Why are Dead Seals Washing Up on Our Beaches?

Since September 28, 146 dead harbor seals have washed up on the shores of Massachusetts, New Hampshire and southern Maine. It is a very upsetting sight and a matter of concern. According to NOAA, this fatality rate is more than three times the average number that typically occurs this time of year.

But this mass die-off is not an isolated event. At least five “unexplained mortality events” or UMEs have affected Gulf of Maine harbor seals since 1980. The cause of death is rarely discovered, apart from two die-offs that were attributed to viral infection. In the current event, five seals tested positive for influenza A virus, while tests for six other viral pathogens and biotoxins were negative. These seals were pups – less than a year old and recently weaned from their mothers. Although the harbor seal population in the northeast is increasing, the pattern of recurring mass mortalities indicates that they are not necessarily healthy.

Two decades of researching toxic contaminants in seals leads me to wonder whether immunosuppressive chemicals play a role in the animals’ susceptibility to viruses and other pathogens. MERI research shows that northwest Atlantic harbor seals are among the most contaminated seals in the world -- and we have documented alarming levels of PCBs, dioxins, flame retardants and other persistent chemicals in their tissues. Much lower levels of PCBs alone have been shown to impair normal immune responses in seals, thus it is plausible that the high body burdens of chemicals in our harbor seals may be compromising their immune systems.

Surprisingly, tests to identify toxic contaminants in seal tissue are not always performed routinely during or after mortality events including this most recent one. Contaminant analysis must be conducted by a specialized lab and takes longer and costs more than testing for biotoxins, trauma or infections. MERI is specialized in the analysis of toxic chemicals in wildlife, but unfortunately, when a UME is officially declared, samples for independent research are not made available. This perhaps reflects a lack of resources presently available to federal authorities. Sadly, it also shows a lack of motivation to identify and document the man-made contaminants that may be indirectly killing our marine wildlife.

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