Key points:- Experts are investigating and searching for any pollutants in the area- There have been several previous incidents of mass seal death- The Caspian Environmental Protection Center says there are just 70,000 Caspian seals leftWarning: Some of the images included in this story may be distressing.Authorities in the Russian province of Dagestan said it was still unclear what had caused the animals' deaths, but that they likely died of natural reasons.However just a day later Zaur Gapizov, head of the Caspian Environmental Protection Center, said after a broader inspection of the coast the number of dead animals was 1,700.Mr Gapizov said the seals likely died a couple of weeks ago, adding there was no sign that they had been killed by poachers.The fisheries agency has said the overall number of Caspian seals is 270,000-300,000, while Mr Gapizov's centre put the number at 70,000."