Dolphins are faced with a variety of impacts that threaten their existence. These threats are mainly a result of human activities. The threats to dolphins include pollution of the environment, climate change, and commercial harvest; dolphins are vulnerable to entanglement in commercial fishing gear. Discarded fishing gear also poses a major threat. Entanglement drowns dolphins when they are unable to reach the surface to breathe. A number of dolphin species are on the verge of extinction for this very reason. River dolphins share their habitat with a large percentage of the world's population which is threatening their existence. The construction of dams, boat traffic, and other waterfront development are destroying their river habitats at an alarming rate. Some dolphin species are harvested as a food source. Despite containing toxic levels of contaminants such as heavy metals in their flesh, some countries continue to consume them. These contaminants have been directly linked to illness and disorders in humans who consume them. As ocean temperatures rise from climate change, prey populations can be affected. Climate change also affects ocean currents altering prey supply, feeding grounds, and traveling pathways. Other threats to dolphins include capture for marine park industries, activity and spills resulting from oil and gas development, disturbance by leisure watercraft, and noise pollution.