Keywords: Species and food webs, Mammals, Marine habitat, Killer whales, Implementation Strategies, Species of concern, Disease

Collisions with boats and other interactions with humans are "significant" causes of death for killer whales in the northeastern Pacific, a recent study says. The findings come from one of the most comprehensive looks at killer whale pathology to date, but scientists say determining how a killer whale may have died is often notoriously difficult.


Several years ago, a killer whale calf in Alaska was learning a special way to catch halibut. A member of the northern residents—the population of killer whales that lives mostly in waters off British Columbia and Alaska—she targeted longlines. When she saw a fish dangling from one of the hooks, she would pluck it off. Many northern residents do this; they can have a stiff economic impact on the Pacific halibut fishery because of it. But since the calf was young, she had not yet perfected her technique, and one day something went wrong: along with some halibut, the calf ended up eating a hook, too.

The hook lodged in the calf’s throat. Its three sharp barbs tore through her oropharynx. Heavy fishing line still attached to the hook dragged back and forth across her mouth and tongue, lacerating her flesh. Bacteria infected the wounds, and between the infection, the swelling, and the hook itself, she may have had difficulty swallowing. For some period of time—weeks, maybe even months—she grew thinner and weaker before she eventually died of sepsis. When her body washed ashore on a beach in Glacier Bay, Alaska, on August 26, 2005, she became one of more than fifty killer whales to have died and stranded in the eastern Pacific Ocean and Hawaii between 2004 and 2013.

Now, these dead whales are helping researchers gain a greater understanding of the threats that killer whales face in the North Pacific. They are included in a recent study published in PLOS ONE that takes one of the most comprehensive looks at killer whale pathology to date.

“We really started think about this issue back in the late 1990s, because we were very interested in the southern resident decline,” says Stephen Raverty, a veterinary pathologist with British Columbia’s Ministry of Agriculture and Lands, and the lead author of the study. “We were wondering how they compared with other killer whales.”

The endangered southern resident population of killer whales may get most of the press in the Salish Sea region, but killer whales as a species are found all over the world. (Whether they are all the same species, or are instead several species, is still being figured out.) And while killer whales die all the time, finding a dead one is rare—their bodies often sink, or are in an advanced stage of decomposition when they float ashore for someone to find. “We did a lot of work with First Nations people in BC, work with other guardians in Alaska,” Raverty says. “They have very strong investment in identifying animals and having them available for post-mortem evaluation.”

In addition to a relative scarcity of carcasses, another issue was differences in pathological reports. Although big dead marine mammals that wind up on beaches are often necropsied, the information reported can depend on who is doing the reporting. That finally started to change in the early 2000s, when a team of wildlife veterinarians developed an 82-page standardized necropsy protocol so results from disparate geographies would be comparable. “Now we could say, ‘If you find a killer whale, here’s what to look for, here’s what to record,’” says Joe Gaydos, the science director for the SeaDoc Society and one of the paper’s co-authors [Joe Gaydos is also a member of the Encyclopedia of Puget Sound editorial board]. “That was really important.”

Illustratio of standard length, girth, and blubber thickness measurements taken from stranded killer whales.

Standard length, girth, and blubber thickness measurements taken from stranded killer whales allow scientists to compare results of necropsies from different locations. Graphic: Raverty et al. 2020

 

For their study, Raverty and his co-authors reviewed pathology reports from 53 dead whales—51 from the west coast of North America and two from Hawaii. (They also used data from an additional 35 killer whales that stranded between 2001 and 2017 to look for associations with body condition.) They were able to determine the cause of death in 22 cases, while nine others had what the researchers called “findings of significant importance for population health.”

“I’ve had human pathologists call me up and say they can’t believe we could only figure out cause of death about thirty percent of the time, but orca pathology is hard!” Gaydos says. "These are very insulated animals that are going to hold heat in cold water." That extra heat speeds up the decomposition process making it harder to determine a proximal cause of death, he says. "This is really wildlife CSI.”

The researchers found that killer whales can die for all sorts of reasons: bacterial or viral infections, birth defects, nutritional stress, and so on. But one finding in particular stood out: blunt force trauma. Six whales—both calves and adults—had died after boats hit them. Another three showed evidence of blunt force.

That killer whales might get run down by boats was not something researchers expected to learn. “Ask the general public, and they would have been skeptical about this,” Gaydos says. Killer whales are fast, they can maneuver well and quickly. It thus stands to reason that if they hear a boat coming they would be able to get out of the way more easily than, say, a blue whale or a humpback.

Gaydos saw this logic at play during the debates last year about the southern residents and whale watching guidelines. “People would say really we’re talking about noise,” he says. “But no, it’s also that killer whales were getting hit by boats.” In one instance, a killer whale swam towards a boat and was subsequently killed when a propellor cut it up. And considering that the southern resident population spends a lot of time in waters around two of the west coast’s largest ports—Vancouver and Seattle—Gaydos says it is vital for management agencies take steps to minimize the risk of ship strikes. “But one of the big things also to remember,” he says, “is that an increased appreciation of vessel strikes doesn’t take away from the big things known to be affecting endangered killer whales. Salmon availability, vessel noise, chemical contamination—these are still the main issues.”

For Raverty, the study shows the value of necropsies not just as a snapshot of killer whale health, but also as a baseline against which to watch for changes in future trends. “Living in such close proximity to humans carries a lot of risks,” he says. “This will help to mitigate the range of impacts we have on our resident killer whale populations.”

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Two southern resident killer whales. Photo courtesy of NOAA.

A 2020 study of pathology reports for 53 stranded killer whales in the northeastern Pacific and Hawaii showed that deaths related to human interaction were found in every age class. Vessel strikes accounted for the deaths of four of the nine endangered southern resident killer whales identified in the study. The findings were published Dec. 2 in the journal PLOS ONE.


About the author: Eric Wagner writes about science and the environment from his home in Seattle, where he lives with his wife and daughter. His writing has appeared in Smithsonian, Orion, The Atlantic and High Country News, among other places. He is the author of "Penguins in the Desert" and co-author of "Once and Future River: Reclaiming the Duwamish." His most recent book is "After the Blast: The Ecological recovery of Mount St. Helens," published in 2020 by University of Washington Press. He holds a PhD in Biology from the University of Washington.

Topic editor: Joe Gaydos

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