Killer whales that feast on salmon in the Pacific Northwest are getting a heaping side of contaminants with each meal. The chinook salmon are heavily dosed with chemicals such as DDT and PCBs, nearly all of which the fish acquire in their years at sea, reports a new study in the January Environmental Toxicology and Chemistry.
“These are some of the most PCB-contaminated mammals on the planet,” says Peter S. Ross, a research scientist with Fisheries and Oceans Canada in Sidney, British Columbia. Ross is advisor to Donna Cullon, a doctoral student at the University of Victoria in British Columbia who led the new work.
The study investigated organic pollutants in chinook salmon to better understand chemical exposure in a population of resident orcas that live around Vancouver Island and Puget Sound. There is concern for both the northern and southern populations, which are both protected.
The southern population, protected under the U.S. Endangered Species Act, has had a harder time. Previous work has shown that this population has nearly four times the amount of polychlorinated biphenyls (PCBs) in their flesh than the northern population, perhaps due to a history of greater pollution in the more-industrialized southern region.
The northern population comprises nine pods and roughly 200 individuals. Three pods make up the southern population, which now numbers 83, down from more than 100 in the early 1990s.
Salmon are roughly 92 percent of the whales’ diet; nearly 70 percent of that is chinook. (Unlike their transient killer whale cousins, resident killer whales do not eat mammals).
Salmon are known to deliver pollutants, especially PCBs, to coastal, freshwater and terrestrial ecosystems. PCBs are a kind of endocrine disruptor, known to interfere with development, meddle with immune system function and cause a host of other problems. The Environmental Protection Agency banned most uses of PCBs in 1979; but the chemicals had been widely used in coolants, pesticides, plastics and other products and are extremely persistent in the environment, cycling through the food web for decades.
To investigate why the two populations had been contaminated differently, the researchers measured persistent pollutants, including PCBs, flame retardants, certain pesticides and other industrial by-products, in chinook salmon along a north-to-south gradient in the southern orcas’ feeding range. Adult fish and the silvery smolts, or young salmon, were collected from five sites — the most northerly fish from the Johnstone Strait and the most southerly from lower Puget Sound.
The researchers found that the southernmost salmon had both the highest concentrations of chemicals and the lowest amount of body fat. Not only are the southern chinook more contaminated, but because they have less fat, the orcas have to eat more of the salmon to meet their energy needs.
“It’s a double whammy,” says Kenneth Balcomb, senior scientist with the Center for Whale Research in Friday Harbor, Wash., which has been studying the area’s orcas since 1976. Calculations based on a whale’s diet of roughly 250 kilograms of salmon per day suggest that the southern orcas are getting 6.6 times more PCBs than the northern orcas.
Adult salmon had a much higher burden of pollutants than the 1- to 2-year-old smolts, suggesting that while areas like Puget Sound have a chemical legacy, chinooks pick up much of their toxic loads at sea. Because they are high in the food chain, orcas get magnified contaminant concentrations.
Adult chinooks spawn in their natal rivers after years in the open ocean. After about a year, young fish head out to live at sea for many years and acquire the majority of their body mass. When they return to spawn (and die) they burn through enormous stores of body fat — nearly 80 percent — leaving much of the fat-soluble contaminants in water as they go. Previous research has found a sevenfold increase of PCB concentrations in Alaskan lakes when returning salmon numbers are high.
“The challenge is when these chemicals move into an area as large as the Pacific, where cycling is happening very slowly,” says Jules Blais, director of the chemical and environmental toxicology program at the University of Ottawa. PCBs in particular are very hard to get rid of — they must be incinerated at very high temperatures. There are 209 ways to build a PCB molecule, which can have one to 10 chlorine atoms attached; the PCBs that accumulate in animal tissues have five to six chlorine atoms.
Female orcas tend to have lower concentrations of the chemicals because mothers offload them to their young, both in the womb and through breast milk. This means at a developmentally fragile time, young orcas get a hefty dose of poisons. “This study has far-reaching implications for the health of marine mammals,” says Blais.
Toxic substances aren’t the only threats the orcas face. Increased noise pollution from shipping, sonar and industrial activities also stress the animals and the crash of salmon stocks in recent years has made it harder for these whales to get a meal, albeit a toxic one.