Grey whales experience massive “boom-and-bust” population cycles due to changing Arctic conditions, reveals new research.
Scientists have identified three major die-offs among the eastern North Pacific gray whale population since the 1980s.
During each of the die-offs, including one that began in 2019 and is still ongoing, the gray whale population was reduced by up to 25 percent over just a few years, according to the findings published in the journal Science.
Study lead author Dr. Joshua Stewart said: “These are extreme population swings that we did not expect to see in a large, long-lived species like gray whales.
“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.
“Even highly mobile, long-lived species such as gray whales are sensitive to climate change impacts. When there are sudden declines in the quality of prey, the population of gray whales is significantly affected.”
He said Eastern North Pacific gray whales are one of the few species of large whales that have recovered to what may be similar numbers that existed before commercial whaling.
As the population has approached levels close to what their Arctic feeding areas can support, Dr Stewart says they have likely become more sensitive to environmental conditions due to competition for limited resources.
But the unfavorable Arctic conditions that led to two die-offs in the 1980s and the 1990s were not permanent, and the population quickly rebounded as conditions improved.
Dr Stewart, an Assistant Professor with Oregon State University’s Marine Mammal Institute, said: “It turns out we didn’t really know what a healthy baleen whale population looks like when it isn’t heavily depleted by human impacts.
“Our assumption has generally been that these recovering populations would hit their environmental carrying capacities and remain more or less steady there.
“But what we’re seeing is much more of a bumpy ride in response to highly variable and rapidly changing ocean conditions.”
Eastern North Pacific gray whales, which currently number about 14,500, migrate over 12,000 miles each year along the Pacific Coast, from the warm waters off the coast of Baja California, Mexico, in the winter months to the cold, productive waters of the Arctic to feed in summer.
Researchers at the National Oceanic and Atmospheric Administration Southwest Fisheries Science Center in California have been conducting long-term population monitoring studies of the species since the 1960s.
They tracked numbers, birth and death rates and monitored body condition of the whales using aerial images.
The research has made gray whales the most closely studied large whale population on the planet, providing a unique window into the population dynamics of the species.
Dr Dave Weller, of the Southwest Fisheries Science Center, said: “This research demonstrates the value of long-term data in understanding not only the species under study but also the environment it depends on.
“When we began collecting data on gray whales in 1967, little did we realise the important role they would play in understanding the effects of climate change on an iconic sentinel species in the Pacific.
“This research would not have been possible without our reliable long-term record.”
The eastern North Pacific gray whale, which was hunted to near extinction before a whaling moratorium was enacted, has been viewed as a conservation “success story” because of the population’s rapid recovery in the post-whaling era.
In 2019, when large numbers of gray whale strandings started along the Pacific coast, Dr Stewart, began looking closely at the long-term data to see if he could learn more about what might be driving the deaths.
By combining long-term data sets on gray whales with environmental data from the Arctic, Dr Stewart and his colleagues determined that the two “Unusual Mortality Events” in 1999 and 2019 were tied to both sea ice levels in the Arctic and the biomass of seafloor-living crustaceans that gray whales target for food.
Dr Stewart also identified a third die-off in the 1980s that followed a similar pattern but was not associated with higher numbers of strandings, likely due to lower reporting rates of stranded whales before the 1990s.
The researchers found that 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, a result of rapid and accelerating climate change, most likely will not be beneficial to gray whales.
Benthic amphipods, the calorie-rich prey that gray whales prefer, are also sensitive to sea ice cover.
Algae that grow underneath sea ice sink to the seafloor, enriching the amphipod population.
Less ice leads to less algae reaching the seafloor, warmer water that favors smaller benthic crustaceans and faster currents that reduce habitat for gray whales’ preferred prey.
Dr Stewart said: “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.”
He says the most recent die-off is still ongoing and has continued “significantly longer” than the two earlier events.
Dr Stewart said: “We are in uncharted territory now.
“The two previous events, despite being significant and dramatic, only lasted a couple of years.
“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.”
But he said gray whales have lived through hundreds of thousands of years of environmental change and have adapted over that time to changing conditions, making extinction due to climate change unlikely.
Dr Stewart added: “I wouldn’t say there is a risk of losing gray whales due to climate change.
“But we need to think critically about what these changes might mean in the future.
“An Arctic Ocean that has warmed significantly may not be able to support 25,000 gray whales like it has in the recent past.”
Produced in association with SWNS Talker
