At least 170 endangered seals have washed up dead over the course of several days on the shores of the Caspian Sea in Russia’s republic of Dagestan, according to researchers.

“These are the dead animals that we saw, photographed and whose GPS coordinates we noted,” Viktor Nikiforov of the Moscow Marine Mammals research centre told the AFP news agency on Thursday.

The Caspian seal is a unique marine mammal and it is found nowhere else in the world, but in the Caspian Sea. Its population has rapidly declined by almost 90 percent and the population is now down to around 100,000 due to both human-induced and natural factors driving their extinction, including unsustainable commercial hunting and toxic emissions in the sea.

Seals have for decades suffered from over-hunting and the effects of industrial pollution in the Caspian Sea. Experts say there are now about 70,000 Caspian seals, down from more than one million in the early 20th century. As well as the seals and other endemic species including the famed beluga sturgeon, the Caspian Sea boasts vast energy reserves. Pollution from the extraction of oil and gas there, along with declining water levels due to climate change, pose a threat to many species and put the future of the sea itself at risk. The UN Environment Programme has warned that the Caspian "suffers from an enormous burden of pollution".

A few weeks ago, the deaths of 350 elephants in the popular Okavango Delta nature reserve in Botswana took the world by surprise. The sudden mass deaths of elephants, which scientists have called a conservation disaster, were first spotted during an aerial survey in early May.

The number of elephants died has since risen to over 350, most of them close to watering holes which have prompted suggestions that they might have died from poisoning. But other scientists pointed out that if the water was indeed poisoned, it would have killed other animals as well.

Authorities have also ruled out illegal poaching by ivory hunters, something that is a cause for concern in the region, as the carcasses had their tusks intact when they were removed by government officials.