Nearly 500 endangered Caspian seals have washed up dead along Russia's Caspian Sea coast, prompting officials in the republic of Dagestan to launch a multi-agency investigation, regional authorities said Wednesday.

Dagestan's natural resources ministry said teams began surveying roughly 300 kilometers (86 miles) of shoreline after receiving the first reports of seal deaths.

"So far, 484 seal carcasses have been found washed ashore," the ministry said, adding that its search had expanded across several coastal districts. Officials notified relevant agencies to collect and dispose of the carcasses and determine the cause of death.

The ministry noted that seal deaths occur annually during migration season and cited recent research suggesting that asphyxiation caused by natural gas releases from the seabed is a leading factor.

Environmental groups, however, argue that authorities downplay human causes. Independent outlets have reported concerns that industrial trawl nets may be suffocating the animals without leaving visible injuries, a suspicion raised during previous die-offs.

Dagestan has seen several mass seal deaths in recent years, including a 2022 event in which more than 2,500 carcasses were found.

The Caspian seal is listed as endangered by the International Union for Conservation of Nature, with populations down more than 90% since the early 20th century and continuing to decline at an estimated 3-4% annually.

The species has long been threatened by overhunting and industrial pollution.

Oil and gas extraction around the Caspian Sea, along with falling water levels driven by climate change, pose a significant threat to many species in the region. The UN Environmental Program has warned that the sea "suffers from an enormous burden of pollution."

The Caspian Sea, the world's largest inland body of water, is bordered by Russia, Kazakhstan, Azerbaijan, Iran and Turkmenistan.