Resident killer whales off the western coasts of Canada and the United States are being poisoned by eating salmon laced with toxic PCBs, says a new study on yet another threat to a species already facing extinction.

The study, led by researchers in British Columbia, tested the whales' main food supply -- chinook salmon -- and found PCBs and other man-made pollutants that are jeopardizing the large orcas that eat them.

The actual levels of PCBs in the salmon are relatively low, says Peter Ross of the Institute of Ocean Sciences in Sydney, B.C., but even in small amounts pose a significant risk to killer whales.

Ross says whales are particularly sensitive because they eat massive amounts of fish over a long lifespan -- killer whales can live for 80 or 90 years -- creating a massive buildup of toxins that their bodies can't get rid of.

That means the whales, particularly the southern resident population, have become one of the most contaminated marine mammals in the world.

"Killer whales are long-lived, top-of-the-food-chain animals, they have small, isolated populations, they have very large habitat needs," says Ross, the supervising researcher on the study which appears in the current issue of the journal Environmental Toxicology and Chemistry.

"If we're trying to protect 84 animals in an area where we've got 8.5 million people, we've got a bit of work to do."

Northeast pacific resident killer whales are divided up into two populations, northern and southern.

The northern population, about 200 whales living off Vancouver Island and in waters off northern B.C., is listed as threatened under Canadian species-at-risk legislation.

The southern population includes just 84 animals living off southern B.C. and Washington state, which are listed as endangered under Canadian and American law.

The latest study followed another in 2000 that found dangerous levels of pollutants in the whales, which can damage their immune system, affect reproduction and lead to developmental abnormalities.

That earlier study found southern whales had four times the amount of toxins such as PCBs -- an industrial chemical that has been banned in North America in the late '70s -- as their northern counterparts. It wasn't clear where the whales were picking up the toxins until now.

Southern resident whales are eating salmon from waters, especially Puget Sound, that have high levels of PCBs. Because salmon in those regions tend to have less fat as they return to coastal streams to spawn, the whales have to eat more of them.

The study appears to confirm that the contaminated whales are getting the chemicals from the salmon they eat, which should inform policy-makers in Canada and the U.S. as they try to protect the iconic orcas, said Ross.

"We were already assuming what was going on or taking steps to mitigate or manage (the effects of the pollutants)," says Ross.

"There are things currently underway to try to reduce noise and disturbance, increase salmon productivity, and reduce chemical contamination. Some of these will take a long time to bare fruit."

However, Ross says there is reason to be optimistic.

In the more than 30 years since PCBs have been banned in Canada and the U.S., contamination levels in killer whales have dropped by 250 per cent, Ross said.

"While we still have extraordinarily contaminated killer whales, things are better," he says.

"We expect them to continue to get better. We continue to do the kind of science that I think will provide more information and facts so we can learn from past mistakes and try to avoid future mistakes."