Biologists still seek cause of mass fish deaths in Luumäki lake

Thousands of fish have died in Urpalonjärvi, a lake in the municipality of Luumäki in South Karelia, and initial water samples have not explained the unusual event, Yle reported.

Volunteers collected 7,000 kilograms of dead fish from the lake and its shores over the weekend. The dead fish included roach, pike and perch.

Biologist Roni Seppä and other researchers took water samples after the mass death was discovered last week. The results for nutrients, humus, suspended matter and oxygen were largely normal, as were the pH levels, Seppä said.

Dead fish have also been sent to the Finnish Food Authority for examination. The results may take up to six weeks.

An exceptional sight

Seppä learned about the fish deaths on September 1 and went to the shores of Urpalonjärvi the following day. He said dead fish were visible everywhere, while the lake water had turned brownish green and there were relatively large amounts of blue-green algae.

According to Seppä, the water sample results do not explain the deaths. Fish can survive in considerably poorer conditions, he said.

Urpalonjärvi has always been shallow and rich in humus. Its ecological condition is considered satisfactory, and its fish population has been healthy, Seppä said.

Seppä described the mass death as an exceptional mystery. Large fish deaths have occurred in Finland before, but they have usually been caused by a lack of oxygen in the water.

Possible explanations include a toxic effect from algae, bacteria or a virus, but there is no certainty about the cause. Seppä said any harmful event in the lake may have been very brief.

Further samples taken from river

Urpalonjärvi drains into the Urpalanjoki river. A sample taken from the river on Monday showed low oxygen and pH levels, and Seppä took new samples on Tuesday in an attempt to identify what may have caused the change.

Volunteers from local fishing associations collected fish from deeper parts of the lake using nets, rakes and buckets. The fish were buried one metre underground in accordance with instructions from the Finnish Food Authority.

A large share of the dead fish has sunk to the bottom, raising concerns about the effect on the lake’s nutrient levels. The lake has 22 kilometres of shoreline, making it impossible to clean all of it.

Not all the fish in Urpalonjärvi have died. Seppä estimates that tens of thousands of kilograms of fish remain in the lake.

Luumäki environmental authorities have advised people to avoid using water from Urpalonjärvi for household or washing purposes and to avoid swimming in the lake for now.