Return of Harbor Porpoise to Puget Sound: Recent Increases in Abundance

Harbor porpoise were reported to be the most commonly observed marine mammal in Puget Sound in the 1940s.  Although little information on their status is available from the next three decades, by the mid-1970s when greater attention began to be paid to marine mammal distribution and abundance in this region, harbor porpoise appeared to have all but vanished from the main basin and south Puget Sound.  A paucity of anecdotal reports, sightings during surveys, or strandings over the following two decades appeared to confirm that this species had essentially disappeared from this area.  While the reasons for their disappearance are unclear, several anthropogenic factors could have been responsible including mortalities in gillnet fisheries, impacts of contaminants, noise, and the degraded state of Puget Sound.  However, in the 2000s, numerous anecdotal reports and a few strandings were documented in the main basin and south Sound.  By 2009, multiple groups of harbor porpoise were being sighted throughout the year in a number of areas of Puget Sound including south of the Tacoma Narrows where historical records from the 1940s reported them as common.  Recent systematic boat surveys of the main basin indicate that at least several hundred and possibly as many as low thousands of animals are now present.  While the reasons for this recolonization are unclear, it is possible that changing conditions outside of Puget Sound, as evidenced by a tripling of the population in the adjacent waters of the Strait of Juan de Fuca and San Juan Islands since the early 1990s, and the recent higher number of harbor porpoise mortalities in coastal waters of Oregon and Washington, may have played a role in encouraging harbor porpoise to explore and shift into areas like Puget Sound. The return of harbor porpoise to these regions of Puget Sound provides encouraging evidence that many of the underlying ecosystem components, e.g., forage fish populations, are functioning at a level sufficient to support these and other top predators. With their return to Puget Sound increased strandings of harbor porpoise have also been recorded in this area, indicating that a number of threats to the long-term viability of this population may still exist.