April 13, 2024

Can the Giants ever recover?

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When large numbers of gray whales began appearing along the Pacific coast of North America nearly six years ago, marine scientists could only speculate as to why: Was it disease? Pollution of the oceans? Increasing ship collisions?

Many of the condemned cetaceans appeared emaciated or emaciated, while others appeared torn apart by orcas. Some clearly died after being struck by a ship or becoming entangled in fishing gear. Others have yet to provide 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 in PLOS One suggests that the culprit was a critical drop in food availability in mammals’ feeding areas in the Arctic and subarctic.

What remains unclear is whether this malnutrition was caused by a change in the ocean or by the whales themselves.

“Did something happen to their food supply in those years that put them under acute nutritional stress and resulted in the deaths of many whales?” asked study co-author Padraig Duignan, a pathologist at the Marine Mammal Center in Sausalito.

“Or did the number of whales in the population reach such a level that they competed with each other for food and, again, a proportion of the population died because they were unable to compete for the available resources?”

The investigation builds on an investigation the National Oceanic and Atmospheric Administration launched in early 2019 after declaring the whales’ deaths an unusual mortality event, or UME. Researchers, observers and stranding coordinators across North America began working together – warning each other about strandings; sending teams to document and collect tissue samples; and performing necropsies (the animal form of an autopsy).

NOAA’s investigation recorded 690 dead whales since January 1, 2019. However, researchers suspect the real number is thousands more than that. Most whales die at sea and sink to the sea floor, 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 gray whale population was reduced by half during the most recent mortality event.

“The population is 14,000, down from 27,000 or so,” he said. “That’s a big drop.”

NOAA recently declared that extinction is over.

Every year, California gray whales make a 13,000-mile round trip from the cold waters of the Arctic to the balmy lagoons of Mexico’s Baja Peninsula and back. During the summer months, they feed on a hodgepodge 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, preparing for the long journey south to the warm, sheltered nurseries of Baja’s shallow estuaries.

Along the way, they dodge ships and fishing gear, navigate polluted waters, and hide from hungry orcas. They also have to contend with biotoxins and infectious diseases.

So, when researchers began observing the bodies of the whales, they tried to determine which of these various calamities was the main cause of the population’s extinction.

Although other gray whale deaths have occurred along the Pacific coast, they have been less researched.

In 1999 and 2000, 651 whales stranded on land, but only three whales were necropsied. Another extinction in the late 1980s has been even less studied.

This time, however, the scientific research team was large – spread across three nations – highly coordinated and had access to new technology, such as drones, which allowed them to create a more complete picture of the whales that died and those that remained. alive.

“I think funding was a big part of it as well,” said Stephen Raverty, a veterinary pathologist at the Marine Mammal Research Unit in British Columbia and lead author of the study.

“It’s really an opportunity to respond to these animals. And so we’re always trying to give information back to the First Nations community or share it with the public. And I think that engages more people to really want to contribute and participate in these efforts.”

He also nodded to co-author Deborah Fauquier, a veterinarian with the National Fisheries Service’s Office of Protected Resources in Silver Spring, Maryland. He said Fauquier was instrumental in organizing information sharing between nations, departments and individuals.

But even with these resources, it is difficult to study whale extinction.

Despite the hundreds of whales that washed ashore, researchers were only able to properly examine 61.

This is because most of the reported carcasses were discovered weeks or months after the animal’s death – and were too decomposed for proper analysis – or were located on remote islands, inaccessible coves or on the bows of ships at sea.

Of the 61 whales examined, researchers determined the cause of death in just over half. Sixteen were severely emaciated and probably starved to death; 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 were trapped in fishing gear.

Overall, 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: the death was not caused by illness.

The death of large animals is often caused by biotoxins, viruses or bacterial infections. The bird flu currently circulating around the world is an example. The domoic acid outbreak during the summer of 2023, which killed hundreds of sea lions and dolphins, is another.

“We found no evidence of anything that appears to be an infectious disease,” Duignan said. “There were no telltale signs of any type of infection. And we did a lot of testing for viruses, bacteria, toxins, and there was nothing significant.”

Now the question is whether the gray whale population will recover, continue to decline or reach a level that is sustainable, given the enormous changes that occur in their summer feeding grounds.

Raverty noted that during this latest investigation, reports of unusual feeding behavior by the whales were observed relatively frequently. While the biological mantra has always been that gray whales feed only on bottom-dwelling organisms in the northern seas during the summer months – and rapidly throughout the rest of the year – reports have come in of gray whales filter-feeding and scavenging. 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 an immediate adaptive reaction to the lack of food, or possibly behaviors that no one had really paid attention to.

That’s partly what’s so interesting about this research, Raverty said. It allowed scientists to build a foundation upon which they could now make comparisons.

“If we look out another five or 15 years, if we have another recurrence,” he said, they will have that data to compare.

As for the future and recovery of the population?

“The way I think about it is… these whales are not going to disappear. They’re not going to go 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 robust and productive Arctic. [seafloor] habitats.”

More information:
Stephen Raverty et al, Gray whale (Eschrichtius robustus) post-mortem discoveries from December 2018 to 2021 during the eastern North Pacific unusual mortality event, PLOS ONE (2024). DOI: 10.1371/journal.pone.0295861

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