There was alarm among Easter visitors to Hillsborough Forest Park when they encountered dozens of dead fish floating on the lake.
According to the Northern Ireland Environment Agency, however, there was no evidence that pollutants had any part to play in what one woman called “a bloodbath massacre of fish”.
“I was horrified to find many dead fish floating in the water,” she said.
“I counted about 50, some in the reeds and some just floating a bit further out.
“It was very disturbing.”
Coming as it did amid reports of an upsurge in the forest park’s rat population, the apparent fish-kill prompted associated discussion online.
Commenting on Facebook about the rising number of rats, Bill Hawthorne said: “That’s not all, there is a lot of dead fish as well in the lake”, and Julie Crothers Potts posted: “Yeah I had two rats run across me on the path this week and the amount of dead fish is awful to see . . .”
With birds apparently steering clear of the fish, one of a number of people to report the matter to NIEA feared something in the water might pose a risk to swans.
“The birds aren’t eating them so they must know there is something wrong with them,” she said.
But during a joint NIEA investigation with the Department of Culture, Arts and Leisure’s Inland Fisheries staff - triggered by initial reports on April 5 - no pollution impact or pollution signs were noted.
A spokesperson said: “DCAL IF determined that the fish mortalities were due to post-spawning trauma, coupled with recent changes in the environmental conditions, this not being an unusual phenomena at this time of year.
“There have been further calls relating to dead fish in the lake since that date with similar cause being determined.”
DCAL IF staff were likewise informed of more recent reports, on april 19 and 20.
A NIEA Water Quality Inspector checked the lake and all the feeder streams were visibly clean.
“He also took a number of readings for oxygen levels and they were satisfactory,” said the spokesperson.
All the dead fish recovered were Roach.