are caught but are not weighed at the point of release. Many highly prized and valuable eels go missing. There is no independent monitoring – IFI is not independent, and who exactly are the “fisheries partnership” people? There is significant bycatch in these nets, with thousands of perch, roach, pollan, trout, and salmon being killed here every year. This is the Lower River Shannon SAC yet no Appropriate Assessment of this programme has been undertaken. The vast majority of silver eels migrating downstream still have to go through through the turbines at Ardnacrusha hydroelectric station. There is no downstream fish pass. T&T is unsustainable – we need new fish passes, fish bypasses, and sustainable water management.

T&T is also very expensive to run. The money spent on this programme and other failed mitigation like the salmon hatchery should be invested instead in engineering solutions. However, silver eel escapement could be immediately improved by using spillways and reducing the abstraction at Parteen Regulating Weir. It is unacceptable that ESB continues to abstract up to 97% of the water from the Lower River Shannon SAC.

None of the mitigation provided by ESB for the Shannon hydroelectric scheme has worked – T&T, hatcheries, and the fish passes have all failed. Yet the ESB continues to operate the Shannon hydroelectric scheme in the same way as they did in the 1930s in breach of the EU Habitats Directive, Water Framework Directive, and Eel Regulation.  

What about elver T&T?

I campaigned for years to get the elver traps at Ardnacrusha upgraded. But when you don’t keep a watch on ESB this is what happens. The next post will provide an update on elver T&T and show the ongoing problems with this.