Changing Arctic ice cover and prey availability are the reasons behind gray whales washing up dead along the Pacific coast since 2019, according to a new study.
Scientists have been perplexed by the surge of gray whale strandings since 2019 with estimates suggesting that the mammal’s population in the North Pacific have declined from a peak of around 27,000 in 2016 to about 14,500 in 2023.
“These are extreme population swings that we did not expect to see in a large, long-lived species like gray whales,” Joshua Stuart from Oregon State University’s Marine Mammal Institute said in a statement.
“When the availability of their prey in the Arctic is low, and the whales cannot reach their feeding areas because of sea ice, the gray whale population experiences rapid and major shocks,” Dr Stuart, author of the new study published in the journal Science, said. The study sheds light on the impact of climate change on these marine mammals.
The new research revealed that even highly mobile, long-lived species like gray whales are sensitive to climate change impacts.
Eastern North Pacific gray whales are one of the few populations of large whales in the world that have recovered to levels similar to those that existed prior to commercial whaling – making it a conservation success story in the post-whaling era.
Their population, which currently numbers about 14,500, migrate over 19,000km (12,000 miles) each year along the Pacific Coast – from the warm waters off the coast of Baja California, Mexico, in winter to the cold waters of the Arctic to feed during summer.
They are also the most closely studied large whale population on the planet, offering unique insights into factors affecting the species.
Researchers found from long term observation that unfavourable Arctic conditions led to two previous die-offs of these whales in the 1980s and the 1990s, which were not permanent as the population quickly rebounded with improved conditions.
But when a large number of gray whale strandings began occurring along the Pacific coast in 2019, scientists were perplexed about what was driving this latest unusual mortality event.
Now, by combining long-term data sets on the gray whale population with extensive environmental data from the Arctic, researchers found that the two “Unusual Mortality Events” in 1999 and 2019 are tied to sea ice levels in the Arctic and the biomass of seafloor-living crustaceans that the whales target for food.
They say years with less summer sea ice in the gray whales’ Arctic feeding areas provided increased foraging opportunities that benefited the population.
However, in the long term, decreasing sea ice cover due to rapid and accelerating climate change may not be beneficial to gray whales, scientists warn.
Deep sea shrimp-like crustaceans called amphipods which are the calorie-rich prey that gray whales prefer, are sensitive to sea ice cover, researchers say.
The amphipod population, according to scientists, is enriched by algae that grow underneath sea ice as it sinks to the seafloor.
But less ice leads to less algae reaching the seafloor, researchers warn.
They say warmer water also favors smaller deep sea crustaceans as well as faster currents that reduce habitat for gray whales’ preferred prey.
“With less ice, you get less algae, which is worse for the gray whale prey. All of these factors are converging to reduce the quality and availability of the food they rely on,” Dr Stuart explained.
Less prey availability ultimately leads to gray whale die-offs such as in the most recent event, which is is still considered ongoing, continuing significantly longer than the two earlier events.
“We are in uncharted territory now. The two previous events, despite being significant and dramatic, only lasted a couple of years,” Dr Stewart said.
“The most recent mortality event has slowed and there are signs things are turning around, but the population has continued to decline. One reason it may be dragging on is the climate change component, which is contributing to a long-term trend of lower-quality prey,” he added.
