Dolphins have inhabited the world’s oceans for 10 million years. Although they prefer more tropical waters, they are also found in colder, polar regions. The 40 known living dolphin species range in size from 4 to 30 feet in length. These social marine mammals travel together in cooperative pods of 2 to 15 animals, and although they do have natural predators, humans are their biggest threat.
Pods of dolphins work together to defend one another, and will circle around weaker pod members to protect them when potential predators are present. For this reason, few animals will bother with dolphins, although sharks sometimes prey on them. Dolphin pods will attack an animal that threatens them, however, and can kill sharks, so all but the largest sharks tend to leave them alone. Researchers have found pieces of dolphins in the stomachs of orcas, but have no way of knowing whether the consumed dolphins were hunted down or simply scavenged.
In some areas, people hunt dolphins for meat. In the Faroe Islands, for example, killing and eating many species of cetacean including dolphins is an ancient cultural tradition. The dolphin is considered a delicacy in Japan, where a single pound of its meat costs as much as $25. In South America, some fishermen kill and sell dolphins because their oceans have been depleted of other stocks of fish. Even where they are not hunted themselves, dolphins are harmed by overfishing that makes it harder for them to find food.
Aside from the harm dolphins suffer through overfishing, large commercial fishing operations use nets that trap dolphins along with the fish they target. The trapped dolphins suffocate when they become entangled in the net and cannot reach the surface to breathe. Dolphins also may break off their fins or otherwise injure themselves in their desperate struggle to free themselves. Whale observers estimate 30 million dolphins have been killed in this way since they began keeping records in the 1960s.
The dolphin’s sensitive hearing is its most important sense, because the dolphin uses echolocation to navigate, compute the distance and depth of objects in the ocean, communicate with other dolphins and find sources of food. Loud, unnatural noises not only cause physical damage to a dolphin’s hearing but also cause them to become confused. Confused dolphins may swim into water that is too shallow and become stranded on shore, or may suffer decompression sickness from surfacing too quickly. Sources of harmful noise pollution include massive shipping vessels, naval sonar, construction, firing exercises and seismic testing to discover offshore deposits of oil and natural gas.
Oil spills, plastics and other chemicals dumped into the ocean can damage dolphins internally as well as killing the fish on which they feed. Heavy metals and organic pollutants weaken dolphins’ immune systems, making them vulnerable to debilitating infections, parasites and diseases a healthy dolphin would be able to overcome.
Most dolphins live in the shallower water close to shore, and these same areas are frequently populated with boats and other recreational watercraft. Dolphin skin is sensitive, and dolphins hit by boats become vulnerable to infections, bacteria and other parasites. The larger the marine vehicle, the more harm it causes to dolphins that are struck.
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