A fish kill in a lake in Co Cavan has set habitat restoration back "by years", a local protection group has said.
An estimated 800 fish were found dead in Lough Sheelin this month.
The Lough Sheelin Trout Protection Association worked with Inland Fisheries Ireland, Cavan County Council and local farmers for decades to improve water quality, for it to be undone almost overnight, treasurer Eamonn Ross said.
"In the last couple of years, we’ve spent well over €100,000 on the rehabilitation of habitat.
"That has had a massive positive impact on the lake. This has set that work back by years.
"If you take 800 fish and half were female, you’d have 400 nests. In a good year you get 2,000, so it’s a big loss to the lake. From our perspective, it is unacceptable that this happens," he said.
A fish kill is where multiple fish deaths are reported from an acute or sudden incident, often caused by the depletion of oxygen in the water.
The Lough Sheelin fish kill was one of three reported to Inland Fisheries Ireland (IFI) in July, with a separate incident in Co Cavan and another in Co Cork.
Around 3,300 fish deaths, mostly trout, were recorded across the three incidents.
Pollution is being investigated as the cause of the fish kill on the Ballinagh River in Co Cavan and the Ballybrack Stream, a tributary of the River Douglas in Cork.
The Lough Sheelin incident was primarily caused by a prolonged spell of hot weather, IFI said, with the Lough Sheelin Trout Protection Association believing it was a contributory factor.
"From the club’s perspective, we don’t believe this is natural," said Mr Ross.
"We believe that something triggered the depletion of oxygen. We accept that water temperatures were high, but we don’t accept that it was water temperature on its own that caused the fish kill," he added.
The club has called for a full investigation into the incident and said that it is asking anglers to voluntarily practice catch and release for the remainder of the season so that stock can be rebuilt.
Multiple factors ‘highly possible’ - IFI
Underlying stresses such as pollution or agricultural practices could contribute to weather-related fish kills, said IFI director of operations Ronan Matson.
"It’s a rare thing that a fish kill in Ireland would be caused purely by low water and high temperatures... If there’s any kind of stress in the river, that could be the straw that breaks the camel’s back and pushes the river over and causes a fish kill," he said.
"A good healthy river would generally be quite robust and resilient to pressures so in a pollution event, a nice, healthy river with good biodiversity, good water quality can generally cope with some of these problems. But a river that has problems ongoing, it can be catastrophic," Mr Matson added.
IFI provides advice to farmers, industry and the public on how to prevent freshwater pollution but also works to strengthen the river network through habitat restoration.
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"That will bring a bit of biodiversity to the rivers, create places for fish to seek refuge, hide out if they need to. If you restore pools into a river, fish have deep water to get into if the water is warm," Mr Matson said.
There is a call for greater enforcement of water quality regulations and stricter penalties on those found to be polluting Irish rivers and lakes.
"The fines are paltry" when IFI does bring polluters to court, said An Taisce’s head of Advocacy Dr Elaine McGoff.
"They are like a slap on the wrist. We need to see far more severe fines for those types of polluters because when you think about it, it's our collective commons, our waterways that they are poisoning. The fine should reflect the severity of the situation they have caused," said Dr McGoff.
Climate change is likely to cause an increased number of fish kills in the future and lead to the degradation of Irish waterways, she said, adding that "warming temperatures are going to really exacerbate underlying pressures on our water bodies".
She said that entire ecosystems could be altered by a major fish kill.
"I heard somebody compare a fish kill to a forest fire, and I think it is a really good analogy. All of the fish can be wiped out from a river and what you’re left with is an entirely different ecosystem and lots of things will depend on those fish," said Dr McGoff.
"It can take years for an ecosystem to recover from that. But the problem is we have ongoing pollution in our water so they’re not getting a chance to recover. So we’re going to just see worse and worse situations. It’s not a great outlook," he said.
