Dolphins are threatened by humans in several distinct ways; by direct hunting, through accidental catches in commercial fishing operations, through habitat degradation and from the new threat of Ecotourism.
Dolphins are still deliberately killed in different parts of the World's oceans for food and oil, or because they are seen to compete for a dwindling fish resource. This occurs in Peru, Chile, the Caribbean, the Canadian Arctic, the Faroe Islands (Pilot Whales), the Solomon Islands, Taiwan and Japan. Indigenous people in some countries hunt dolphins and other whales, as part of their traditional culture and not for commercial gain.
Accidental entrapment in fishing operations occurs in a number of ways. At the beginning of the 1990's tens of thousands of kilometers of drift nets would be set in the Pacific Ocean each night, like giant vacuum cleaners, "sucking in" every living thing in their path. The actual toll of drifts nets on dolphins has been estimated at over 5 million, and their carcasses have been dumped back into the ocean as useless waste. It is clear that the sheer magnitude of drift net fishing has depleted, if not endangered, some populations. World-wide outrage at the use of these nets has led to bans of their use in coastal fishing zones and the eventual phasing out in the longer term. The sad thing is that discarded nets still float around in the world's oceans acting as eternal death traps for bottom-dwelling species.
Purse-seine tuna fishing in the Pacific has killed millions of dolphins in the last few decades. For reasons that we do not understand, huge schools of dolphins in the Pacific are accompanied by schools of yellowfin tuna that swim directly underneath. The "Purse-seiners" find the tuna by first locating the highly visible dolphins and trap them both together. Many of the dolphins simply sink and drown in a type of "sleepy" panic state. In the 1960's, millions of dolphins would die this way each year. The American government acted to change the types of nets and practices so that the annual mortalities dropped to less than 100,000 dolphins per year. It is believed that the death rate has again risen because of tuna boats that have become re-registered and are no longer under American control. Some tuna fisheries off the Australian coast use hooks to catch their fish and consequently are "Dolphin Safe".
Habitat destruction is perhaps the largest threat to dolphins. It is estimated that the dolphin population in the Black Sea has plummeted to just 1% of the number recorded in 1946. It is thought that this is the result of heavy pollution and the massacre of dolphins by the communist regime. It is feared that there will be no dolphins left in the Black Sea by the turn of the century. Elsewhere, we still use the sea as a bottomless sewer and toxins build up through the food chain to reach alarming levels in some dolphins. It is thought that levels of organochlorines (particularly PCB's) can suppress the immune system. Organochlorines have been linked to the poor reproduction rate and high rate of disease in beluga whales in Canada. We also compete directly with dolphins for food and there is little doubt that the depleted coastal fisheries in many areas directly threaten dolphin populations.
The stranding of whales and dolphins on our beaches is probably a natural phenomenon and something that poses little threat to natural populations. The sight and sounds of beached whales evokes emotions and concern from the public and teams of enthusiastic volunteers may work around the clock in rescue attempts.
Strandings of isolated individuals are usually sick or aged animals that come ashore to die. There is little that can be done for animals in this situation apart from minimizing their suffering.