Seals carry heavy pollution burden
Printer Friendly Page

By Maureen Kelly

AFTER FOUR YEARS of study on U.S Atlantic coast seals, a Maine researcher and her colleagues have found that seals in the region bear high levels of PCBs, DDT and other pesticides in their blubber, and that substances banned in the United States as long ago as the 1970s are still cycling in the environment.

Conducted by Dr. Susan Shaw, founder and executive director of the Marine Environmental Research Institute in Blue Hill, Maine, the studies tested 60 stranded and captured seals. The research provides the first extensive look at levels of persistent organic pollutants (POPs) and other contaminants in the region's harbor seals since the 1970s.

POPs are man-made chemicals such as dioxins, furans, dioxin-like PCBs and pesticides such as DDT that are slow to degrade and can linger in the environment for decades. Animals that feed at the top of the food chain accumulate POPs in their fatty tissues from eating contaminated fish or other organisms. Mothers pass the toxins on to their young in the womb and through nursing.

Since 2001, Shaw, an environmental toxicologist and public health professional, has been gathering data on what she calls "the cocktail of pollutants that are in all wildlife and humans" to see if there is an association between the levels of toxins in seal blubber and health.

The harbor seals that live along the coast from Maine to New Jersey have multiplied over the past two decades to a seemingly robust population of about 100,000 animals. But Shaw cautions, "Just looking at numbers doesn't tell you much about health."

Several mass die-offs in the region over that same period suggest that the seal population in the region may be susceptible to recurrent viral disease outbreaks, according to Shaw.

In 1979 and 1980, influenza spread from Cape Cod through the Gulf of Maine killing 500 harbor seals. Phocine distemper virus (PDV), a pathogen related to canine distemper virus, infected harbor seals from southern Maine to New York in 1991 and 1992.

Most recently, during the summer of 2004, about 300 harbor seals - mostly pups - turned up dead on the coast of southern Maine over the course of several weeks. The cause is still undetermined.

In trying to understand the cause of mass seal mortalities in other parts of the world, many scientists have considered exposure to POPs a factor that may predispose seals to disease by weakening their immune systems. Some POPs, notably the polychlorinated biphenyls (PCBs), are "endocrine disruptors" that interfere with the hormones that trigger the growth of an organism and can cause immune system dysfunction, developmental problems and sexual and reproductive abnormalities. PCBs and dioxins are also cancer-causing chemicals in animals and humans.

After PDV decimated the seal population in northwestern Europe in 1988 leaving over 20,000 dead, a Dutch study investigated the role contaminants might have played. The researchers found that seals fed a diet of herring from the notoriously polluted Baltic Sea had higher levels of POPs than seals fed on Atlantic herring. The Baltic seals also showed signs of immune deficiencies.

It is unclear whether pollutants played a role in the die-offs along the U.S. Atlantic coast, but Shaw's research shows that the burden of PCBs and pesticides carried by some seals is enough to impair immune function and effect reproduction and endocrine function.

PCBs were the most prevalent contaminant Shaw found in the blubber of the stranded seals she studied. Adult males had the highest levels followed by pups, yearlings, adult females and fetuses. The higher amounts in adult males, as opposed to females, could be attributed to the females transferring some of their toxic burden to their pups when nursing.

Compared to other industrial areas of the world, U.S. Atlantic coast seals have PCB burdens in "the high-middle range of the contaminant spectrum," Shaw said. Seals from the Baltic, Dutch Wadden and Caspian Seas have the highest PCB loads in the world.

Other pollutants are also of concern. Mercury levels, obtained from hair and liver samples, suggest that the metal may be increasing in the region's harbor seals. Levels in liver samples of adult seals from Maine exceeded threshold levels for liver damage in mammals. Young seals exposed to mercury in the womb or from their mothers' milk may be particularly vulnerable, as are young humans, to neurological effects.

Shaw will be expanding her research to study more free-living seals and to look at other chemicals that are accumulating in marine mammals. She will be investigating flame retardant compounds, PBDEs (polybrominated diphenyl ethers) that are widely used in electrical appliances, upholstery, and foams. PBDEs which are banned in Europe, are rapidly building up in the environment. “Concerns about PBDEs have arisen because of their increasing presence in human tissues in Europe, Canada and the United States, and their association with endocrine disruption and reproductive and developmental toxicity including neurotoxicity in young mammals,” Shaw said.