Scientists studying an endangered population of orcas resident off the Pacific north-west coast of Canada and the US have recorded a “strong increase” in skin lesions on the animals’ bodies, which they believe is due to the decreasing ability of their immune systems to deal with disease.

The lesions appear on the whales as grey patches or targets, or black pin points. Some resemble tattooed skin. Their presence on the animals’ graphically black and white bodies is “increasing dramatically”, according to Dr Joseph K Gaydos of the SeaDoc Society at the school of veterinary medicine at the University of California, lead author of the scientific paper.

“The health significance … is not clear,” says the report, but the possible relationship between the lesions and “decreasing body condition in an endangered, non-recovering population is a concern”. The lesions appear to be the symptom of serious underlying conditions affecting the whales’ health – from immune suppression to infertility – and their presence, as in any cetacean, is a warning sign that things are not well.

With 99% of the animals studied exhibiting the lesions, the consequences could be disastrous for the fragile population of southern resident orcas, who now number fewer than 75 individuals. “It’s worrisome,” said Gaydos. “Now we need to try and isolate the potential infectious agent.”

The study comes as orcas are increasingly in the global news as result of the impact of human activities on their lives.

The co-authored paper discusses possible causes as warming coastal waters, poxvirus, the effects of persistent organic pollutants or an increasing scarcity of prey reducing their immunity, but cannot yet draw any firm conclusions. It is certain, however, that the mystery infection will be of urgent concern to a worldwide network of scientists and environmentalists who study orcas, a species that exhibits so many similarities to humans.

The southern resident orcas have been studied intensively by the Center for Whale Research since 1976, identifying individual whales by the black and white markings on their bodies and by the shapes and scars on their fins. It is the comprehensiveness of the centre’s dataset that has allowed Gaydos and his collaborators to assess the rapid decline in the animals’ health from images taken from 18,697 sightings.

Southern resident orcas are subject to multiple factors created by humans. A decrease in chinook salmon numbers is decreasing their fertility, and calves now being born to the group usually die within three years. In ideal circumstances, female orcas can live up at least 90 and even possibly 100, while males reach 60 years old. The Center of Whale Research also believes that the impact of new pipeline projects in Canada and consequent sound pollution and disruption could push the whales’ closer to extinction.

Orcas have played an important role in the culture of the First Nations peoples in the region. But in modern western culture, the whales been taken into captivity. In one incident in 1970, more than 80 of the southern resident population were rounded up off Washington State. Five drowned, and seven were taken into captivity.

Orcas held in oceanariums can exhibit skin infections as a result of unsuitable water conditions.

The effect of marine pollution on whales has long been a concern. Coastal waters where orcas forage are susceptible to the run-off of agricultural and industrial chemicals. Salmon farms have also been implicated due to their use of antibiotics which can end up in wild salmon. However, Gaydos told the Guardian that his study had found “no evidence that the increasing prevalence of these skin lesions is associated with antibiotic use or any other management action associated with salmon farming”.

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Orcas occupy the top of the marine food chain and pollutants accumulate through the fish they eat. Like humans, their culture is defined by their diet, and the southern resident population are almost exclusively salmon-eaters. Wild salmon stocks in the area have been overfished by humans. Some observers believe that a similar resource conflict over blue fin tuna with orcas foraging in the strait of Gibraltar plays a part in the recent increase of reports of their physical interactions with vessels in the area.

The UK’s only resident orcas, found on the Scottish west coast, area seriously affected by the impact of marine pollution. PCBs and organchlorines in the marine environment have degraded their immune systems, to the extent that they have not produced a healthy calf in 20 years.

Meanwhile there were reports this week of an orca pod off Iceland having “adopted” a young pilot whale calf, in an incident similar to one recorded a year ago. The pairing raises new questions about the complex social behaviour of these animals, and how they are adapting to oceanic waters increasingly dominated by human activities.


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