Harbour porpoises usually occur in small groups of less than 8-10 individuals, although larger groups of up to several hundred have been recorded (Jefferson et al, 1993; Hoek, 1992). Larger group aggregation may be associated with migration or feeding behaviour (Jefferson et al, 1993). They are rarely found in close association with other cetacean species (Read, A.J., 1999).Porpoises are difficult to approach and follow, generally avoiding boats and other human activity. Though fast swimmers, they are not acrobatic, rarely breaching or leaping. They make brief dives, usually lasting for less than five minutes. They will often lie motionless at the surface on calm days, either resting or scanning the water below (Watson and Gaskin, 1983). Dive depths in excess of 200 metres have been recorded (Westgate et al, 1995).
Harbour porpoises feed on a wide variety of fish and cephalopods. Many prey species are benthic or demersal (Jefferson et al, 2008). Prey species vary by location, with small, schooling clupeoid and gadid fishes forming a major part of their diet (Read, A.J., 1999). Harbour porpoises forage independently and seldom use co-operative strategies for concentrating or obtaining prey (Würsig, 1986).