Friends of the Irish Environment and Save Bantry Bay first requested a copy of the Marine Engineering Division report earlier this year.
But Agriculture Minister Simon Coveney refused and interested parties in West Cork appealed his decision under internal departmental mechanisms.
Responding to the appeal, the department said the “internal report” was not completed and public interest would not be served by its publication.
Storms in February last had caused the fish cage to break free from its holdings and burst open at Gerahies, leading to hundreds of thousands of farmed fish being let loose into the open bay, where they pose a risk to wild species.
The Dromagowlane, Coomhola, Owvane, Meelagh, Glengarriff and Adrigole rivers are all within 20km from the escape site.
According to Alec Donovan, of action group, Save Bantry Bay, the farmed fish “can interfere with the genetic integrity of protected species and the farmed salmon may inflate catch-based spawning stock estimates to such an extent that the stock appears either to be healthy or recovering, the consequences of which are that conservation measures are either relaxed or not strengthened, or new measures not being introduced”.
However, Richie Flynn of the Irish Farmers Association (IFA) insisted it was “the view of the aquaculture industry that the farmed fish were killed”.
“I can’t comment on a report that I haven’t seen but Murphy’s Seafood suffered immensely in the storm. This was a tragedy and it’s the first time that one of their cages was damaged since 1985 when it first occupied the bay,” he said.
Mr Flynn said that Murphy’s Seafood, which also owns a fish-processing plant, employs up to 60 people in the area and storm damage had put jobs at risk.
Meanwhile, Friends of the irish Environment (FIE) director Tony Lowes said: “The department announced it has given permission for the operators to restock this site. How can they do this if the report on the accident is not complete?”
A similar report on the loss of 80,000 fish at Clew Bay, four years ago, had concluded if “rigorous frequent mooring inspections programme had been in place, it is possible.... there would have been earlier detection which would therefore have avoided the November 2010 failures”.
Mr Lowes said: “We believe this report will show that in spite of the 2010 recommendations, nothing has been done to ensure that these installations are inspected to ensure their viability.” That is why the minister is refusing to release this report.” Mr Flynn said he hopes Murphy’s Seafood “gets every support” so it can overcome the damage to its cages.