Climate change opens up new prey for Alaska’s orcas | Polarjournal
Orcas roam wider areas in the U.S. Pacific Arctic due to the lack of sea ice and encounter new prey. Photo: Holly Fearnbach, NOAA Fisheries

Originally, orcas rarely visited arctic waters, as sea ice was too dangerous for them. However, with climate change and retreating sea ice, Orcas have been advancing further into the Arctic in recent decades, where the table is richly set for them: Bowhead whales, narwhals, belugas and various seal species are among their prey. Off the north coast of Alaska, bowhead whales were spared for a long time, according to indigenous hunters. A recent study could now prove for the first time that orcas have been hunting bowhead whales in the region for several years.

Of all the great whales, the bowhead whale is best adapted to life in the Arctic. They live almost exclusively in seasonally ice-covered waters and protect themselves from the icy temperatures with a layer of fat up to 50 centimeters thick, called blubber. Thanks to their massive skulls which they use to break ice floes up to 60 centimeters thick, they are able to venture far into the ice. In spring and fall, the bowhead whales migrate along the north coast of Alaska in the western Beaufort Sea and eastern Chukchi Sea, where indigenous communities hunt them for subsistence.

In recent years, the Arctic ecosystem has changed rapidly due to warming and loss of sea ice. “Sea ice extent during the summer and autumn […] has declined dramatically in recent years. In this region there has also been earlier seasonal sea ice melting, later seasonal sea ice freeze-up, and thinner sea ice,” said Amy Willoughby, University of Washington scientist and study leader.
Thus, Greenland whales are increasingly lacking sea ice as a refuge where they are safe from orcas. Unlike orcas in Antarctica, Arctic orcas avoid sea ice.

Bowhead whales prefer areas with sea ice where they are protected from predators like orcas. Photo: NOAA Fisheries

“Until now, bowhead whales in the U.S. Pacific Arctic were thought to experience minimal predation pressure from killer whales. Our study suggests that is no longer the case,” said Willoughby. “We now know that during 2009–2018, killer whale predation was the primary cause of death observed for bowhead whales in the region.”

First direct evidence

In 2009, the Aerial Surveys of Arctic Marine Mamals (ASAMM) project, funded by the Bureau of Ocean Energy Management and implemented by NOAA Fisheries, began with the photo documentation of whale carcasses. Images of bowhead whale carcasses were sent to experts in the North Slope Borough to determine the cause of death.

Wildlife veterinarian Raphaela Stimmelmayr of the North Slope Borough Department of Wildlife Management in Utqiaġvik (formerly Barrow) explained the process:
“Killer whales are remarkable hunters of diverse marine prey. They use variable hunting strategies, either cooperative or as individuals, depending on the prey type. Much of what we know about the hunting techniques of killer whales on baleen whales and the resulting bodily injuries has been informed by eyewitness reports of killer whale attacks from indigenous hunters, biologists, and commercial whalers.
When examining photos as we did in this case, one follows the trail of evidence by assessing the key forensic signs. We look at the type of injury, bite characteristics, and bodily location to determine if the evidence at hand is consistent with orca predation. Sometimes there is no doubt given the extent of external injuries such as missing jaws, missing tongue, semi-circular bite marks, or rake marks. In other cases no conclusion can be reached and the cause of death remains a mystery.”

A – The carcass of a bowhead whale calf, which provided the first evidence of an attack by orcas. B and C – Carcasses of young bowhead whales with fatal injuries to mouth and jaws caused by orca attacks. Photo: NOAA Fisheries

Especially one photo from 2015 showing the carcass of a Bowhead whale calf caused the researchers to review all photos of Bowhead whale carcasses between 2009 and 2018. “As far as we know, that was the first documented case of killer whale predation on a bowhead whale in the eastern Chukchi and western Beaufort sea,” said Willoughby. “In the years that followed, we saw more bowhead whales with injuries consistent with killer whale predation. That made us wonder if there were earlier accounts of predation that had not been noted.”
In the course of their analysis, scientists classified 18 of 33 documented carcasses as ‘severely injured by killer whales’ between 2009 and 2018.

Warming opens up new hunting opportunities for killer whales

Observations suggest that orcas have only begun to hunt bowhead whales off the north coast of Alaska in recent years. Prior to this study, the successful hunting of bowhead whales by orca in the study area had not been documented neither by scientific research nor by the knowledge of the indigenous people. The fact that orcas hunt gray whales in the region and bowhead whales in the Russian Arctic was well known. Indigenous hunters in Alaska, however, observe scars from non-lethal orca attacks on bowhead whales that they have killed.

Although photos of bowhead whale carcasses have only been taken since 2009 as part of the ASAMM project, there was no evidence of orca attacks on live bowhead whales in the 30 previous project years. The first sighting of orcas was in 2012.

The study area off the north coast of Alaska in the eastern Chukchi Sea and the western Beaufort Sea. The carcasses of bowhead whales are color-coded by year and symbol shapes indicate the category corresponding to cause of death. Killer whale sightings are marked with white stars. Graphic: NOAA Fisheries

“We have been monitoring scars on harvested bowheads for 35 years. We noted scars from killer whale attacks, line entanglement, and ship strikes. We knew bowheads were attacked by killer whales. Native hunters on St. Lawrence island, Alaska, had reported bowhead carcasses likely killed by killer whales. But we had no evidence of successful predation within the eastern Chukchi and western Beaufort seas. However, for decades we found dead gray whales that were victims of killer whale attacks along the U.S. Chukchi coast,” said Craig George, retired senior biologist at the North Slope Borough Department of Wildlife Management in Utqiaġvik, Alaska. “We now have direct evidence of killer whales taking bowheads in U.S. Arctic waters over the last 10 years.”

Looking to the future in a changing Arctic

“We don’t yet know what the long-term effects of killer whale predation on bowhead whales will be,” said Willoughby. However, she sees a number of potential effects on West Arctic bowhead whales and the subsistence communities that have developed around them.

Along with the removal of bowhead whales from the population, orca predation pressure could influence the behavior of bowhead whales. It might force bowhead whales to migrate further offshore or they could stay in shallow water near the coast. The presence of killer whales could disrupt feeding opportunities for bowhead whales. Or, they may not be affected at all.

“Gray whales are common in the northeastern Chukchi Sea, and more recently, fin and humpback whales appear to be moving farther north into the Arctic. Killer whales may target these species instead,” said Willoughby.

As the Arctic continues to warm, other factors could have an increasing influence. “As sea ice melts, marine traffic and commercial fishing is expected to increase in these waters. That could further impact bowhead whales in this region. Warming may also affect bowhead whale food sources or other biological functions,” Willoughby expplained. “Our study provides insight for interpreting future information on bowhead whale mortality and causes of death.”

In the next step, the researchers will evaluate photos from 2019, the last survey year for the project. According to Willoughby, preliminary results suggest that 2019 – one of the lightest ice years they have seen in these surveys – had the highest predation yet.

Julia Hager, PolarJournal

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