New research in Science suggests that major mortality events in eastern North Pacific grey whale populations were likely caused by dynamic and changing Arctic Ocean conditions.
“These are extreme population swings that we did not expect to see in a large, long-lived species like grey whales,” says Joshua Stewart, an assistant professor with Oregon State University’s Marine Mammal Institute in the US, and the study’s lead author.
“When the availability of their prey in the Arctic is low, and the whales cannot reach their feeding areas because of sea ice, the grey whale population experiences rapid and major shocks.
“Even highly mobile, long-lived species such as grey whales are sensitive to climate change impacts. When there are sudden declines in the quality of prey, the population of grey whales is significantly affected.”
These whales were hunted to near extinction in the early 1900s by commercial whaling but have since rebounded to estimated pre-whaling population levels.
However, an increased number of strandings in 2019 prompted Stewart, a researcher at the National Oceanic and Atmospheric Administration’s (NOAA) Southwest Fisheries Science Center at the time, to begin looking more closely at what might be driving the unusual event.
During the summer eastern North Pacific grey whales travel to areas in the Pacific to feed on sea-floor dwelling crustaceans.
Stewart and collaborators combined more than 50 years of data on grey whale abundance, calving, body condition, and strandings with extensive environmental data to determine that the “Unusual Mortality Events” declared by NOAA in 1999 and 2019 (which is still ongoing) were tied to both sea ice levels and the biomass of the crustaceans the whales hunt for food.
When low prey biomass coincided with high ice cover, the grey whales population reduced by about 15-20%.
While years with less summer sea ice in the whales’ feeding areas actually benefited the population due to increased foraging opportunities in the short-term. Unfortunately, decreasing sea ice cover as a result of rapid and accelerating climate change will likely lead to reduced availability of the food they rely on in the long-term.
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This is because grey whale’s crustacean prey are also sensitive to sea ice cover, feeding on the algae that grows underneath sea ice and then sinks to the seafloor. Less ice leads to less algae reaching the seafloor, warmer water favours smaller crustaceans than them, and faster currents also reduce available habitat.
“With less ice, you get less algae, which is worse for the grey whale prey. All of these factors are converging to reduce the quality and availability of the food they rely on,” says Stewart.
“I wouldn’t say there is a risk of losing grey 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 grey whales like it has in the recent past.”
