13. Feb, 2020

The surviving porpoise

The name porpoise is a strange assembly of fish and swine. It’s stocky body and bulbous head with a small blunt snout was probably why the species was called porcopiscus in Latin which is a compound of porcus (pig) and piscis (fish). The species' taxonomic name, Phocoena, is derived from the Greek phōkaina, which in turn comes from φώκη (phōkē) or seal. Suggesting that the ancients might have mistaken the porpoise for a seal.

Left: Harbor porpoise. Picture  by F.Graner

Porpoises are small toothed whales (Odontoceti) that are closely related to oceanic dolphins. The porpoise, however, has always remained more mysterious than its relative the dolphin. Perhaps because they are less wide-spread over the world than the dolphins, and are also not easily spotted in the open sea.  Porpoises rarely jump out of the water like dolphins and even then you are not likely to see more than the top part of its back with its small triangular dorsal fin when surfacing for a breath of air. Porpoises also have a compact body shape with a stiff neck while the dolphins have a  long beak and flexible head.

Seven extant species of porpoises The porpoises belong to the order of the toothed whales,  consisting of around 70 species. Odontocetes feed largely on fish and squid, not rely on their sense of sight, but rather on their sonar to hunt prey. They echolocate by creating a series of clicks emitted at various frequencies. There are now only seven extant species of porpoises that fall in the family of Phocoenidae. They are in respective order:

  • Genus Phocoena (four species): the harbor porpoise, vaquita, spectacled porpoise, and Burmeister’s porpoise
  • Genus Neophocaena (two  species): the finless porpoise and  narrow-ridged finless porpoise
  • Genus Phocoenoides (one species),  the Dall’s porpoise

As said, porpoises are not widespread, with some species specializing near the polar regions, usually near the coast. The most frequent species, the harbor porpoise  (Phocoena phocoena)  lives in the shallow, relatively cold northern coastal seas. One believes that there were once huge populations living in these waters, when their favorite food, the anchovies, was still abundant. The reason why the species was called harbor porpoises was that they probably often followed the fishing boats into the harbors. Because it is most commonly found in bays, estuaries, harbors, and fjords the species is particularly vulnerable to gillnets and fishing traps, pollution, and other types of human disturbance, such as underwater noise.

In past decades, pollution, particularly PCBs, caused a sharp decline in the population of harbor porpoises along all of the coastal areas of the southern North Sea. According to a recent study an increasing number of stranded harbor porpoises have been found in the last three decades on the beaches of Belgium and Holland: an amount of 16.000, mostly young male species*.

 Porpoises were also highly affected by bycatch. Many porpoises, mainly the vaquita, are subject to great mortality due to gillnetting. The vaquita is a species of porpoise endemic to the northern part of the Gulf of California that is on the brink of extinction. The Dall's porpoise from the northern Pacific is still extensively harvested for meat in Japan. Stranded porpoises often got killed by drowning after they became entangled in fishing nets. The ‘pinger'',  a loudspeaker tied to a floating buoy,  is used by fishermen to keep the harbor porpoises away from their boats. But these devices could also serve to tell the porpoise that ’dinner is ready’, so rather attracting the animals than keeping them at a distance.  Along the northern Dutch coast porpoises have been found with bites resembling the tooth of the grey seals, which often show up in the North Sea. Suggesting that adult grey seals are true predators, even preying on porpoises. The good news is that in the open North Sea the porpoises now seem to be back again, feeding mostly on herring, sprat and mackerel, and even smaller species like gobies. The estimate is that there are presently living around  250.000 species living in the North Sea, making them the most common cetacean in these waters.