Dolphins have a few predators in the natural world, essentially some larger sharks and orcas — and they take relatively few dolphins. Over the last couple of centuries Man has created a much wider range of threats which have dramatically altered the risks that dolphins face.
Modern fishing techniques are vastly more effective than those used one hundred years ago. As a result, many of the species that dolphins traditionally preyed upon have been dramatically reduced in their abundance and about one-third of all fishing stocks worldwide have collapsed. In fact, overfishing has depleted fish populations to the point that in most of the world large scale commercial fishing is not economically viable without government assistance. The fall in fish stocks is a significant concern for humans too, but one that is not widely understood.
During the early years of purse seine fishing, huge numbers of dolphins routinely drowned in nets, estimated at over 350,000 each year. Eventually, consumers concerned about dolphins started boycotting tuna in the late 1980s. In response to public pressure, major canneries stopped buying tuna caught using nets set around dolphins and in the 1990s “dolphin-safe” labels started appearing on tuna cans indicating that modified techniques had been employed which released most of the dolphins inadvertently caught.. Although this dramatically reduced the number of dolphins killed by the tuna fishing industry, it has been estimated that around 60,000 cetaceans are still trapped in nets each year, and the number is thought to be increasing. While gill-nets are now the main concern, but purse seines, beach seines, trawl nets, driftnets, and longline gear also kill whales and dolphins.
Dolphin hunts occur in Asia, parts of Africa and South America. Most of the dolphins killed are used for food but in some cases they are killed because local fishermen believe that they compete with them or damage fishing nets. Dolphin drive hunting is a method of hunting dolphins by driving them together with boats and then usually into a bay or onto a beach, as was featured in the movie The Cove. Many thousands of dolphins are killed by man this way each year, yet remarkably, they never fight back against their aggressors.
Given that the dolphins’ most important sense is hearing it is clear that any reduction in hearing ability can be very significant. This may take the form of simple background noise pollution, or it may involve physical damage to the dolphins’ hearing. Noise can come from shipping, ocean research, the seismic testing techniques used to find oil and gas deposits and military activities. Some military technologies now use powerful sounds that may radiate over thousands of kilometres.
Confusion or actual damage caused by noise is considered to be a possible reason for mass strandings by dolphins.
Enormous amounts of waste products enter the sea. Some of these create a simple physical hazard and huge numbers of marine mammals become entangled in marine debris. Others ingest items like plastic bags in mistake for food.
Heavy metals and organic pollutants in the oceans may well be the reason that recently we have found that dolphins are now growing cancerous tumours. There is also evidence that pollution is weakening the immune systems of marine mammals. Persistent Organic Pollutants (POPs) including pesticides such as DDT, and industrial chemicals; most famously the Polychlorinated biphenyls (PCBs) are a major hazard. PCBs are linked to immune system suppression and reproductive failure, and have been found in high levels in the bodies of dead dolphins in many parts of the world. To learn more about these problems and what you can do to help, visit Project AWARE or the Whale and Dolphin Conservation Society.