BREMERTON, Wash. - As top predators in the food chain, killer whales are "general sentinels of ecosystem health" -and things are not looking so good at the moment, a toxicologist said.
Peter Ross, of the Canadian Department of Fisheries and Oceans, joined a group of experts to discuss how toxic chemicals are working their way up the food chain.
The mystery that still stumps researchers is why the southern resident orcas - which frequent Puget Sound - contain about three times the levels of PCBs in their blubber as the northern residents, which stay in Canada and Southeast Alaska.
Both groups of whales eat fish, mainly salmon. Neither takes seals or other marine mammals.
Since salmon put on most of their weight in the ocean, they may be picking up most of their toxic loads from atmospheric deposits of PCBs.
"Toxic chemicals," Ross said, "are globally distributed through atmospheric processes."
PCBs have been banned in the United States, but they're still produced in some countries. It takes just seven to 10 days to get from smokestacks in China to the Northwest coast. And PCBs have been found in snowpacks of the Rocky Mountains.
"It's important for us to clean up our back yards," Ross said, "but we are very much a part of the global village."
Why southern resident orcas show higher levels of PCBs than northern residents is hard to understand, given their similar food supplies. But it may be that southern residents have a greater preference for chinook salmon, which have a higher fat content. PCBs are stored in fat. Or it could be that southern residents are eating more bottom fish, which contain overall higher PCB levels.
Jim West of the Washington Department of Fish and Wildlife described during the recent discussion how bottom fish living in urban bays show much higher levels of PCBs than fish from more pristine areas.
Puget Sound is a textbook study of bioaccumulation, he said.
Herring eat plankton, which pick up PCBs from the water. Salmon eat plankton. Killer whales eat salmon. Each step increases the level of PCBs in the tissues of the next predator.
Humans who eat a lot of salmon, such as Native Americans, show higher levels of PCBs than the average person, though nowhere near the levels found in orcas.
In female killer whales, PCB levels generally decline during their reproductive years. That's when they off-load the toxins through their milk to nursing offspring. After age 45, the levels start climbing again.
Since PCBs mimic hormones, scientists suspect the chemicals may be interfering with immune and reproductive systems of the whales. Either could contribute to the sudden decline in population the past few years.
PCBs in orcas are above levels known to have serious effects on seals.
While northern and southern resident orcas stick with fish, a group of wider-ranging whales, called transients, goes after marine mammals, such as seals.
Because seals accumulate more PCBs than salmon, the transients have the highest levels of PCBs of all the orcas.