Seals that rely on sea ice off Alaska's northwest coast have been dying at uncommon rates. The federal marine mammal biologists declared it as an "unusual mortality event". The cause of death for nearly 300 ringed, bearded and spotted seals since June 1, 2018, is not known. However, biologists are trying to rule out probable causes behind these mysterious deaths.

NOAA on possible causes of deaths

According to the fisheries arm of the National Oceanic and Atmospheric Administration (NOAA), the management agency for the marine mammals:

"We're looking at a broad spectrum of possible causes and trying to rule out what we can and narrow it down," NOAA Fisheries spokeswoman Julie Speegle said on Thursday.

READ: In Small Alaska City, Native Women Say Police Ignored Rapes

Viruses, bacteria and algal blooms are possible causes. Water temperature in the northern Bering Sea and the Chukchi Sea the last two summers have been higher than normal. The agency is looking at possible ecosystem influences, including diminished sea ice, Speegle said. Alaska Native coastal communities hunt all three seals for meat and hides as part of subsistence life. NOAA Fisheries received reports of 282 dead seals, almost five times more than the average number of reported strandings, according to the agency. The count included 85 bearded seals, 66 ringed seals, 40 spotted seals and 91 seals that were not identified. The deaths occurred over all age groups. The deaths both years occurred mostly from June to September. Some stranded seals were missing fur or were skinnier than normal, but the time between death and discovery of a carcass hinder investigators.

"Many of the seals have been pretty badly decomposed so it's been hard to identify exactly what the fat layer is," Speegle said.

'Alien' Sea-creature Found In Alaska; Video Gathers Over Million Views

Federal law defines an unusual mortality event as strandings that involve a significant die-off and demand an immediate response. The declaration allows the agency to focus resources on an investigation. Bearded seals get their name from short snouts covered with thick, long, white whiskers. Bearded seals give birth and rear pups on drifting pack ice. Ringed seals are the smallest of Alaska's ice seals and are the main prey of polar bears. They get their name from small, light-coloured circles on their coats. Ringed seals are the only seals that thrive in completely ice-covered Arctic waters. They use strong claws to maintain breathing holes in thick ice and give birth in lairs dug in the snow that accumulates on top of the ice.

READ: Alaska Man Discovers 50-year-old Message In Bottle From Russian Navy

More information by NOAA

Young pups cannot survive in cold water until they have grown a blubber layer. Spotted seals are medium-size seals with light coats and dark spots. They often are found at the outer margins of shifting ice floes. NOAA Fisheries also declared an unusual mortality event following abnormal moulting and deaths of ice seals and walruses from 2011 to 2016. A definitive cause was not identified. But biologists don't believe that event, marked by animals with hair loss and skin lesions, is connected to the current event.

READ: Climate Change, Trade Woes Reshape Frankfurt Auto Show To Protect Cars

The agency estimated that 657 seals were affected over those six years. Ringed seals and some populations of bearded seals are listed as threatened species because of the loss of sea ice due to climate warming. The Center for Biological Diversity in June sued the Department of Commerce and the National Marine Fisheries Service for not designating critical habitat for threatened ringed and bearded seals.