Alo Laas, associate professor of hydrobiology and fisheries at the Tartu-based University of Life Sciences (Maaülikool), investigating the incident, said: "Fish dying at this time of late spring into early summer is not at all common. Normally, fish die due to either pollution in the water, or a lack of oxygen in the lower layers, or for example a bloom of toxic blue-green algae — but all of these can currently be ruled out. This spring has been extremely long and somewhat chilly. So we do need to study them a bit to see whether there are any clear signs, for example, of a disease."

Laas added that clarity on the case is hoped for by the end of this week.

Jakob Kose, owner of the Sauga fishing village, said the dead fish found in his local river, in this case Common roach, was an unusual sight.

"On Sunday the kids came here to fish, then in the morning we saw the outlook was bad, so we took nets and started collecting the roach and white bream. We collected about three boxes of fish on the Sunday. I've run this Sauga fishing village for 11 years, always been by the water, and actually something like this in spring has never happened before," Kose said.

The fish had likely been dead for several days, though no new cases have been seen so far since then, he added.

The Environmental Board said dead fish have also been found in the nearby Reiu and Pärnu rivers, reiterating that disease rather than pollution seems the likely culprit.

Romes Saard, the board's senior inspector of environmental protection, said: "Right now, the Environmental Research Center has taken a water sample to rule out the possibility of pollution, because given that only roaches have died so far, it's unlikely to be pollution but rather some infection or disease. We also took a few dead fish from here as samples, which we will send to the University of Life Sciences for studies to determine what diseases or injuries or whatever they have that might have caused their deaths."

The agency has also received reports in the past month about dead white bream found along the beaches of Pärnu Bay, Saard added, again noting there is no reason to suspect pollution in these cases.

Members of the public who find significant numbers of dead fish should report the case on the national information hotline on 1247, operators generally speak English.

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