Human Threats to Marine Mammals

What are marine mammals?
Marine mammals are mammals that spend most or all of their time in the ocean. Marine mammals include dugongs, seals and dolphins. Because they are not fish, they need air so have to come to the surface to breathe.

Fishing Nets
A great danger to marine mammals is being caught accidentally in fishing nets. If they cannot come to the surface to breathe, they drown.

Some kinds of nets used by commercial fishermen pose a particular danger to marine mammals. Nets such as gill nets, trawling and drift nets capture any marine life that swims or floats into a huge net towed behind a boat. Sometimes a mammal that is tangled in fishing net manages to escape, but the net prevents it from hunting and it starves to death.

These seals are tangled in fishing net and will starve to death because they can't hunt

Purse-seine nets are spread in a ring around fish and then the bottom of the ring is closed up by pulling a drawstring, just like a purse. The net is then dragged up onto the fishing boat.

In the 1960s, Yellowfin tuna were caught in vast purse-seine nets and a method known as "dolphin fishing." This developed because people noticed that schools of yellowfin tuna follow some kinds of dolphin. The dolphins were easy to spot because they surface to breathe. Fishermen spread their nets around the dolphins, and sent speedboats or helicopters to herd the dolphins into the purse-seine nets, knowing that the tuna would go also. The dolphins got tangled in the nets and drowned. It is estimated that as many as 250,000 dolphins died each year, about 7 million altogether through the 1960s. In 1972, laws were passed that tried to stop dolphin deaths through this method of fishing.

In 1988 there were still many dolphin deaths through tuna fishing, and people around the world refused to buy tuna tinned by three major companies who bought tuna fished in this way. They controlled 70% of the tuna industry. In 1990, all three companies agreed to buy only 'dolphin safe tuna', or tuna that was not caught by purse-seine dolphin fishing or drift nets.

Competing with humans
Fisheries also threaten marine mammals by fishing for the same food that the animals depend upon for survival. In many places, marine mammals are seen as competition for fish and are poisoned and killed to keep the fish for humans.

Pollution
All through the world's oceans pollution is found, caused by human sewage, oil spills (like the one at left), poison leakage and dumping, and also noise.

Scientists studying marine mammals are increasingly worried about the effect of noise. Marine mammals originally developed in oceans when there was no human sound. Today, the oceans are filled with the sounds of ships, dredging and construction work, oil and gas drilling, exploration, explosions, and geological studies. Loud sounds can cause hearing damage and low-frequency sounds can affect communication, the detection of prey or predators, and navigation. Marine mammals depend on sound in most of their regular activities.

Conservation and protection
In some places, marine mammals are killed for meat, oil and leather, or killed because they are seen as competition for fish. In some countries it is against the law to hurt, harass or kill a marine mammal, and in some countries the methods of fishing that harm marine mammals has been banned.

Many marine mammals have been listed internationally as endangered (in danger of becoming extinct) or vulnerable/threatened (likely to become endangered in the near future). The animals that are listed as endangered are those that are in danger of extinction in the near future in all of their main habitats.

A recovery plan is designed for all endangered and threatened species to help their conservation and recovery. Protected species in international waters include the gray whales of the Western North Pacific population, the Gulf of California harbor porpoise, the Southern right whale, the Mediterranean monk seal, and the Ringed Seal.

Click here to find out about the ocean environment
http://www.abc.net.au/oceans/seals/environments.htm

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Updated May 2007