The number of gray whales off the shore of western North America has continued to decrease over the past two years, according to US researchers. This decline resembles previous population swings which have been occurring over the past many decades. 

The assessment, released Friday, was published by the National Oceanic and Atmospheric Administration Fisheries. The most recent population count for the whales is 16,650 - a number which has decreased 38% from its peak between 2015 and 2016. 

In addition, since scientists began counting whales' calves in 1994, the specie has not been reproducing as it had before. 

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Whales have been appearing washed up on the shore in increasing numbers on the US west coast, with the agency noting that this marks an "unusual mortality event" in 2019. 

Researchers say that climate change is the culprit behind the deaths, with global heat strongly affecting the level of sea ice as well as the location and availability of prey. The assessment noted that many if not all of the whales washed up on the shore appeared to be malnourished. 

Stranded whales died from other causes, such as being hit by ships or being hunted by killer whales. The number of stranded whales peaked in 2019 before decreasing afterward. 

In December last year, US scientists suggested that killer whales have expanded their hunting parameters due to the alarming fact that ice in the Arctic sea is melting at unprecedented levels due to global warming. For this reason, killer whales are spending more time in polar waters.

Killer whales, otherwise known as orcas, are animals at the top of the food chain, and the expansion of their hunting grounds - as a result of their increasing hunger - means preying on endangered species, potentially creating an "ecological imbalance" in the Arctic region. This warning was issued by the University of Washington.  

"Through acoustic surveys, we have detected orcas in the Barents Sea in November between Svalbard and Franz Josef Land, so they are clearly following the edge of the ice," Marie-Anne Blanchet of the Norwegian Polar Institute said.

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