A 6-year, mass die-off of grey whales has been declared over by scientists

A mass die-off of grey whales in the Pacific has been declared over by researchers, who say a combination of starvation, vessel strikes, and attacks from killer whales are to blame for thousands of whale deaths over a roughly six-year period.

The "unusual mortality event" was first declared by the National Oceanic and Atmospheric Administration in early 2019, and deemed over at the end of March. Now, new research published in PLoS One is shedding light on the 690 whales that became stranded in that period.

Study author Stephen Raverty, a veterinary pathologist and adjunct professor at the University of British Columbia, said researchers have tracked past mortality events involving grey whale populations, with the most recent one declared over in 2000.

The 2019 to 2024 event was triggered by an increased number of whales becoming stranded or washing up on the shores of Mexico, the continental United States,  B.C., and Alaska, oftentimes with the bodies in rough shape, emaciated or scarred from attacks or collisions.

"Between 2019 and 2023, what was really dramatic is the population was observed to have declined from 28,000 animals down to an estimated 14,000," he said.

"So this was a significant decline in the population that occurred quite suddenly."

Now, there are signs the grey whale population is growing, with the number of calves born double the number from the year before — offering a chance at renewal.

Every year grey whales make their migration from Arctic waters to Mexican lagoons, where they give birth in the shallow, warm waters. Before making the journey south, the whales have five to six months up north to take in as much nutrition as possible before making the 17,000-kilometre round-trip swim.

Andrew Trites, the director of the University of British Columbia's Marine Mammal research Unit, said a failure to take in enough energy before starting the long migration can spell disaster, especially for pregnant whales. 

"If they leave the Arctic and the gas tank is not full, they're not making it back. There's nowhere to stop off to top up the reservoir," he said.

"It's all or nothing."

Raverty said because of changing ice conditions, whales are expending more energy to dive deep to the ocean floor, where they eat amphipod crustaceans, tiny, shrimp-like animals that live in the sediment. 

"Because of climate change and the receding ice up there, it's thought the nutrient value of some of the prey items has actually declined," he said.

"And so there's a combination of increasing costs with having to forage further and further away from their historic areas, as well as a decline in the nutritional quality of the species themselves."

Massive whales washing ashore provide dramatic scenes to onlookers, from the beaches of Mexico, California, Tofino and further north. 

The study reveals that subsequent necropsies on the stranded whales identified wood chips, bark, eel grass, kelp and inorganic debris in some of their stomachs, potentially from their attempts to forage in sub-optimal habitats as nutrients ran low.

The research also suggests an increased number of vessel strikes could be a result of whales foraging in shipping lanes, outside their usual feeding grounds.

Killer whales 'lying in wait'

The necropsies also found grey whales marked by teeth marks, a telltale sign of attacks from killer whales. Some grey whales became stranded in shallow waters as they attempted to flee.

Trites said narrow bays and inlets where the grey whales must cross leave them particularly vulnerable to attacks from killer whales. Mothers with their calves are particularly at risk in narrow passages.

"They're sort of swimming the gauntlet when they're trying to cross there. All the calves and moms have to pass through this little tiny passage and the killer whales are lying in wait for them to come through," he said.

According to the study, necropsies found a cause of death in 33 of the whales examined. Sixteen were severely emaciated, while 11 were struck by vessels, and 19 showed signs of attack from killer whales. But Raverty and Trites emphasize that all the factors are connected — with a lack of nutrition leaving grey whales more vulnerable to every peril they face in their migration.

A starving whale won't have the energy to out-swim a predator or get out of the way of a fast-moving vessel. Or, as they attempt to flee those dangers, they may end up stranded. When strandings happen in areas where people come across the bodies, they are quickly reported.

"We realize that even the ones we get on shore are a small portion of the ones that have actually died, but it does give us an index of when something really strange has happened," said Trites.