There are 28 species of dolphin in the waters off southern Africa, all members of the family Delphinidae, small toothed whales.

As dolphins are mammals they breathe air and have a constant body temperature, about the same as man's. Dolphins mate at sea and the female bears a single calf, tail first so that it doesn't drown. The calf immediately swims to the surface to breathe. Soon after birth the calf suckles for the first time. This also occurs underwater, so suckling takes place in short bouts between breaths.

Dolphin milk is about four times richer than human milk, which allows the calf to grow very quickly. Calves suckle from their mothers for about 18 to 24 months, although they may begin eating fish and squid at about six months. During the long suckling period, the mother teaches the calf how to hunt and behave, much like humans teach their offspring.

WHAT DO DOLPHINS EAT?

Dolphins hunt for fish and squid and often cooperate in catching these prey. A group will surround a school of fish and dash in one at a time to gorge themselves.

THE SENSES OF DOLPHINS

Although dolphin eyesight is good, long distance vision is reduced underwater. Dolphins use a sound "seeing" system, much like bats, called echolocation. Echolocation is as important to dolphins as sight is to most land mammals. It provides information such as water depth and the position of food and rocks. There is even evidence that some dolphins use powerful sound waves to stun their prey, making the prey easier to catch.

Dolphins do not have a sense of smell, but they can taste quite well and their skins are very sensitive. With these senses they are aware of where they are through feeling different water temperatures or tasting water types. This is useful if sight or echolocation are restricted.

MAN AND DOLPHINS

In the open seas and in coastal waters, dolphins are in danger.

* Pollution: The accumulation of waste, such as plastic, and toxic substances in the oceans cause problems for dolphins. In many parts of the world, stranded dolphins are often found to have swallowed plastic bags or have nylon straps wrapped around their tail flukes. Fortunately this is uncommon off the South African coast, but as our population and the production of plastics and other wastes increases, our dolphins may still have to face this problem. Toxic pollutants come from two main sources, agriculture and industry. Agricultural pollutants include DDT, dieldrin and lindane, all of which are still used in pest control and public health. Dolphins accumulate these pollutants and levels in dolphins off KwaZulu/Natal are a cause of concern.

The main industrial pollutants are PCBs, a group of highly toxic substances. High levels of PCBs are found in dolphins off South Africa, and KwaZulu/Natal particularly, and they probably lead to the death of many newborn dolphin calves.

Recent research suggests that dolphins, particularly the males, are unable to rid themselves of toxic pollutants and because of this and the continued accumulation of pollutants in the marine environment, some dolphin populations may become extinct within the next 100 years.

Pollution also includes non-toxic effluent such as sewage and waste water which is pumped into rivers and the sea by industry and cities. Bad agricultural practises causing soil erosion result in soil and silt accumulation in the inshore region. Silt and effluent laden water smothers reefs and prevents light from reaching underwater plants, thus limiting their growth and ability to provide food for the animals on which dolphins feed.

* Gill nets: used in coastal and deep sea waters, gill nets are a very cheap and efficient method of catching fish or squid. However they are not selective and trap any animal that comes into contact with them. Current estimates are that between one and three million dolphins are accidentally captured in gill nets each year. The problem is so serious that many dolphin populations face depletion and extinction.

Fortunately only a few gill nets are used in South Africa's coastal environment, the most notable of these being KwaZulu/Natal's shark nets. Captures of bottlenose and humpback dolphins in these nets threaten the survival of both species in KwaZulu/Natal's waters within the next few decades. In other African countries, gill nets are used extensively in coastal waters to feed expanding human populations. In the deep sea waters off southern Africa the situation is bad. Currently some 400 ships using gill nets are known to operate in the southern Indian and Atlantic oceans and these ships catch uncounted numbers of dolphins each year.

IF YOU FIND A DOLPHIN STRANDED ON THE BEACH:

If alive get help as quickly as possible (see contacts below). While waiting for help be quiet and keep the dolphin cool and wet by covering with a wet blanket or seaweed. Be careful not to get anything into the blowhole (usually on the top of the head) through which the animal breathes. If dead, photograph and measure the animal and report it to one of the contacts listed. Information from strandings is of vital importance to dolphin research.

FURTHER READING

MARINE MAMMALS. V.G. Cockroft. De Jager Haum Insight Series, Pretoria, 1988.

WHALES, DOLPHINS AND PORPOISES. R. Harrison and M. Bryden (eds). Timmins Publishers, Cape Town, 1988.

THE GREENPEACE BOOK OF DOLPHINS. J May (ed). Struik Timmins, Cape Town, 1990. All books available from Russel Friedman Books, PO Box 73, Halfway House, 1685. Tel. 011-7022300/1.

USEFUL ADDRESSES

Centre for Dolphin Studies. Port Elizabeth Museum, P O Box 13147, Humewood 6013. Tel. 041-561051

MRI Whale Unit. c/o South African Museum, P O Box 61, Cape Town 8000. Tel. 021-243330

Natal Sharks Board. Private Bag 2, Umhlanga Rocks 4320. Tel. 031-5611017.

Oceanographic Research Institute. PO Box 10712, Marine Parade 4056. Tel. 031-373536, a/h 031-373530

Dolphin Action and Protection Group. PO Box 22227, Fish Hoek 7975. Tel.021-7825845.

Natal Parks Board. See directory for nearest zone office.