Roughly 2,500 seals have been found dead on the Caspian Sea coast in the southern Russian province of Dagestan, officials said Sunday.
The carcasses were discovered at various locations in and around Makhachkala, the coastal capital of the republic.
Authorities said it was unclear why the seals washed ashore but that it was likely due to natural causes.
There was no evidence of foul play and it appeared the seals died a couple of weeks ago, Zaur Gapizov, head of the Caspian Environmental Protection Center, said. He said there was no sign that they were killed or caught in fishing nets.
The Caspian seal is an endangered species and is under special protection, with its population dropping by around 90% in the past 100 years, according to animal rights activists.
There is increasing pollution in the Caspian Sea, where oil, for example, is extracted and according to sea life experts, the seals are perishing from oil leaks, poaching and overfishing.