The Baikal seal is the smallest in the world, and exactly how and when the species colonized the ancient Siberian lake is still a mystery.
“There were about 130 animals found dead” over the past few days, said environmental ministry spokesman Nikolai Gudkov.
“We took water samples to understand whether we can talk of water pollution as the reason,” he told AFP, though results have not yet been processed.
Scientists have also taken biopsies of the animals, he said.
The animal is not endangered and Gudkov said the species’ population has actually increased in recent years, growing to around 130,000.
Preliminary theories about the die-off did not suggest pollution is the reason, he added.
Lake Baikal, a UNESCO World Heritage Site which has thousands of endemic species, has been suffering from a string of detrimental phenomena over recent years.
These include depletion of fish stocks, death of endemic sponges and explosion of growth of Spirogyra algae unnatural to the lake which scientists say is caused by pollution.