Harbour seals are good swimmers. The pinipeds serve as rudder. The body and back pinnipeds cause propulsion. The seal is highly adapted to speed with its torpedo-like body and pinna. Observations made from ships revealed that they generally swim just 10 m beneath the water surface and that only rarely will they hunt under –20 m. Young harbour seals are born on tide-sandbanks between the end of June and mid July, and are required, almost immediately (during high-tide) to be able to swim.
In general, harbour seals feed on benthic fish (such as flounder, sole, cod and whiting), but they must largely be quite opportunistic in regard to what they eat. They may also feed on mussels, crabs and cephalopods.