Unfortunately, one of the characteristics marine mammals share is that most are, or have been, exploited by humans. Whales, sea otters, sea cows, and seals have been historically overexploited, which has resulted in the extinction of some species (e.g. Stellar sea cow, Caribbean monk seal, and the Atlantic gray whale) and the endangered status of many. Though there are laws that protect marine mammals, not all countries abide by these laws. So, the direct harvesting of whales, seals, walruses, dugongs, and dolphins still occurs today in various places throughout the world.
Another threat to marine mammals is entanglement in fishing gear and nets. Many marine mammal species are accidentally caught in nets used by fishermen. This is called “bycatch”, which is the non-target portion of the catch. Bycatch includes not only marine mammals, but also other species of fish, invertebrates, reptiles, and even birds and is a very wasteful practice. Passive, or discarded, gear and trash cans also cause a significant amount of marine mammal mortality each year. Lost and discarded nets wreak havoc on marine organisms as they drift for many miles and sometimes many years. When a marine mammal gets entangled in a net, they often lose the ability to swim to the surface to breathe and then drown.
Marine pollution, specifically single use plastics, cause injury or death to many marine species every day. Also, toxins, can cause serious health issues in marine mammals. Chemicals such as PCBs, organochlorides, pesticides, and heavy metals are related to reduced immune function (i.e. the ability to ward off infections and disease), an increase in the mortality of newborns (also called “neonates”), sterility, fewer births, birth defects, skeletal abnormalities, various types of cancer, and neurological dysfunction.
Oil spills suffocate marine environments and cause serious issues for many marine organisms. Marine birds and marine mammals tend to suffer heavily from exposure to oil, because they lose their ability to float and insulate themselves when their feathers/fur become soaked with oil. The long-term effects on marine mammals from oil spills are associated with the ingestion of the oil and the inhalation of hydrocarbon vapors. The Exxon Valdez spill in Alaska in 1989, resulted in the death of approximately 250,000 seabirds, sea lions, 2800 sea otters, 300 harbour seals, 22 killer whales, and billions of fish. The recent BP oil spill is said to be the equivalent of 13 Exxon Valdez spills as of July 2010.
Other threats to marine mammals include sound pollution (interferes with communication, breeding, diving, migrating, and feeding), collision with vessels (particularly manatees but also occurs between dolphins and whales and large ships), river regulation devices (causes drowning of manatees and river dolphins), habitat destruction, and climate change (e.g. global warming and the loss of polar bear habitat and prey).