River dams create barriers that affect freshwater dolphin movement and their prey. Dams may isolate dolphin communities which could endanger genetic diversity and limit their survival. Coastline development often creates more runoff pollution such as sewage bacteria and toxic chemicals which can harm dolphins and their habitat environment. In addition, plastics may be ingested by dolphins which they mistake for food, or they may become entangled in disposed plastic.
It is illegal to feed dolphins under the Marine Mammal Protection Act. Human-fed wild dolphins have their instinctive role as hunters disrupted. Seeking to be fed by fishermen, they could be injured by boats, get caught in fishing nets or ingest fishing hooks. If teased while eating from humans, dolphins may become aggressive and literally bit the hand that feeds them.
Dolphins rely on the sense of sound to communicate with each other, navigate and to find natural food supplies. Sound travels through water, so outside noise may affect these abilities. Examples of noise are construction, boat traffic and drilling. High-powered sonar used by naval organizations and the oil/gas industry is being studied in relation to deaths of wild dolphins.
Fishing nets are major threat for coastal dolphins. They don't see or detect fishing nets well so they can swim into a net, be trapped and subsequently drown. This has been known to happen for the Ganges River dolphin, harbor porpoises in the Black Sea, Irawaddy dolphin and the Hector's dolphin off the New Zealand coast. In Japan and the Solomon Islands, "drive fisheries" round up and kill dolphins for their meat and because they are seen as a threat to the fishing industry.