Killer whales are dying off the western Canadian coast - and some scientists are blaming man-made toxins. A new study shows that killer whales are among the most chemically polluted marine mammals in the world. 'Based on the weight of the evidence, we suspect that PCB [polychlorinated biphenyl] levels present a risk to the killer-whale population,' said Peter Ross, of the federal Institute of Ocean Sciences in Sidney, British Columbia. The Centre for Whale Research in Friday Harbour, in the US state of Washington, has made a much stronger link. It believes that a dramatic increase in the death of killer whales around Vancouver Island is related directly to PCB contamination. US biologists estimate half the newborn and young whales in the area have died in the past few years. 'I would say [the PCB link] is a certainty, but a lot of scientific documentation has yet to be done,' the centre's Ken Balcomb said. PCBs are toxic, fat-soluble chemicals that were used for industrial purposes, such as in electrical transformers. They were banned in North America in the early 1970s, but residues remain in soil and water. Mr Balcomb said the PCBs may have leaked or been dumped in the sea and subsequently made their way into the marine food chain.