US researchers say the number of gray whales off western North America has continued to dwindle during the last two years, a decline that resembles previous population swings over the past several decades but is still generating worry.
According to a National Oceanic and Atmospheric Administration Fisheries assessment released on Friday, the most recent count put the population at 16,650 whales – down 38% from its peak during the 2015-16 period. 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”, or UME, in 2019. Researchers are still investigating the die-off, but they say factors probably include climate change, its effects on sea ice and the location as well as availability of prey.
Many – but not all – of the whales that washed up appeared malnourished, the assessment said.
According to the Noaa, some of the stranded whales died from other causes such as being hit by ships or getting hunted by killer whales. The number of stranded whales peaked in 2019, only to then decrease in the following years, suggesting that most of the population decline probably happened in the years shortly after the declaration of the UME.
“There is no one thing that we can point to that explains all of the strandings,” said Deborah Fauquier, a veterinary medical officer in Noaa Fisheries’ Marine Mammal Health and Stranding Response Program. “There appears to be multiple factors that we are still working to understand.”
The population recovered from the days of commercial whaling before a similar population drop of 40% occurred in the late 1980s and early 1990s. Officials removed gray whales from the endangered species list in 1994.
The population rebounded before a spike in whales washing up on beaches prompted the declaration of another UME in 1999 and 2000, when the number of whales fell by a quarter.
Scientists say that although the current population swing so far fits within historical patterns, it is nevertheless concerning.
“We need to be closely monitoring the population to help understand what may be driving the trend,” said David Weller, a director at the Southwest Fisheries Science Center in San Diego.
The majority of gray whales migrate between feeding grounds in the Arctic during the summertime and the lagoons in Baja California, Mexico, in the winter as they nourish their calves.
“This annual roundtrip of more than 10,000 miles exposes them to many stressors along the way,” the Noaa said.
Researchers count the whales as they return from their summer feeding grounds in the Arctic to the lagoons.
Typically, the counts are conducted over a two-year period. But to better monitor the population, Noaa Fisheries is adding a third year to the current survey, counting the whales as they pass the central California coast from late December to mid-February 2023.
The calves are counted as the whales head north to the Arctic. There were 217 calves in the count that finished in May, down from 383 the year before, marking the lowest number of calves since 1994.
