Harbour porpoises are observed several times a year

The harbour porpoise is the only toothed whale species to be seen annually in Finnish coastal areas. The individuals observed in Finnish waters are originally from a harbour porpoise population from the Baltic Proper.

Observing a harbour porpoise requires luck

The harbour porpoise (Phocoena phocoena) is one of the smallest toothed whale species in the world. Adult females are about 160 centimetres in length and weigh approximately 60 kilograms. Compared to females, adult males are somewhat smaller, measuring approximately 145 centimetres in length and weighing around 50 kilograms.

The harbour porpoise has a short, rounded body with a small head that lacks the typical beak of many dolphin species. Its colouration varies from dark-grey to almost black, and the flanks are light grey at the front with a white underbelly. On each side of the body, a narrow dark stripe runs from the corners of the mouth to the pectoral fins. The dorsal fin is low, triangular and slightly concave at the back. 

It can be very difficult to detect harbour porpoises, particularly if there are any waves. In general, porpoises are only glimpsed as they roll on the surface, during which time, only part of the back and the triangular dorsal fin show above the surface. The head and tail of a porpoise are rarely seen. It can also sometimes be observed resting motionless near the surface, particularly in calm weather.

Porpoises usually move alone or in groups of a few individuals. They do not seek out the company of people or ships, which makes their detection even more challenging.

Harbour porpoises eat mainly fish

From a global perspective, there are major regional and temporal variations in the main food items and the composition of the prey species of porpoises. The porpoise is an opportunist in its diet, meaning it usually uses the best available food at any given time. For example, in the Finnish marine area, the diet of porpoises is rich in herring and sprat.

A harbour porpoise near the surface.
The harbour porpoise is a rare sight in Finnish marine areas.

The porpoise population of the Baltic Sea Proper is critically

The harbour porpoise is not classified as being under threat worldwide. However, by the mid-20th century, the harbour porpoise population in the Baltic Sea had collapsed due to hunting, by-catch mortality in commercial fisheries, and environmental toxins. This population has not yet recovered.

The population in the Baltic Sea Proper is now classified as extremely endangered. Based on four years of acoustic data collection and analysis by the international SAMBAH (Static Acoustic Monitoring of the Baltic Sea Harbour Porpoise)-project, the population size of the Baltic Sea Proper harbour porpoise is estimated to be approximately 450 individuals.

From November to April, the SAMBAH-project obtained the acoustic- or sound observations of porpoises in the open sea areas off the southwest Finnish coast. On the other hand, visual observations are most common in Finnish marine areas during the summer, when more people are by the sea. These results indicate that although harbour porpoises are few in number, they also occur regularly in Finnish coastal areas. 

The most significant threat to the survival of the species in the Baltic Sea is estimated to be by-catch mortality from commercial fishing, especially in the southern Baltic Sea. In the marine areas of Finland, the by-catch of harbour porpoises is extremely rare. In December 2018, one individual porpoise became entangled in a fisherman's net near the village of Nagu, in the Archipelago Sea. Probably the most effective way of reviving the harbour porpoise population is to limit harmful fishing practices in areas where porpoises are known to occur regularly.

Today, porpoises in the Baltic Sea are also threatened by environmental toxins in their food, which is accumulated in their tissues, as well as underwater noise from human activities. The protection of Baltic harbour porpoises is included in several international treaties and regulation, as well as in Finnish national legislation.