The smallest cetacean regularly seen off the US
The harbour porpoise is small even among porpoises, shy and quick, surfacing briefly and rarely riding a boat’s wake the way dolphins often do. It tends to travel alone or in loose small groups rather than the large, active pods common dolphins form. That adds to how easily it goes unnoticed even in busy coastal waters. It favors cold, shallow coastal water over the open ocean, which makes it one of the easier whales or dolphins to miss entirely unless you know exactly where to look.
Two coasts, always close to shore
Records span both US coasts, adding up to nearly 8,000 confirmed sightings, with the heaviest concentrations off California and Washington in the Pacific and Maine and North Carolina in the Atlantic. Nearly every sighting sits right at the surface near the coastline rather than out in deep water. Records rise through spring and summer, peaking in August, then drop off sharply once fall arrives.
Fish-only diet, and a surprising predator
Harbour porpoises hunt fish almost exclusively, taking Atlantic cod, herring, mackerel, and capelin depending on the coast. Great white sharks and orcas both take porpoises, but so do grey seals. Grey seals have been documented actively hunting and killing harbour porpoises rather than simply competing with them for the same fish, an unusual predator for a marine mammal to have.
Breeding nearly every year
Females mature around age three, males closer to four, and pregnancy runs about ten and a half months. A female typically raises one calf and can give birth again the following year, a faster reproductive pace than most whales manage. Wild harbour porpoises have been recorded living to 20.
Conservation
The IUCN lists the harbour porpoise as Least Concern globally. Some regional populations do face real pressure from fishing net entanglement, since a porpoise this small can drown in gear built for much larger catch. US records suggest a species that remains widespread along both coasts.
