THE SOUTHERN RESIDENT ORCAS of the Salish Sea are much beloved in the Pacific Northwest. Yet despite scientific scrutiny and ongoing conservation efforts, their population has remained stagnant for decades, and no one really knows why. Scientists believe they may have discovered a means to better monitor the orcas’ health, deepening our understanding of how disease affects them and ultimately improving their conservation.

“We know so much about these animals, but the population is just teetering,” said Joseph Gaydos the chief scientist for the University of California Davis SeaDoc Society Program, in Washington, and senior author of a recent study that looks at skin disease as a possible health indicator for the marine mammals.

The work done with orcas may also extend to other cetaceans, as rising rates of skin disease have been observed globally in whales, dolphins, porpoises, and other species.

The Southern Resident orca has been listed as an endangered species for decades. Its population fell to a low point of 71 animals in 1975 and has barely recovered, to some 75 animals now. While pollution and declining food availability play a role in this stagnation, Gaydos noted that data related to many possible population health challenges was too sparse to be used in forming solid conclusions.

Even less is known about the effects of factors such as disease, which can be challenging to measure directly in orca. So to get a sense of disease rates in the Southern Resident population, Gaydos and his team examined close to 19,000 photos of these whales taken from 2004 to 2016, looking for signs of skin lesions that might indicate the presence of disease.

“We’re looking for ways to say what are signs of poor health and good health,” Gaydos said.

Gaydos and his team gathered enough data to estimate the number of individual orcas with evidence of disease at any given time during the study period. These so-called point prevalences could indicate changes in the overall health of individuals, pods, and possibly the entire population over time.

Their results, published recently in the journal Public Library of Science, showed that skin lesions appeared on nearly all orca photographed during the study period — 99 percent — and that more animals displayed them over the years. A rising rate of point prevalence over time, he said, “was probably the most worrisome thing that we saw.”

One possible concern with such studies is that evidence of skin lesions in any given batch of photos could be more representative of factors unrelated to orca health, such as the number of photographers available to capture more orca photos at a given time and location.

“If you go out and you you take a sample, you might say, 90 percent of the animals have this [disease],” Gaydos said. “But if you don’t go back year after year, month after month, was there just something going on for that one month? Was the water temperature hot or is that really a situation where all those animals seem to have that disease?” Still, he added, “because things were collected systematically over time and there’s a real small number of people doing the photographs, we were confident that what we were seeing was an increase in point prevalences.”

The Southern Residents displayed six distinct types of skin lesions over the study period: cephalopod lesions, erosions, gray patches, gray targets, orange on gray, and pinpoint black lesions. Gray patches and gray targets being the most prevalent.

The underlying causes of two highly correlated lesions remain uncertain. They could, for instance, be the results of distinct pathogens or they might represent two components of a single underlying disease process. Fundamentally, though, the challenge is to understand why infection is occurring in the first place.

“We think it’s a symptom of everything else that’s going [on] with this population,” Gaydos said. “I think it’s a little scary that this symptom is getting more and more prevalent.”

WITH STUDIES SCATTERED across the globe and a relatively small research community spread across many species, it can be difficult to know whether observations amount to a real trend or merely reflect better data.

Marie van Bressem, a researcher with the Cetacean Conservation Medicine Group and the Peruvian Center for Cetacean Research, recalls taking note of widespread skin disease among dolphins off the Peruvian coast as far back as the late 1980s and early 1990s. At the time, van Bressem was studying the impact of fisheries on dolphin and porpoise populations.

“Cetacean poxvirus…was already very high among the dolphins and porpoises of Peru,” she said.

Echoing Gaydos, van Bressem said that while she and her colleagues never really knew why prevalence of infection among dolphins and porpoise there was so high, it may have been due to, or influenced by, stress from intensive fishing practices.

“That was the hypothesis,” she said. “But it’s difficult to prove.”

Much of the difficulty comes from challenges inherent in directly measuring the impact that various environmental factors have on cetaceans’ immune responses. Researchers do know, however, that chronic stress, much as in humans, degrades a body’s ability to mount a robust immune reaction, which can result in greater susceptibility to infection.

One possible source of chronic stress could be toxins that enter the water from industrial waste. In more recent research, for instance, van Bressem noted that high concentrations of halogenated organochlorines, which are compounds used as pesticide, correlate with more severe disease manifestations among cetaceans around the globe.

Alisa Hall, a professor of biology at the Scottish Oceans Institute at the University of St. Andrews, attempted to assess the effects of PCB exposure on Southern Resident orca health in 2018. “We concluded that based on the limited amount of data available on Southern Resident killer whales that PCB exposure could still be a factor preventing the recovery of the population,” she said via email. “This was based on the inferred effects of PCBs on calf survival as well as immune function.” She cautioned, however, that whatever might be driving the skin disease seen in the orca, and to what degree, remain unclear.

The orca in the Salish Sea are exposed to high levels of compounds such as PCBs, DDT, organic chlorine, and more, Gaydos said. “But those haven’t been going up over the same time frame that would make us think that they were correlated or even that there was causation [between] contaminants and disease.”

A joint report from the US Environmental Protection Agency and Climate Change Canada on the health of the Salish Sea backed Gaydos up, finding little change in toxins within the area’s food web. It did, however, report declining marine water quality, as measured by falling levels of dissolved oxygen in the waters of Puget Sound and the Strait of Georgia and generally declining Marine Water Condition Index scores throughout the Salish Sea. It also noted falling salmon populations, noting that Chinook salmon populations have fallen by 60 percent since the Pacific Salmon Commission began tracking salmon abundance in 1984.

Gaydos sees the falling salmon populations as a prime suspect in the mystery of the orcas’ apparent poor health. Orca are specialist eaters, hyper-focusing on a preferred food source to near exclusivity. And the Southern Residents prefer the Chinook.

“Our [Salish Sea] salmon is just a fraction of what it used to be,” he said. “We’ve been spending 200 years kind of dismantling salmon, and Chinook, the biggest of the big salmon…are endangered. So you’ve got an endangered species eating an endangered species.”

Gaydos figures that monitoring the health of the Southern Residents by tracking skin infection prevalence will help those studying the orca and working to conserve them better focus their efforts. His group is currently developing tools that they hope will enable them to conduct more detailed health exams on the Southern Residents.

“We’re trying to have a bunch of tools in our toolbox for diagnosing problems and also for treating things,” he said. “This goes to the benefit of being able to diagnose problems and being able to assess population health from individuals.”