Updated: January 16, 2009
Puget Sound killer whales are not only suffering from exposure to a contaminated food supply, but the salmon they're eating appear to be less nutritious than scientists previously assumed.
A new study in the a journal called Environmental Toxicology and Chemistry reported that Puget Sound chinook salmon contained a lower percentage of fat than chinook along the west coast of Canada. This finding provides an additional explanation of why Puget Sound's orcas are loaded with more toxic chemicals than their salmon-eating cousins in northern British Columbia.
It has been known for several years that the average chinook returning to Puget Sound contains more chemicals than fish returning to rivers up north, said Sandra O'Neill, who works for the state Department of Fish and Wildlife and is a co-author of the study. Part of the reason, she said, is that a significant percentage of the fish in Puget Sound stay in the waterway, picking harmful chemicals from urban bays. In contrast, Canadian salmon stay in less polluted rural waters or swim out to sea.
The result is that the Southern Resident orcas, which spend time in Puget Sound, tend to contain more contaminants than Northern Residents, which frequent the northern end of Vancouver Island, where the fish are cleaner. For example, Southern Residents have been found to harbor four times the concentration of polychlorinated biphenyls (PCBs) in their blubber than Northern Residents. PCBs are a group of long-lived chemicals believed to impair the immune and reproductive systems of many mammal species.
The latest report shows that Puget Sound chinook also contain less body fat, which means that the Puget Sound killer whales probably eat more fish to maintain their energy levels and that results in an even greater uptake of contaminated chemicals.
"It's a combination of things," O'Neill said. "The fish (from Puget Sound) are smaller, a little less fatty and they're more contaminated."
Toxic chemicals are a problem for the entire Puget Sound food chain, she said. Orcas pick up contaminants from salmon and other fish they eat. Salmon pick up contaminants from herring and other fish they eat. Herring pick up contaminants from plankton ... and so on.
Persistent chemicals tend to accumulate in animals higher in the food chain and Puget Sound's killer whales are known to be among the most contaminated marine mammals in the world.
To reduce the risk, experts call for cleaning up contaminated sediments in Puget Sound and preventing further deposition through stormwater and sewer discharges. The issue also needs to be addressed worldwide, because a significant contributor is atmospheric deposition from industrial smokestacks in Asia and other parts of the world.
For a discussion about water-related issues, check out the blog Watching Our Water Ways at kitsapsun.com.
© 2008 The E.W. Scripps Co.