Fishery board officials estimate the death toll may run into the tens of thousands as the numbers of reported dead fish in river systems escalates. Salmon and trout have been literally suffocating as the unusually high evening temperatures have reduced oxygen supplies to dangerous levels. Oxygen is naturally dissipated at night and the hotter the water is, the more quickly this occurs.

An estimated 5,000 fish died of oxygen starvation in Lady’s Island lake in Wexford last week. Fishery board officials said that if the hot weather continues this week more fish will die in other areas. Lakes in Kerry, Cavan and Louth are experiencing major algae blooms, which also reduce oxygen levels to dangerous levels.

The problem in inland water systems is exacerbating the existing threat to salmon stocks from a rise in sea temperatures. Salmon are sensitive to temperature changes, which affect the currents that immature fish use to journey around the Atlantic. In the past decade the number of salmon numbers making their way back to Irish rivers has declined by a third.

Low oxygen levels in rivers and water courses are typically caused by organic pollutants containing micro-organisms that compete for oxygen. The biggest sources include agricultural pollution, known as “diffuse pollution”, caused by bad drainage and the “run-off” of slurry and old sewage plants.

Such pollution has killed salmon in smaller numbers for years, but there is concern that, combined with an increase in seasonal temperatures, the death toll could dramatically rise. Some experts predict a rise in global annual average temperatures of between 1.5C and 5.8C by 2100.

The Avoca River system in Co Wicklow, which has only seen the return of a sizeable salmon run in recent years, has been particularly affected by unusually hot weather. Water levels have fallen so low in a stretch of the river that it can no longer support life.

Donnachadh Byrne, an environment officer with the Eastern Regional Fisheries Board (ERFB), said: “We have been working very hard to return this river to a condition in which it can sustain trout and salmon stocks. The recent hot weather has resulted in such low water levels that pollution has become concentrated making a large part of the river uninhabitable.

“As a result, salmon and trout have become stranded in the upper reaches, where for the time being they can survive.”

The ERFB is also monitoring Lady’s Island lake. “The saturation of dissolved oxygen in the water should be around 100%, but when we were measuring levels there earlier in the week they were down to around 40%, and this can’t support fish life. Plants and algae add oxygen to the water during the day, but during the night they respire, and it is at this time that oxygen can fall to dangerous levels,” said Byrne.

An inquiry by the Environmental Protection Agency into conditions at Lady’s Island was halted when an oxygen-level indicator was stolen from the lake. The loss of the valuable equipment will further hamper attempts to monitor the situation throughout the summer.

Commercial angling fisheries, where fish are bred in shallow lakes or ponds, have been recording the most deaths. Dave Robinson, owner of Rathbeggan Lakes, an angling centre in Co Meath, said: “As water temperatures have gone above 20C its ability to hold oxygen declines. I have installed in my lake system an artificial oxygen replacement system, but even with this a few fish are being lost every day. Other water bodies with murkier water or even a minor amount of pollution are seeing much greater losses which will take a long time to replace, if it is possible at all.”

The loss of game fish may hit the angling tourism industry, which is worth €55m annually.

Fish stocks in Scotland have also been adversely affected by the hot spell, where experts warn stocks of wild salmon could be extinct within a decade. Mike Donaghy, of the nature conservation body WWF Scotland, said that temperatures in many of Scotland’s rivers are reaching “lethally” high levels.