Thousands of tourists visit the islands every summer hoping to catch a glimpse of a killer whale.  Hundreds of local fishermen grumble about competing with harbor seals for salmon.  Love them or hate them, marine mammals are a big part of island life.  But little attention is paid to one of our most intelligent and successful marine mammals: the harbor porpoise.

It is estimated that the San Juan Islands are home to about one-third of the harbor porpoises in the inland waters of western Washington.  Harbor porpoises were also once a common sight in south Puget Sound, but declined rapidly after 1940 and are rarely seen there today.  Reasons for their decline are unclear, but accumulation of toxic compounds such as PCBs is suspected.  Declines in herring and other favorite prey of porpoises have also been greater in the south Sound than here in the islands.

Harbor porpoises are small, sleek and fast with dark backs and white bellies.  The largest are about six feet long and 200 pounds, the size of a human adult.  They travel in pairs or small groups, but aggregations of more than 50 have been reported where there is a rich lode of prey such as a swarm of migrating herring.

Harbor porpoises are year-round residents in the Sound but tend to congregate in the islands in summer, like orcas.  They prefer relatively shallow waters, typically diving to less than 200 feet, and feeding on many of the same “forage fish” that attract migrating salmon and seabirds.  Energetic swimmers, harbor porpoises consume about 10 percent of their body weight in fish every day.  There is some evidence that local populations of porpoises are genetically distinct and behaviorally specialized, passing down “cultural” prey preferences to their offspring, as has been documented in orcas.

Like orcas, harbor porpoises vocalize a great deal.  They avoid areas of boat noise that may affect their ability to hear one another, but may also use the acoustic shadow of noisy boats to evade orcas.  Young porpoises are born in summer after a gestation period of 11 months, and have a life expectancy of eight-ten years if they avoid stranding, starvation, entanglement in derelict fishing gear and hungry orcas.  Stranding is a relatively frequent occurrence.  Herding by killer whales and flight from noisy boats are two possible causes.

Just as resident orcas in the islands have suffered from declining Chinook salmon runs, harbor porpoises and seabirds in the islands have had less herring to eat, most critically in spring when they are feeding young.  Many more herring used to spawn in the islands, and spawned earlier than the remaining large herring population associated with Cherry Point on the mainland.  We know very little about how porpoises here have adapted.  In the north Atlantic, harbor porpoises shifted from a diet of herring to juvenile cod and other ground fish, but they are reportedly less healthy.

Salish Sea harbor porpoises are highly contaminated with toxics such as PAHs, at levels comparable to orcas.  Recent studies suggest that toxic loads lead to higher rates of infection, as well as reproductive failure.

Why are harbor porpoises neglected, when they face the same threats as orcas in the islands?  They are almost certainly more numerous than orcas, but their actual status and numbers are less studied and it is uncertain whether they are increasing or declining.  Perhaps we should take more interest in them now, while they still have a good chance to survive and thrive in our waters.