Persistent Organic Pollutants enter the world's waters from a variety of anthropogenic (human-caused) sources. Chemicals such as PCBs (polychlorinated biphenyl), the pesticide DDT (dichlorodiphenyltrichloroethane) and PBDEs (polybrominated diphenyl ethers), flame retardants used in items such as mattresses and children's clothing, are dumped as industrial waste. Heavy metals such as iron, copper and zinc arrive in the oceans from oil spills, road runoff and other manufacturing processes. Fishing practices such as cyanide fishing, which stuns fish with the poison cyanide, also add toxins to ocean ecosystems.
Since dolphins, like their cousins the whales, exist at the top of the marine food chain, toxins entering the food chain at a lower level accumulate upward, so that dolphins consume all the concentrated levels of pollutants absorbed by creatures all the way up the chain. Pollutant poisoning, particularly from PCBs, can kill dolphins outright or sicken them, making them vulnerable to other threats and causing mass fatalities in areas of heavy saturation.
In addition to poisoning dolphins, chemical pollutants can have hidden, long-term effects on dolphins' immune and reproductive systems. Animals with compromised immune systems have little or no resistance to disease, and reproductive damage leads to reduced populations or to the birth of damaged or deformed individuals. Pollutants may also be linked to phenomena such as strandings or disorientation, as toxins attack dolphins' brains.
Pollutants damage marine habitats, indirectly harming dolphins as well as other species. As chemicals create imbalances in ocean ecosystems, fish and marine plants die and bacteria flourish, causing disease and disruptions in the dolphin food chain. Toxic algae outbreaks caused by these imbalances can reduce oxygen in the water, driving dolphins from safe areas. Marine debris, including plastic bags, tarps and other non-degradable objects dumped along shorelines and in coastal areas can trap or choke dolphins, especially young animals.