Biologists have been observing sporadic die-offs, diseased and emaciated animals and reduced reproductive success among marine mammals for some time, according to WWF. Studies of bottlenose and striped dolphins suggest that viral outbreaks may be related to the presence of synthetic chemicals. Troubling findings of clusters of cancers in beluga whales signal long-term immune system weaknesses, according to the group. The WWF report is taken from a larger scientific study to be released in 1999, and underscores the urgent need to develop a clear picture of chemical impacts on whales and other marine mammals. It details how persistent organic pollutants pose significant health threats to the world's cetaceans (whales, dolphins and porpoises) and their offspring. Chemical contamination is linked to disease, reproductive failure and sporadic die-offs among the world's whales, dolphins and porpoises, according to the report from the World Wildlife Fund. "Synthetic chemicals are the stealth bombers of the oceans -- able to travel undetected across continents and drop their deadly loads on the most unsuspecting victims," said the director of International Wildlife Policy. However, long-term impacts of toxic pollutants are often difficult to detect because mother whales transfer chemicals accumulated in their tissue to their offspring during gestation and lactation, the most sensitive period of development. There is increasing evidence that chemicals can interfere with a whale's ability to develop normally, to breed and to cope with stress and disease. PCBs,linked with hearing loss in animal studies, causing great concern among cetacean scientists, because of the unique dependence of cetaceans upon their auditory system for navigation and communication; Mercury,findings suggest that the immune systems of some cetaceans, particularly beluga whales, are exceptionally sensitive to mercury; Plasticsand plastic components, (e.g. bottles and packaging) until recently considered inert are now recognized as having characteristics that make them biologically active. In the laboratory, several of these globally dispersed compounds undermine the development of the reproductive tract in the offspring of pregnant mice fed exceedingly low doses. Pesticides,many of which are designed to inhibit photosynthesis, travel thousands of miles in the atmosphere to the Arctic, where they could potentially destroy the food supply of cetaceans by killing algae.