Harbour porpoises live either solitarily or in groups, normally of three to five individuals, although sometimes more. Normally individual porpoises or couples (mother and calf) are observed and high numbers can be seen during migrations. Like dolphins, the harbour porpoise orientates itself by means of echo-sounding or ‘sonar’. It has a particular way of swimming with a ‘rolling’ movement (imagine a turning wheel); part of the back, including the dorsal fin surfaces briefly before re-entering the water giving the appearance of a rotating motion. This movement will normally be repeated three to four times, prior to a long dive.
The diet of an adult harbour porpoise is made up of various pelagic and demersal fish, as well as invertebrates. In the Southern North Sea, it consists mainly of small benthic fish, herring, cephalopod, whiting and cod. A daily meal would normally consist of about 5 kilos fish (around 10% of the body