by When large numbers of green whales began washing up along North America’s Pacific Coast nearly six years ago, marine scientists could only speculate as to the cause: Was it disease? Ocean pollution? Increase ship collisions?
Many of the grieving cetaceans looked thin or emaciated, while others looked like ripped orcas. Some apparently died after being hit by a ship, or entangled in fishing gear. Still others had no discernible clues.
Now — after more than 700 gray whales have washed ashore in Mexico, Canada, California and other US states since late 2018 — new research published Tuesday in PLOS One suggests the culprit may have fallen significantly with the availability of food in the Arctic and in submammals. Arctic seabed feeding stations.
What is unclear, however, is whether the change in the ocean, or the whales themselves caused this malnutrition.
“Did something happen to their food supply during those years that put them under severe nutritional stress and resulted in many whales being in poor condition and dying?” 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 high that they competed with each other for food and then again, part of the population died because they could not compete for the available resources?” he said.
Read more: Something is killing green whales. Is it a sign that the oceans are in danger?
The research builds on an investigation launched by the National Oceanic and Atmospheric Administration in early 2019, after it declared whale deaths as an unusual mortality event, or UME. Researchers, observers and stranding coordinators across North America began working together — alerting each other to tradies; send teams to document and collect tissue samples; and performed necropsies (an animal form of autopsy).
A NOAA investigation recorded 690 dead whales since January 1, 2019. However, researchers suspect that the true number is thousands more than that. Most whales die at sea and sink to the bottom of the sea, far beyond the 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 green whale population was halved during the latest mortality episode.
“The population is 14,000 down from 27,000 or so,” he said. “That’s a big fall.”
NOAA announced last week that the death had ended.
Every year, California green whales make a 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 eat a smorgasbord of bottom-dwelling invertebrates, such as crustaceans such as shrimp, which thrive in the mud and sand of the Bering, Chukchi and Beaufort seas. Here they mate and fill their bellies, preparing themselves for the long journey south to the warm, protected nurseries of Baja’s shallow estuaries.
Along the way, they avoid ships and fishing gear, 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 figure out which of these various calamities was the main cause of the population’s death.
Although other green whale deaths have occurred along the Pacific Coast, they have been less researched.
In 1999 and 2000, 651 whales washed ashore, but only three whales were necropsied. Another death was studied even less in the late 1980s.
Read more: The death of gray whales on the West Coast has stopped, NOAA declares
This time, however, the large scientific investigation team – spread across three nations – was highly coordinated, and had access to new technologies, such as drones, which enabled them to create a more thorough picture of the whales. who died, and those who remained. alive.
“I think funding was a big part of it as well,” said Stephen Raverty, a veterinary pathologist with the Marine Mammal Research Unit in British Columbia, and lead author of the study. “There was an opportunity to really 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 community. And I think more people want to participate and really participate. in these efforts.”
He also gave a nod to co-author Deborah Fauquier, a veterinary officer with the national fisheries service’s Office of Defense Resources in Silver Spring, Md.
But even with such resources, studying whale bones is difficult.
Despite the hundreds of whales that washed ashore, researchers could only properly examine 61.
That’s because most of the carcasses reported were found weeks or months after the animal died — and were too decayed to be properly analyzed — or were located on remote islands, inaccessible coves, or on a ship at sea.
Of the 61 whales they examined, researchers determined the cause of death in just over half. Sixteen were severely injured and probably died of starvation; 11 died from blunt force trauma – although two of those whales were also underweight; at least three suffered fatal wounds from killer whales and two were entangled in fishing gear.
In total, 18 whales were considered emaciated, 27 considered “thin,” nine medium and two fat. In the other five, nutritional status could not be determined.
One thing was clear: Death was not caused by disease.
Large animal deaths are often caused by biotoxins, viruses or bacterial infections. One example is the bird flu that is currently circulating around the globe. Another outbreak of domic acid occurred during the summer of 2023, which killed hundreds of sea lions and dolphins.
“We found no evidence of anything that looks like an infectious disease,” Duignan said. “There were no significant signs of any kind of infection. And we did a lot of testing for viruses, bacteria, toxins, and there was nothing significant.”
Read more: California wildlife is at risk of an avian flu ‘apocalypse’. What is driving the spread?
Now, the question is whether the green whale population will recover, continue to decline, or have reached a sustainable level, given the massive changes taking place in their summer feeding grounds.
Raverty noted that during this latest investigation, reports of unusual feeding behavior by the whales were frequently observed. Although the biological mantra has always been that green whales feed only on bottom-dwelling organisms in the northern seas during the summer months — and quickly throughout the year — reports of green whales screening and surfing from the surface, in places like San Francisco Bay.
Green whales have long been known to be adaptable – but the frequency with which these behaviors occurred to humans suggested that there was an immediate adaptive reaction to a lack of food, or perhaps behaviors that no one had ever noticed.
That’s part of what’s so exciting about this research, Raverty said. It has enabled the scientists to build a baseline against which they can now make comparisons.
“If we look at another five or 15 years, if we get another recurrence,” he said, they will have this data to compare with.
About the future and population recovery?
“The way I feel about this is … these whales are not going away. 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 we did in the past when we had a really strong, productive Arctic. [seafloor] habitats.”
This story originally appeared in the Los Angeles Times.
