The incident happened at a hydroelectric power station in Ballyshannon in April 2014.
More than 300,000 juvenile eels, called elvers, died when traps at the bottom of the dam at Cathleen’s Falls became overwhelmed.
A scientist for ESB said the incident was “quite small”.
He said otherwise it had been a good year for elver ‘recruitment’.
Another eel expert said it was one of the largest fish kills ever recorded in Ireland and has called for changes to eel conservation policy.
Commercial eel fishing was banned on Lough Erne in 2009 as part of European wide efforts to reverse a dramatic decline in eel stocks.
As part of an eel management programme, ESB operates a trap and transport scheme.
Eels are caught by former commercial fishermen, put into tanks, and driven around the hydroelectric powers stations between Belleek and Ballyshannon.
Since 2009, more than 130 tonnes of silver eels have avoided the power turbines in this way.
2014 has been a record year with almost 44 tonnes of eels trapped on Lough Erne, exceeding ESB’s targets.
After being released downstream in Ballyshannon, the eels make their way thousands of miles across the Atlantic Ocean to the breeding grounds of the Sargasso Sea.
The life-cycle continues with the return of elvers to European waters. They are trapped below the dam and then driven upstream to be released back into Lough Erne.
In April 2014, hundreds of thousands of elvers died in the traps below the dam at Cathleen’s Falls.
Despite reports that the traps had been left unchecked for several days over the Easter holiday weekend, ESB said they had become overwhelmed with a sudden run in one night.
ESB confirmed that 112 kg of elvers died. The total elver catch for the year was 533 kg, representing a loss of about 20%.