When large numbers of gray whales began washing up along the Pacific Coast of North America about six years ago, oceanographers could only speculate about the cause: Was it a disease? Ocean pollution? Are ship collisions increasing?
Many of the doomed marine mammals appeared emaciated or emaciated, while others appeared to have been mauled by orcas. Some had clearly died as a result of being struck by a ship or entangled in fishing gear. Still others offered no discernible clues.
Now — after more than 700 gray whales have stranded 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 decline in food availability for mammals in the Arctic and lower regions. suggests. Arctic seafloor feeding grounds.
But what remains unclear is whether this malnutrition is due to a change in the ocean or the whales themselves.
“Did something happen to the food supply in those years that put them under acute nutritional stress and caused a lot of the whales to be in really bad shape and die?” said study co-author Padraig Duignan, a pathologist at the Marine Mammal Center in Sausalito.
“Or did the number of whales in the population increase so much that they competed with each other for food, and then again some of the population died because they couldn’t compete for available resources?” said.
Read more: Something is killing gray whales. Is this a sign that the oceans are in danger?
The research builds on an investigation the National Oceanic and Atmospheric Administration launched in early 2019 after declaring the whale deaths an unusual mortality event (UME). Investigators, observers, and stranding coordinators across North America began working together; They warned each other about running aground; Sending teams to document and collect tissue samples; and performing necropsy (the animal form of autopsy).
NOAA research has recorded 690 dead whales since January 1, 2019. But researchers suspect the real number is thousands more than that. Most whales die at sea and sink to the seafloor, 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 of the paper, estimates that the gray whale population fell by half during the most recent die-off event.
“The population dropped from 27,000 to 14,000,” he said. “That’s a big drop.”
NOAA announced the death toll last week.
California gray whales make a nearly 13,000-mile round trip each year, from the frigid waters of the Arctic to the tranquil lagoons of Mexico’s Baja Peninsula and back again. During the summer months, they feed on a buffet of bottom-dwelling invertebrates, such as shrimp-like copepods, that thrive in the mud and sand of the Bering, Chukchi, and Beaufort seas. Here they mate and feed themselves, preparing themselves 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.
When researchers began examining the whales’ bodies, they tried to determine which of these various disasters was the main cause of the population’s demise.
Although other gray whale deaths have occurred along the Pacific Coast, they have been less closely investigated.
In 1999 and 2000, 651 whales were stranded, but only three whales were necropsied. Another death in the late 1980s was even less investigated.
Read more: NOAA announces end to West Coast gray whale die-off
But this time the scientific research team was large (spread across three countries), highly coordinated, and had access to new technologies such as drones; This allowed them to build a more comprehensive picture of deceased and surviving whales. live.
“I think funding is a big part of this as well,” said Stephen Raverty, a veterinary pathologist at the Marine Mammal Research Unit in British Columbia and the study’s lead author. “We’re really provided with an opportunity to respond to these animals. And then we’re always trying to get the information back to the First Nations community or share it with the public. And I think that makes more people really want to contribute and participate in those efforts.”
He also gave a nod to co-writer Deborah Fauquier; He was a veterinarian with the National Fisheries Service’s Office of Protected Resources in Silver Spring, Maryland. Fauquier was instrumental in organizing knowledge sharing across countries, departments and individuals, she said.
But even with such sources, whale deaths are difficult to study.
Despite hundreds of whales washing ashore, researchers were only able to properly examine 61 of them.
This was because the majority of reported carcasses were either discovered weeks or months after the animal died and were too decomposed for proper analysis, or were found on remote islands, inaccessible bays, or on the prows of ships at sea.
The researchers determined the cause of death for just over half of the 61 whales they studied. Sixteen were severely emaciated and probably starved to death; 11 died from blunt force trauma; but two of these whales were extremely weak; At least three suffered fatal injuries from killer whales, and two were entangled in fishing gear.
Overall, 18 whales were determined to be emaciated, 27 to be “thin”, nine to be average and two to be fat. In the other five, nutritional status could not be determined.
One thing was clear: The cause of death was not disease.
Large animal deaths are often caused by biotoxins, viruses or bacterial infections. Bird flu currently circulating around the world is an example of this. The domoic acid epidemic that caused the death of hundreds of sea lions and dolphins in the summer of 2023 is another.
“We found no evidence of anything resembling an infectious disease,” Duignan said. “There were no signs of infection. We also did numerous tests for viruses, bacteria and toxins and nothing significant came up.”
Read more: California wildlife vulnerable to bird flu ‘apocalypse’. What causes the spread?
The question now is whether the gray whale population will recover, continue to decline, or reach a sustainable level, given the major changes in summer feeding grounds.
Raverty noted that reports of unusual feeding behavior by whales were observed relatively frequently during this latest survey. While the biological mantra has always been that gray whales feed exclusively on bottom-dwelling organisms in the northern seas during the summer months and feed briskly the rest of the year, there have been reports of gray whales filter-feeding and skimming krill from the surface. In places like San Francisco Bay.
The adaptability of gray whales has long been known; but the frequency with which these behaviors occurred suggested to some that they were an immediate adaptive response to lack of food, or possibly behaviors that no one really cared about.
That’s partly what makes this research exciting, Raverty said. This allowed scientists to establish a baseline from which they can now make comparisons.
“If we look out another five or 15 years, if we encounter another relapse,” they will have this data to compare with, he said.
As for the future and the recovery of the population?
“I think these whales are not going to disappear. They’re not going to become extinct,” Stewart said. “But if the environment becomes much more marginal, we may not see as many whales as we did in the past in the really robust and productive Arctic. [seafloor] living spaces.”
This story was first published in the Los Angeles Times.