Authorities in the Russian province of Dagestan said it was unclear why the mass die-off happened but it was likely because of natural causes.

Regional officials initially reported on Saturday that 700 dead seals were found on the coast, but the Dagestan division of the Russian Ministry of Natural Resources and Environment later raised the figure to about 2,500.

Zaur Gapizov, head of the Caspian Environmental Protection Center, said in a statement the seals likely died a couple of weeks ago. He added there was no sign that they were killed or caught in fishing nets.

Experts from the Federal Fisheries Agency and prosecutors inspected the coastline and collected data for laboratory research, which did not immediately spot any pollutants.

Several previous incidents of mass seal deaths were attributed to natural causes. Kazakhstan, which has a long Caspian coastline, reported at least three such incidents this year.

Data about the number of seals in the Caspian vary widely. The fisheries agency has said the overall number of Caspian seals is 270,000 to 300,000, while the Caspian Environmental Protection Center put the number at 70,000.