A new study shows that gray whales in the eastern North Pacific, which once flourished before commercial whaling nearly drove them to extinction, have faced significant population drops due to the changing conditions in the Arctic Ocean.
According to the study, there have been three major mortality events since the 1980s, one of which started in 2019 and is still ongoing. These events reduced the gray whale population by an astonishing 25% within a short span.
Joshua Stewart, the lead author of the study and an assistant professor at Oregon State University’s Marine Mammal Institute, expressed concern over these findings.
“These are extreme population swings that we did not expect to see in a large, long-lived species like gray whales,” Stewart said.
“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.”
Surprisingly, these die-offs weren’t permanent. After the die-offs in the 1980s and 1990s, the gray whale population quickly rebounded as the Arctic conditions improved. However, this recovery brought with it its own set of challenges.
As the population has approached levels close to what their Arctic feeding areas can support, they have likely become more sensitive to environmental conditions due to competition for limited resources, explained Stewart.
“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,” he said.
“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.”
The eastern North Pacific gray whale population, currently estimated at around 14,500 individuals, embarks on a massive migration each year. Traveling over 12,000 miles, they move from the warm waters of Baja California, Mexico, during the winter to the chilly, nutrient-rich waters of the Arctic in the summer.
The National Oceanic and Atmospheric Administration Southwest Fisheries Science Center in California has been closely monitoring this population since the 1960s. This extensive data provides a profound insight into the species’ population dynamics.
Dave Weller, the director of the center’s Marine Mammal and Turtle Division, highlighted the significance of these long-term studies.
“This research demonstrates the value of long-term data in understanding not only the species under study but also the environment it depends on,” said Weller.
“When we began collecting data on gray whales in 1967, little did we realize 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.”
Based on long-term data, Stewart, discovered a correlation between the two “Unusual Mortality Events” declared by NOAA in 1999 and 2019 and the sea ice levels in the Arctic.
In addition, these events were closely tied to the biomass of crustaceans on the seafloor, which gray whales mainly feed on.
Interestingly, Stewart identified another die-off event in the 1980s, not associated with increased strandings, possibly due to lesser reporting of stranded whales back then.
Lesser summer sea ice in the gray whales’ Arctic feeding zones initially benefits the whale population by providing more foraging opportunities. However, in the grander scheme of things, decreased sea ice, a direct result of rapid climate change, might be detrimental.
Benthic amphipods, the primary prey of gray whales, rely on algae that grow beneath the sea ice. Fewer ice means less algae sinking to the seafloor, leading to warmer waters that favor smaller crustaceans and faster currents. These conditions reduce the habitat for the gray whales’ preferred prey.
“We are in uncharted territory now. The two previous events, despite being significant and dramatic, only lasted a couple of years,” said Stewart.
“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.”
While gray whales have survived environmental changes over hundreds of thousands of years, the ongoing conditions have raised concerns.
“I wouldn’t say there is a risk of losing gray whales due to climate change,” he said. “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.”
The research is published in the journal Science.
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