When large numbers of gray whales began washing up along North America’s Pacific coast nearly six years ago, marine scientists could only speculate about the reason: was it a disease? Ocean pollution? Increasing ship collisions?
Many of the doomed cetaceans looked thin or emaciated, while others looked torn apart by killer whales. Some had clearly died after being struck by a ship or entangled in fishing gear. Still others gave no observable clues.
Now — after more than 700 gray whales have washed ashore in Mexico, Canada, California and other U.S. states since late 2018 — new research published Tuesday in PLOS One suggests the culprit is a critical drop in food availability in the Arctic and sub-region of the mammals. Food grounds on the Arctic seabed.
What remains unclear, however, is whether this malnutrition was caused by a change in the ocean, or by the whales themselves.
“Did something happen to their food supply during those years that put them under acute nutritional stress, which resulted in many whales being in very poor condition and dying?” said co-author Padraig Duignan, a pathologist at the Marine Mammal Center in Sausalito.
“Or did the number of whales in the population become so high that they competed with each other for food and then part of the population became extinct because they could no longer compete for available resources?” he said.
Read more: Something is killing gray whales. Is this a sign that oceans are in danger?
The research builds on one the National Oceanic and Atmospheric Administration launched in early 2019 after declaring the whale mortality an unusual mortality event (UME). Researchers, observers, and stranding coordinators across North America began working together and warning each other of strandings; sending crews to document and collect tissue samples; and performing necropsies (the animal form of an autopsy).
The NOAA survey has recorded 690 dead whales since January 1, 2019. However, researchers suspect that the actual number is thousands more. Most whales die at sea and sink to the seabed, far out of sight or reach of humans.
Joshua Stewart, a quantitative ecologist at Oregon State University’s Marine Mammal Institute who was not an author on the paper, estimates that the gray whale population was reduced by half during the most recent die-off.
“The population is down by 14,000, down from about 27,000,” he said. “That’s a big drop.”
NOAA declared last week that the die-off was over.
Each year, California gray whales make a round-trip journey of about 13,000 miles from the cold waters of the Arctic to the balmy lagoons of Mexico’s Baja Peninsula, and back again. During the summer months they feed on a variety of bottom-dwelling invertebrates, such as shrimp-like copepods, which thrive in the mud and sand of the Bering, Chukchi and Beaufort Seas. Here they mate and fill their bellies as they prepare for the long journey south to the warm, protected nurseries of the shallow Baja estuaries.
Along the way, they dodge ships and fishing equipment, navigate polluted waters and hide from hungry orcas. They also have to deal with biotoxins and infectious diseases.
So when researchers started looking at the whales’ bodies, they tried to determine which of these various calamities was the primary cause of the population’s extinction.
Although other gray whale deaths have occurred along the Pacific coast, they have been less thoroughly researched.
In 1999 and 2000, 651 whales stranded on land, but only three whales were dissected. Another die-off in the late 1980s was even less studied.
Read more: The West Coast gray whale extinction is over, NOAA declares
This time, however, the scientific research team was large – spread across three countries – well coordinated and had access to new technologies, such as drones, that allowed them to get a more thorough picture of the whales that died and those that remained. empathize.
“I think funding was a big part of it too,” says Stephen Raverty, a veterinary pathologist at the Marine Mammal Research Unit in British Columbia, and lead author of the study. “It really provides an opportunity to respond to these animals. And then we always try to get the information back to the First Nations community or share it with the public. And I think this will encourage more people to actually want to contribute and participate in these efforts.”
He also nodded to co-author Deborah Fauquier, a veterinary physician with the National Fisheries Service’s Office of Protected Resources in Silver Spring, Maryland. He said Fauquier played an important role in organizing information sharing between countries, departments and individuals.
But even with such tools, studying whale mortality is difficult.
Despite the hundreds of whales that washed ashore, researchers were only able to properly examine 61.
That’s because the majority of reported carcasses were either discovered weeks or months after the animal had died – and were far too decomposed for proper analysis – or they were located on remote islands, inaccessible bays or on the bows of ships at sea .
Of the 61 whales they examined, researchers determined the cause of death in just over half. Sixteen were severely emaciated and probably died of starvation; 11 died from blunt force trauma – although two of these whales were also extremely underweight; at least three suffered fatal injuries from killer whales and two became entangled in fishing gear.
In total, 18 whales were considered emaciated, 27 ‘skinny’, nine average and two fat. The nutritional status of the other five could not be determined.
One thing was clear: the deaths were not the result of disease.
Deaths of large animals are often the result of biotoxins, viruses or bacterial infections. The bird flu currently circulating around the world is an example of this. The summer 2023 domoic acid outbreak, which killed hundreds of sea lions and dolphins, is another.
“We haven’t found any evidence of anything resembling an infectious disease,” Duignan said. “There were no telltale signs of infection of any kind. And we did a lot of testing for viruses, bacteria, toxins, and there was nothing significant.”
Read more: California’s wildlife is vulnerable to an avian flu ‘apocalypse’. What causes the spread?
The question now is whether the gray whale population will recover, continue to decline, or have reached a level that is sustainable given the enormous changes taking place in their summer feeding grounds.
Raverty noted that during this latest study, reports of unusual feeding behavior by the whales were observed relatively frequently. Although the biological mantra had always been that gray whales feed only on bottom-dwelling organisms in the northern seas during the summer months – and rapidly the rest of the year – reports emerged that gray whales filter their food and take krill from the surface skimming. in places like San Francisco Bay.
The ability of gray whales to adapt has long been known – but the frequency with which this behavior occurred suggested in some an immediate adaptive response to lack of food, or possibly to behavior that no one had ever really paid attention to.
That’s part of what’s so exciting about this research, Raverty said. It allowed scientists to build a baseline against which they can now make comparisons.
“If we look at another five or 15 years and if we have another recurrence,” he said, they will have this data to compare to.
As for the future and population recovery?
“The way I think about this is … these whales are not going to disappear. They are not going to become extinct,” Stewart said. ‘But if the environment becomes much more marginal, we may not see as many whales as in the past, when we had a really robust, productive Arctic. [seafloor] habitats.”
This story originally appeared in the Los Angeles Times.