Thousands of fish have died in the River Lee in Co Cork over the last number of weeks because of the hot weather.
Salmon, sea trout, brown trout, pike, bream and other species of fish fell victim to a combination of high water temperatures and decreased water flow brought on by the weather.
The conditions bring on stress in the fish, which in turn leads to fish diseases and death.
The South Western Regional Fisheries Board (SWRFB) is investigating the fish deaths and has sent water samples away for analysis.
The ESB at Inniscarra last night spilled additional water through a dam in an effort to increase water flow and reduce water temperature, a SWRFB statement said.
The statement added that no improvement was expected until the weather cools down and rainfall occurs.
