The gray whale population has plunged 24% along the West Coast since the last estimate in 2016, estimates released Tuesday show.
The population of eastern north Pacific gray whales decreased by more than 6,000 whales, from 26,960 in 2016 to 20,580 estimated from counts made during the whales’ southbound migration in the winter of 2019-20, according to the National Oceanic and Atmospheric Administration’s Southwest Fisheries Science Center in San Diego.
The population has been increasing overall since counting began in 1967, when only 13,426 of the mighty grays were counted.
The eastern north Pacific gray whale undertakes one of the longest migrations of any mammal, traveling 10,000 miles from its feeding grounds in cold Arctic seas to calving lagoons in Mexico.
The whales are a conservation success story, surging back in numbers after the enactment of federal protection under the Marine Mammal Protection Act in 1972. Gray whales neared extinction in the 1950s after excessive commercial hunting, and were listed as an endangered species. The whales were removed from the endangered species list in 1994.
Commercial hunts remain illegal.
The cause of the most recent die-off in the population, called an “unusual mortality event” by NOAA, is not known. It is possible the population reached carrying capacity. Another possible cause is disruption of the ocean food web caused by climate change.
The whales have rebounded from an “unusual mortality event” before, weathering a decline of nearly the same size, from 1999-2000.
A total of 651 gray whales were reported dead in that event, with 283 deaths in 1999 and 368 in 2000 along the West Coasts of the U.S., Mexico and Canada.
The cause was not determined but some stranded whales were in poor body condition, leading to the question of whether the population had reached carrying capacity at 21,000 grays estimated in the eastern Pacific.
But the population rebuilt to even stronger numbers by 2016 — only to decline again.
Big fluctuations for these big whales are not rare, and short-term declines have not resulted in any detectable longer-term impacts on the population of the species, according to the technical memorandum reporting the population estimates.
The new population estimate is based on counts of eastern north Pacific gray whales migrating southward off the Central California coast between December 2019 and February 2020. The counts were made from a shore-based watch station south of Carmel by 21 trained observers over 48 survey days. The highest daily count of whales was 169 grays on Jan. 17, 2020.
Senior research biologist John Calambokidis, a founder of the nonprofit Cascadia Research Collective in Olympia, said the count confirms what scientists suspected, that the whales observed washed up on the beach during the most recent “unusual mortality event” were only a fraction of the number dying.
The short- term crash also paints a picture of a population experiencing wider fluctuations than many scientists had expected, Calambokidis said in an email, even if part of the reason is that the population is at or near carrying capacity.
“Perhaps these wider fluctuations are one of the impacts of the dramatic changes going on in the Arctic as a result of climate change, sometimes benefiting gray whales by opening up more ice-free areas for gray whales but also resulting in wider fluctuations in the prey available to them,” Calambokidis said.
“I think the key now will be whether the mortality even remains short (1-2 years) as was the case in 1999-2000 and is not the start of a longer decline.”