Why are there no salmon in the upper Shannon ? : Reason 1 Smolt Turbine mortality: The floods of May take the smolts away; delay and carnage at Ardnacrusha
The smolt run on the River Shannon peaks in May every year, usually coinciding with a May flood and a rise n water temperature. Smolts from the Shannon upstream of the Shannon dams migrate downstream along the head race canal and meet Ardnacrusha. The smolts here are mainly derived by restocking of juvenile salmon from Parteen hatchery, with small proportion of naturally spawned fish – in all cases now the progeny of previous restocking.
Smolts at Ardnacrusha have two choices; wait in front of the turbine intakes, or make the decent thought the turbines. There is no fish pass to cater for downstream migrants at Ardnacrusha.
The smolts have a problem however. Smolts will normally descend obstacles such as natural cascades and waterfalls at night. This is a genetic behavioural adaptation to help them avoid predators by migrating though obstacles like this under the dark of night. Unfortunately peak hydro generation takes place at Ardnacrusha during the day, with no generation or just the smaller Kaplan turbine working on a partial load at night. ESB introduced a “night time generation protocol” to encourage smolts to pass through the turbines at night. However, there is no evidence that ESB actually do this, and when it does occur only the Kaplan turbine is used at a reduced load.
When turbines are run at lower than full capacity then fish passage mortality increases. It is also noteworthy that Kaplan turbines have a higher mortality rate than Francis turbines (the other 3 turbines at this facility). Notably when ESB did turbine mortality passage tests in the 1990’s they cynically used the Francis turbines at full load – a scenario that smolts rarely face when migrating at night. You would still be looking at an instantaneous mortality rate of 10%, and overall smolt passage mortality is much higher.
If you want to know why there are (practically) no salmon in the rivers above the Shannon dams, smolt delay and mortality is one of the main reasons. Up to 120 cormorants forage every morning at Ardnacrusha feeding on the dead silver eels during the winter, and then the dead smolts in the spring and early summer. Even a short delay above the turbines increases stress on these fish, and causes condition loss. They are also subjected to increased predation when lined up in front of the turbines.
The floods of May take the smolts away? delay and carnage at Ardnacrusha!
Nothing is being done at present to address this serious issue.
So what could be done?
- Arthur Went proposed discharging water through the boat lock at night. He used lights to attract smolts in the fore bay and then opened the locks to flush them out, during the 1960’s. This seemed to work, but was discontinued by ESB due to costs – water in particular that was lost to generation. This could be started again immediately, and should be an interim approach.
- A new water management regime could also be introduced where water would be spilled through Parteen weir during the smolt run. This would also have significant hydro-geomorphological and ecological benefits for the Old River Shannon. This would be costly to ESB in terms of losses to generation, but would not involve any capital costs. This would be a preferred approach.
- Install a smolt bypass system at Ardnacrusha. This would be the most expensive option and would bring limited benefits to the Old River Shannon.
Remember, it does not have to stay this way. There should and can be salmon in the upper Shannon.