Cetaceans – dolphins and porpoises

0

Bottle-nosed dolphins leap out of the water.

THE word cetacean is derived from the Greek ketos meaning whale, which for many centuries, observers thought of as aquatic mammals, for they saw whales as giant fish. Ancient historians did not realise that these are marine mammals with wombs, in which they bear their offspring, releasing a single calf underwater. Just as in mammalian terms, there are bulls and cows which breed to produce a calf.

There are 43 species of dolphin worldwide, of which five species are river dolphins. They are small whales, characterised by their very long dorsal fin and a pronounced horny beak.

In the far southwest of England, I loved to watch, from on top of the high granite cliffs, schools of dolphins chasing shoals of mackerel in summer time. Sadly, owing to the dolphin’s intelligence and affinity to man, they may be seen worldwide in dolphinaria performing tricks and allowing children to ride on their backs. Personally, I have shunned such ridiculous ‘shows’, as I shun any circus today with performing animals of any sort. Sometimes, in a restaurant, you may be tempted to order off the menu the exotic dish described as ‘dolphin’ but do not be fooled, for you will actually be served with the dolphin fish better known as Dorado.

Common dolphin (Delphinus delphis)

These were the dolphins I saw in British waters in summer time, but later in the year they migrate to warmer waters. They are often seen in schools or pods numbering up to 1,000. Loving to leap out of the water as they intake air, they provide an extra summer’s income for boat owners during the holiday season as passengers are charged a fee to see dolphins. Dark grey on top with a pale grey underbelly, they display alternating light and dark bands on their flanks.

Bottle-nosed dolphins (Tursiops truncatus)

These species are the easiest to tame (much like the Asian pink dolphin) and hence their appearances in dolphinaria. This dolphin will frequently pop out of the sea alongside swimmers and yachtsmen. Slate grey in colour with distinctive white throats and bellies, they have a relatively short beak. Since time immemorial, children swimming in shallow waters of the Mediterranean Sea have leapt on their backs for a fun ride.

In a visit to the ancient Roman excavations at Pompeii, I saw a mural depicting a ride on a bottle-nosed dolphin’s back. Pompeii, once a major roman city was overcome by volcanic ash and mudflows during the eruption of Mount Vesuvius in AD 73. I have also seen this species in the Indian Ocean on a voyage from Mombassa to Madagascar.

Irrawaddy dolphins break the surface at Santubong. – Photo by Bernard Dupont

Irrawaddy dolphin (Orcaella brevirostris)

With its Latin name as the short-nosed species of whale, it is related to the killer whale and the Australian snub fin dolphin. It is seen mostly in coastal Indian Ocean locations from India to Indonesia. In Bornean waters, it is sometimes wrongly referred to as ‘the finless porpoise’. This species, with its blunt nose and no beak, together with its grey to slate blue upper body and its rounded flippers with no dorsal fin, is easily distinguishable.

Six months ago I saw a pod of three Irrawaddy dolphins whilst I took a ferry across the very wide Ayeyarwady River in Myanmar. Breaking the surface of the river for air at intervals of between one and three minutes, they truly were a sight to behold. They just love sheltered inshore maritime waters, which receive freshwater inputs and are never seen more than a few kilometres offshore. They mainly feed on crustaceans, cephalopods and small fish from muddy, brackish estuarine waters.

Daily boat trips from Santubong and Damai or kayak tours will guarantee a glimpse of these beautiful mammals here on our doorstep. To see them spit water out of their ever-smiling mouths and observe their bulging foreheads is a real treat. Their average lifespan is 28 years and with a gestation period of between nine and 14 months, one calf is produced every three years despite the polygamous bulls.

It is reckoned that only 6,000 of this species of dolphin are left in our world, for their main threats are manmade fishing gill nets, bacterial infections of pregnant cows and toxic agro-pesticides seeping into rivers. The Irrawaddy dolphin varies on the International Union for Conservation of Nature (IUCN) threatened species lists from vulnerable to critically endangered. Sarawak Forestry has led the world in establishing a protected area for these dolphins between Santubong and Damai where gill net fishing is totally banned.

Harbour porpoises are known to follow migratory herring shoals.

Porpoises

In South West England local fishermen refer to porpoises as ‘herring hogs’, as they follow after shoals of herring along the English Channel. Like dolphins, porpoises are members of the whale family, which descended from land mammals into the seas 50 million years before present (BP). About 15 million years BP they are thought to have diverged from their ancestors, for their two-chambered stomachs are almost identical to hoofed mammals on the land today. There are 14 species of porpoise worldwide.

Porpoise or dolphin?

With distinctly bulging heads and tail fins together with their very blunt snouts, they are smaller both in weight and length. Unlike dolphins, they use blowholes to expel stagnant air from their lungs in a spout and then inhale fresh air. Torpedo-like in shape, they are faster movers through water, darting here and there with constant flipper movements and using their triangular shaped dorsal fin for steering. With eyes planted on the sides of their heads, their field of vision is superb. Another distinguishing feature is their layer of whale-like blubber.

Types of porpoise

They vary enormously in bodily size and weight from Dall’s porpoise (Phocoenoides dalli) at 2.3 metres long and up to 220kg in weight, to the Vaquita porpoise (Phocoena sinus), found only in the Gulf of California, at 1.4 metres long and weighing only 54kg. This is the world’s most threatened species. The Dall’s porpoise can swim at a speed of 74km an hour.

Porpoises only mate every two to three years and, after a gestation period of 12 months, a calf receives its mother’s milk in a series of squirts into its mouth. The milk has a consistency not unlike that of toothpaste. Calves follow their mothers for up to two years before seeking independence.

Little is actually known about the diets of porpoises other than the Atlantic (harbour) porpoise (Phocoena phocoena) which follow the migratory herring shoals, and Dall’s porpoise which feeds mainly on sardines.

Threats to porpoises

Worldwide, their main threat comes from fishing gill nets where, if entrapped in the mesh, they drown. In the Faroe Islands, the Solomon Islands, Peru and Japan they are hunted for their meat in drive hunts with fishing boats encircling a pod of porpoises and forcing them into the shallow waters of bays or onto a beach to be slaughtered.

Heavy shipping traffic and military exercises also take their toll. Submarine sonar systems confuse porpoises and they panic in rushing too quickly upwards to the surface, thus dying of decompression sickness or ‘bends’.

Furthermore, the use of transponder echo sounding on coastal vessels attracts porpoises and they become churned up in ships’ propellers. Add to these threats toxic chemicals from the land which are disgorged into the sea at river estuaries, and porpoises seem to be fated.

All is not lost

China and Mexico have created marine reservation areas for porpoises with very strict laws established to eliminate illegal fishing and hunting of porpoises. In one of China’s many reserves the porpoise population has increased eightfold in 25 years. In the upper Gulf of California, where once up to 80 Vaquita porpoises were annually killed, the Mexican government has banned fishing and thus compensates local fishermen for the loss of their catches.

I sincerely hope that all countries will follow such laws to protect both dolphins and porpoises which are so friendly to humans. Sarawak has certainly made its mark in setting an example of marine conservation between Santubong and Damai to protect its dolphins. Do take a local boat ride to see dolphins at play.