Virtually every orca in an endangered North American group is suffering from mysterious skin lesions, say scientists

• There has been an increase in skin disease in Pacfic north-west killer whales, a new study found.
• There is a possible connection between the skin lesions and the declining condition of the orcas.

Scientists are concerned after a recent study found that there has been a "dramatic" increase in skin disease in an endangered North American population of orcas, and they don't know why.

In a recent study published in PLOS ONE, researchers studied a "strong increase" in skin lesions in the population of southern resident orcas from 2004 to 2016.

The exact underlying cause of the skin disease — often in the form of gray patches and targets — is unclear, and the researchers are worried about the implications for the fragile population of orcas living on the Pacific north-west coast.

The "possible" relationship between these lesions and the "decreasing body condition" of the endangered, non-recovering population is a concern, according to the study. The study found that 99% of the animals examined exhibited the skin disease.

"Before we looked at the data, we had no idea that the prevalence of these skin lesions was increasing so dramatically," Joseph K Gaydos, lead author of the study, said in a statement. "It's worrisome. Now we need to try and isolate the potential infectious agent."

The scientist studied a vast number of photographs obtained by the Center for Whale Research that showed nearly 20,000 whale sightings in the Salish Sea — off the coast of British Columbia and Washington state.

After ruling out certain environmental factors, the researchers suspect that the cause of the legions comes from an "infectious agent," which might indicate a decrease in "immunocompetence" — a reduction in the orca's immune system to fight disease — which they found "concerning," according to the study.

The paper explores possible causes, including warming coastal waters and the effects of man-made organic pollutants or an increasing scarcity of chinook salmon numbers reducing their immunity. Still, it cannot yet draw any firm conclusions.

Fewer than 75 animals remain in the endangered population of the southern resident orcas, so a better understanding of the impact of skin lesions on their health is crucial, according to researchers.

The killer whales' failure to reproduce is another indicator of troubling times for the orcas of the Pacific north-west, and calves now being born to the group usually die within three years, per The Guardian.

This comes during a growing interest in orcas after increasing killer whale boat interactions in other waters.