Orcas' deadliest predators don't live in the ocean, study finds

Depending on your mindset, you either consider orcas deadly killer whales or a devastatingly intelligent, endangered species.

A new study published Wednesday in the journal PLOS ONE undercuts the idea killer whales are an apex predator without parallel — and gives yet more reason to treat the orca with compassion

The greatest threat to the killer whale may be a species we are all too familiar with.

A Deadly Impact — In a first of its kind study, researchers sought to solve the mystery of what is killing the killer whales. They analyzed reports on the deaths of 53 stranded killer whales between the Eastern Pacific Ocean and Hawaii.

The researchers reviewed each wild whales' condition at the time of the death — something older research has just not done. Most of the studies on the causes of killer whale deaths have involved orcas in captivity, which have ample medical records for scientists to draw upon. When you're looking at dead whales out in the wild, it's hard to get such a full picture of what went down.

"For wild animals, aside from opportunistic or targeted field observations and use of drones to study body condition, oftentimes, there is little information available regarding the clinical history of wild stranded animals," Steve Raverty, lead author on the study and veterinary pathologist for the Canadian Ministry of Agriculture, tells Inverse.

So Raverty and his colleagues had to find ways to work around the lack of information. To do this, they turned to what evidence they could gather from the dead whales themselves. And much like a toxicology report or an autopsy might reveal the killer in a human murder, the whales' blubber holds secrets to their tragic demise. Each whale's blubber's thickness ties back to a metric known as the body condition index, which measures the whale's health.

"BCI is similar to the body mass index in people and has been developed to provide a quantitative assessment of the nutritional status of killer whales," Raverty says.

Researchers measured the whales' blubber thickness to assess the length, health, age, and manner of death of the killer whales.

Who's the real Killer? — Killer whales which died of blunt trauma — as opposed to an infection — had a higher score on the body condition index.

"With catastrophic traumatic injuries, we would anticipate an acute death of an animal; whereas, with infections, tumors or extralimital events, loss of body condition related to insufficient caloric consumption may result in decline of the nutritional condition of animals," Raverty explains.

Together with photographs from each scene, the researchers were able to suss out the cause of death for 22 of the 53 whales.

Baby whales most often died of infectious diseases and lack of nutrition. Younger and adult whales however suffered from toxic bacterial infections, disease, and blunt trauma. But the greatest common denominator, regardless of age, was death by humans.

A whale might ingest a fishing hook or get tangled in a fishing net, for example. They might suffer parasitic infections (toxoplasmosis) from polluted water.

Or they might die of blunt trauma after being hit by a fishing vessel, known as a vessel strike. The study reconfirms these vessel strikes not only happen but can prove fatal, underscoring humans' deadly impact on the marine world.

"With regards to ship strike, there have been anecdotal reports and published accounts of vessel and propeller strikes in killer whales, and results from the case series provide a forensic account or documentation to describe the effects of lethal impact with animals," Raverty says.

Raverty notes 53 whales is a limited sample, so the researchers can't quantify how great a threat ship strikes pose to orcas.

"As only a small proportion of those killers that die in the wild are identified floating or are beach cast and available for necropsy, the samples we examined are biased," Raverty admits. But the study does "provide some important baseline information of the health of the individual, and potentially, the overall population," he says.

Wake Up Call — The study is a wake-up call for marine conservationists invested in protecting the killer whale. Southern-resident killer whales are an endangered species.

The United States and Canadian governments already sponsor some efforts to protect the orca, including a whale-tracking network, which helps reduce ship traffic near orca whales.

"There are a series of submerged acoustic arrays in critical habitats and areas frequented by killer whales and when vocalizations are detected, a notification is issued to ship captions with a recommendation to reduce vessel speeds," Raverty says.

"These efforts, as well as public education campaigns, may reduce the number of vessel strikes," Raverty adds.

But vessel strikes aren't the only cause for concern. Humans are indirectly killing the killer whale through polluted toxins we use at home making their way into the ocean. On that front, there is also hope, but it will likely take a concerted global outreach to make change happen, as toxins can travel from the other side of the world. Inverse has a guide to how you can reduce pollutants coming from your own home water use.

"Many of these [toxins] appear to emanate from southeast Asia, transit the Pacific Ocean, and are deposited along the coastal mountains with rainfall. Efforts to minimize industry discharge in these source areas as well as cleaning up industrial sites along the coast may reduce exposure to marine mammals," Raverty says.

Abstract: Understanding health and mortality in killer whales (Orcinus orca) is crucial for management and conservation actions. We reviewed pathology reports from 53 animals that stranded in the eastern Pacific Ocean and Hawaii between 2004 and 2013 and used data from 35 animals that stranded from 2001 to 2017 to assess association with morphometrics, blubber thickness, body condition and cause of death. Of the 53 cases, cause of death was determined for 22 (42%) and nine additional animals demonstrated findings of significant importance for population health. Causes of calf mortalities included infectious disease, nutritional, and congenital malformations. Mortalities in sub-adults were due to trauma, malnutrition, and infectious disease and in adults due to bacterial infections, emaciation and blunt force trauma. Death related to human interaction was found in every age class. Important incidental findings included concurrent sarcocystosis and toxoplasmosis, uterine leiomyoma, vertebral periosteal proliferations, cookiecutter shark (Isistius sp.) bite wounds, excessive tooth wear and an ingested fish hook. Blubber thickness increased significantly with body length (all p < 0.001). In contrast, there was no relationship between body length and an index of body condition (BCI). BCI was higher in animals that died from trauma. This study establishes a baseline for understanding health, nutritional status and causes of mortality in stranded killer whales. Given the evidence of direct human interactions on all age classes, in order to be most successful recovery efforts should address the threat of human interactions, especially for small endangered groups of killer whales that occur in close proximity to large human populations, interact with recreational and commercial fishers and transit established shipping lanes.