Wildlife Spotlight: Harbor Porpoise

Harbor Porpoise. Photo: Ari Friedlaender

By Cori Hatch

When most people hear the term “cetacean”, they imagine majestic whales and playful dolphins.  This is understandable: the giant whales simply astound us with their size, while dolphins command our attention and spark our curiosity with their startling intelligence.  Indeed, the Gulf of the Farallones National Marine Sanctuary and its neighboring marine sanctuaries are blessed with many familiar species of whales and dolphins.

However, the sanctuaries also host the other, more frequently overlooked member of the cetacean family—the porpoise.  Both Harbor Porpoises and Dall's Porpoises live in the Gulf of the Farallones.

Porpoise or Dolphin?

The term “porpoise” is often used interchangeably with “dolphin”, and indeed the two species are very similar.  Besides their similar physical characteristics, both are large-brained toothed wales who use echolocation, a sort of sonar, to avoid obstacles and detect food.   However, http://dallsporpoise.org provides the following criteria for distinguishing the two:

Porpoises are generally much smaller and stockier than dolphins and tend to live shorter lives. They lack the prominent beak that dolphins usually have and their dorsal fins are usually triangular shaped compared to the generally curved dorsal fins of dolphins.

Porpoises have flat spade-shaped teeth with long, sharp upper edges while dolphins have pointed cone shaped teeth. Another difference is that there are more fused neck vertebrae in porpoises than in dolphins, which limits the range of mobility in the porpoise neck.

Physical Characteristics

On average, harbor porpoises reach a length of 5 to 6 feet.  They eat non-spiny fish such as herring, cod, pollack, sardines, whiting and squid, and lots of them—they must consume about ten percent of their body weight per day in order to support metabolic function.  They can swim at speeds of up to 35 miles per hour. 

They become sexually mature at three or four years of age and then breed approximately every two years after that.  The gestation period for porpoises is about eleven months.  Harbor porpoise are non-social animals; they are often spotted alone, sometimes in pairs, other times in small groups of up to five. 

Porpoises, like dolphins, are often victims of bycatch.  All marine mammals in the United States are safeguarded under the Marine Mammals Protection Act, so porpoises are not targeted by fishermen for food.  However, gillnets and trawls entangle porpoises while they comb the seas for other species.

Enigmatic Beings

Porpoises, like so many ocean residents, remain creatures of mystery.  Our knowledge is limited to what we can observe.  Unlike dolphins, porpoises adapt poorly to captivity.  In fact, the second porpoise birth ever to occur in a tank just happened in May 2009.  The challenges of holding porpoises in zoos, aquariums, or research facilities mean that studying their natural social behavior is extremely difficult. 

So, while we can observe that the bold Dall's porpoises are more apt to put on a show for human observers than bashful harbor porpoises, we still have much to learn about these intelligent mammals of the sea.