October is the month of one of the smaller and sometimes most often overlooked cetacean, the harbour porpoise (Phocoena phocoena). The word porpoise is derived from the latin for pig, Porcus, and these little creatures used to be referred to as "puffing pig's" because of the sneeze-like puffing sound they make when they breathe.
Usually found in small groups or alone, although known to come together to feed, harbour porpoises are small (less than 2m in length), have no beak, a robust dark body with a paler underside and a triangular dorsal fin. These features and its characteristic slow rolling swimming style makes it easy to recognise and distinguish from other cetaceans that share its distributional range. They are not a demonstrative species and are rarely surface active so can be quiet difficult to study, especially as they have a tendency to avoid boats.
Currently, four sub-species of harbour porpoise are recognised; (1) P. p. phocoena in the North Atlantic; (2) P. p. vomerina in the eastern North Pacific; (3) P. p. relicta in the Black Sea; and (4) an unamed sub-species in the western North Pacific. Although no definitive global population figure is available, there have been several estimates suggested and a figure of approximately 700,000 individuals is the most current estimate. The IUCN list the species as of 'Least Concern' but the population in the Baltic Sea is listed as 'Critically Endangered' and the population in the Black Sea is listed as 'Endangered'. They are also a species of concern at an international level and populations of harbour porpoise in the North and Baltic Seas, the western North Atlantic and northern west Africa, appear on Appendix II of CMS. Found in coastal waters of the sub-Arctic, and cool temperate waters of the North Atlantic and North Pacific, the majority of sightings occur within 10km of land. They are the only member of the porpoise family found in European waters and there have been up to 13 separate harbour porpoise populations proposed for the North Atlantic alone.
The distribution of harbour porpoises in southern Europe and along the Atlantic African coast is poorly understood, with tentative assessments suggesting a discrete West African population with a northern limit around the Straits of Gibraltar. Porpoises are known to be present off West Africa in the coastal waters of Mauritania and Senegal, but are rare or absent further south. Recent genetic analysis of samples from stranded animals suggests that, on a regional scale, the porpoises of Mauritania should be considered separate from all European populations.
The number of harbour porpoises in some areas have declined in the past few decades due to human activities Historically, they were hunted in large numbers but the biggest current threat to harbour porpoises throughout their range is incidental capture in fishing nets with thousands of casualties each year. They may be especially vulnerable to bottom-set nets because they forage on the seabed and there is substantial research into mitigation measures for this threat including the use of 'pingers', an acoustic deterrent attached to the nets and intended to keep the porpoises away. This is crucially important work as in many areas annual bycatch rates may exceed sustainable levels; i.e. - more are being caught than are being replaced. Additional threats to harbour porpoises include boat traffic, hunting, habitat loss, human disturbance and noise pollution and prey depletion. Chemical pollution has also recently been shown to adversely affect harbour porpoise health in the Northeast Atlantic.
We'll be hearing more from harbour porpoise researchers and projects around the world, so stay tuned for more on these little guys in the coming weeks ...!!!