Summer habitat appears to be in the northern Gulf of Maine and the Bay of Fundy. During the winter, porpoises move south to waters off of New England to the mid-Atlantic. Despite their name, they often occur in deep waters. Mating occurs from March through July, and calves are born after an 8-10 month gestation period. Calves are approximately 2 feet and 15-20 pounds at birth.
Sexual maturity is reached at four years, and females calve every other year.
Harbor porpoises are typically sighted in small groups of 3-15 animals, but aggregations of more animals than that in an area are not uncommon. It is unknown if these are stable groups or not. One recent estimate indicated that if current catches continue, the stock will become extinct in 100 years. The National Marine Fisheries Service had proposed threatened status for the population, but withdrew the proposal in January 1999, hoping that new regulations would eliminate the decrease in the population. However, they may reconsider it in the year 2000.