Pollution of the marine environment is one of several human activities that has an impact on marine mammals. In early 1988 nearly twenty-five thousand harbor seals in the North and Baltic Seas—60 to 70 percent of the population—abruptly died of a distemper virus. The seals’ deaths appeared to be part of a disturbing trend. In the late 1970s an influenza virus killed 450 harbor seals off the coast of Cape Cod, Massachusetts. During 1987 and 1988 the morbillivirus killed about 750 bottlenose dolphins along beaches from New Jersey to Florida. Studies revealed that the bodies of these marine mammals contained high levels of persistent organic pollutants (POPs) that are found in a wide variety of industrial chemicals, pesticides, herbicides, and manufacturing byproducts. These pollutants are called persistent because they do not decompose easily and once released into the environment last...