The harbour porpoise is one of the smallest marine mammals. Adults grow up to only 1.4-1.9m length. They tend to surface in a gentle rolling motion and travel alone or in small groups of up to five.
The species prefers cool waters such as the Northern Atlantic, Northern Pacific and Black Sea, most often spotted in the North Sea region. They stay close to coastal areas and river estuaries, and might even venture up rivers, making them easier to watch.
Their small size and a poorly demarcated beak make them easy to distinguish. They can be seen all year round in the waters of Ireland and Britain; however, most sightings are inshore between June and September.