Since the 1980s, the dynamic environment of the Arctic has caused mass mortality of gray whales. Although gray whales are highly adaptable, climate change poses new challenges that affect their prey and may affect future populations. A new study finds that dynamic changes in Arctic Ocean conditions likely contributed to three major mortality events in eastern North Pacific gray whale populations since the 1980s.
In each of the die-offs, including one that began in 2019 and is still ongoing, gray whale populations have declined by as much as 25% in just a few years, said Joshua Stewart, an assistant professor at Oregon State University's Marine Mammal Institute and lead author of the study.
"These extreme population fluctuations are something we didn't expect to see in a large, long-lived species like gray whales," Stewart said. "When prey availability in the Arctic is low and whales are unable to reach feeding areas because of sea ice, gray whale populations can experience rapid and large shocks. Even a species as mobile and long-lived as gray whales is sensitive to the effects of climate change. When prey quality suddenly declines, gray whale populations can be greatly affected."
The findings have just been published in the journal Science.
The eastern North Pacific gray whale is one of the few large whale species that has recovered to pre-commercial whaling numbers. As gray whale populations approach levels that their Arctic feeding areas can support, they are likely to become more sensitive to environmental conditions due to competition for limited resources, Stewart said.
The adverse Arctic conditions that led to two whale kills in the 1980s and 1990s were not permanent, and whale populations quickly rebounded as conditions improved.
"It turns out we don't really understand what healthy baleen whale populations look like when they're not severely depleted by human impacts," he said. "Our usual assumption is that these recovering populations will reach their environmental carrying capacity and remain more or less stable. But what we're seeing now is that these populations are much more bumpy as they deal with highly variable and rapidly changing ocean conditions."
There are currently about 14,500 gray whales in the eastern North Pacific. They migrate more than 12,000 miles along the Pacific coast every year, migrating from the warm waters off the coast of Baja California, Mexico, in the winter to the cold and fertile waters of the Arctic, where they feed in the summer.
Researchers at NOAA's Southwest Fisheries Science Center in La Jolla, California, have been conducting long-term population monitoring studies of these whales since the 1960s, tracking their numbers, birth and death rates, and monitoring their body condition using aerial imagery. This extensive study makes the gray whale population one of the best-studied of the large whale populations in the world, providing a unique window into the species' population dynamics.
"This study demonstrates the value of long-term data not only for understanding the species being studied, but also for understanding the environment in which that species thrives," said Dave Weller, director of the Marine Mammals and Sea Turtles Division at the Southwest Fisheries Science Center. "When we began collecting data on gray whales in 1967, we had little awareness of the important role gray whales play in understanding the effects of climate change on the Pacific's iconic sentinel species. This study would not have been possible without our reliable long-term records."
The gray whale population in the eastern North Pacific was hunted to the brink of extinction before the whaling moratorium was issued. However, in the post-whaling era, the gray whale population has rapidly recovered and is therefore regarded as a successful example of gray whale protection.
In 2019, as large numbers of gray whale strandings began to appear along the Pacific coast, Stewart, then a researcher at the Southwest Fisheries Science Center, began looking more closely at the long-term data to see if she could learn more about what might be driving the unusual mortality events.
By combining long-term data sets on gray whale populations with extensive environmental data from the Arctic, Stewart and his collaborators determined that two NOAA-declared "unusual mortality events" in 1999 and 2019 were related to Arctic sea ice levels and the biomass of seafloor crustaceans that gray whales feed on.
Stewart also found a third mortality event in the 1980s that followed a similar pattern but was not associated with higher stranding numbers, possibly due to lower reporting rates of stranded whales before the 1990s.
The researchers found that years with less summer sea ice in gray whales' Arctic feeding areas benefit gray whale populations by providing more foraging opportunities. However, gray whales will likely not benefit from reduced sea ice cover in the long term due to rapid and accelerating climate change.
Benthic amphibians, the calorie-rich prey favored by gray whales, are also sensitive to sea ice cover. Algae growing under sea ice sink to the seafloor, enriching amphipod populations. Less ice will result in less algae reaching the seafloor, warmer water temperatures favor the growth of small benthic crustaceans, and faster currents will reduce habitat for gray whales’ favored prey.
"With less ice, you have less algae, which is worse for gray whale prey. All of these factors come together to reduce the quality and availability of the food that gray whales rely on to survive," Stewart said.
For gray whales, loss of prey ultimately leads to death. The most recent incident is still considered ongoing, lasting significantly longer than the previous two incidents.
"We are in uncharted territory right now. The first two incidents, although serious, only lasted a few years," Stewart said. "Mortality events have slowed down recently and there are signs that things are improving, but numbers continue to decline. One reason may be climate change factors, which are causing a long-term trend of declining prey quality."
Gray whales have experienced hundreds of thousands of years of environmental change and have adapted to changing conditions, making them unlikely to become extinct due to climate change, Stewart said.
"I wouldn't say gray whales are at risk of extinction due to climate change," he said. "But we need to think hard about what these changes might mean in the future. The Arctic Ocean, which is already significantly warmer, may not be able to support 25,000 gray whales as it did in the past."