Gray whales are one of the most remarkable marine mammals, migrating thousands of miles every year between their feeding grounds in the Arctic and their breeding grounds in Mexico.
However, their survival depends largely on the availability and quality of their prey in the Arctic, which is influenced by changing ocean conditions.
A new study by Oregon State University and NOAA Fisheries reveals how gray whales experience major population swings as a result of Arctic conditions.
Three Major Die-Offs Since the 1980s
GUILLERMO ARIAS/AFP via Getty Images
The study, published in the journal Science, analyzed long-term data on gray whale abundance, birth and death rates, and body condition collected by NOAA Fisheries since the 1960s.
The researchers found that gray whales suffered three significant mortality events in the eastern North Pacific since the 1980s, each reducing the population by up to 25% in a few years.
The first die-off occurred in the mid-1980s when a strong El Niño event disrupted the ocean circulation and reduced the productivity of the Arctic ecosystem.
The second die-off happened in the late 1990s when an unusually warm and persistent sea ice cover prevented gray whales from accessing their preferred feeding areas.
Meanwhile, the third die-off began in 2019 and is still ongoing, coinciding with a rapid decline in sea ice extent and duration in the Arctic.
The study's lead author, Joshua Stewart, an assistant professor with Oregon State University's Marine Mammal Institute, said that these were extreme population swings that were unexpected for a large, long-lived species like gray whales.
He explained that when the availability of their prey in the Arctic was low, and the whales could not reach their feeding areas because of sea ice, the gray whale population experienced rapid and major shocks.
Gray whales are one of the few populations of large whales that have recovered from commercial whaling, which nearly drove them to extinction in the 19th and early 20th centuries.
The current population of about 14,500 individuals is estimated to be close to what existed before whaling.
However, this also means that they are approaching the carrying capacity of their environment, which makes them more vulnerable to environmental fluctuations due to competition for limited resources.
The study challenged the assumption that recovering populations would hit their environmental carrying capacities and remain more or less steady there.
Stewart said that what they were seeing was much more of a bumpy ride in response to highly variable and rapidly changing ocean conditions.
The study also suggests that gray whales have a remarkable ability to adapt to changing conditions, as they have rebounded quickly after each die-off when the Arctic conditions improved.
Gray whales have a diverse diet, ranging from amphipods and polychaetes on the seafloor to krill and mysids in the water column.
They also have a flexible behavior, switching between different feeding strategies and habitats depending on the availability of prey.
An evolutionary biologist at the University of California, Berkeley, David Lindberg, who was not involved in the study, said that gray whales had a lot more evolutionary plasticity than anyone imagined.
