US researchers say the number of gray whales off western North America has continued to fall over the last two years, a decline that resembles previous population swings over the past several decades.

According to an assessment by the National Oceanic and Atmospheric Administration Fisheries, the most recent count put the population at 16,650 whales - down 38 percent from its peak in 2015-16. The whales also produced the fewest calves since scientists began counting the births in 1994.

An increase in the number of whales washing up on West Coast beaches prompted the fisheries agency to declare an “unusual mortality event” in 2019. Researchers are still investigating the die-off, but they say climate change and its effects on sea ice and prey availability and location are likely factors. Many, but not all, of the whales that washed up appeared malnourished.

The population recovered from the days of commercial whaling before a similar population drop of 40 percent occurred in the late 1980s and early 1990s. Gray whales were removed from the endangered species list in 1994. (AP)