Authorities in the Russian province of Dagestan said it was unclear why the mass die-off happened but that it was likely due to natural causes.Regional officials initially reported 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 that the seals likely died a couple of weeks ago.Experts of the Federal Fisheries Agency and prosecutors inspected the coastline and collected data for laboratory research, which didn't immediately spot any pollutants.The fisheries agency has said the overall number of Caspian seals is 270,000-300,000, while the Caspian Environmental Protection Center put the number at 70,000."