In 1988, white-sided dolphin carcasses washed up along the coast of Maine, killed by the same virus. And since 1990, more than 1,000 Mediterranean striped dolphins have died of a similar ailment. Other fatal illness also appear to be on the rise among marine mammals. In 1989, a mysterious fungus was discovered in 274 dolphins that washed up dead on along the coast of the Gulf of Mexico. And in 1991, analysis of 39 harbor seals that died in New York harbor showed unusually low numbers of white cells in their blood.
White cells play a key role in the immune system's ability to fight infection.
But until the marine mammal epidemics, these toxic pollutants had never been persuasively associated with immune deficiencies that made animals vulnerable to infectious diseases. To medical researchers, the association between marine mammal fatalities and high tissue levels of organochlorines was intriguing, but it was not proof of cause and effect. The proof came from researchers with the National Institute of Public Health in the Netherlands. Shortly after the Baltic seal epidemic, Dutch researchers captured 22 seal pups along the coast of Scotland and for two years, fed them their normal diet of herring. Half the pups ate herring caught in the relatively clean North Atlantic, while the other half's herring came from the industrially polluted Baltic.