Return of harbor porpoise to Washington State's Puget Sound as documented through 20 years of aerial seabird surveys
Anderson, D, J Evenson, B Murphie, T Cyra, J Calambokidis. 2015. Return of harbor porpoise to Washington State's Puget Sound as documented through 20 years of aerial seabird surveys. Abstract (Proceedings) 21st Biennial Conference on the Biology of Marine Mammals, San Francisco, California, December 14-18, 2015.
Harbor porpoise (Phocoena phocoena) habitation of coastal waters exposes them to frequent interactions with fisheries, pollution, boat traffic and other anthropogenic threats. Many harbor porpoise populations throughout their range are in decline, while others show signs of recovery, with animals commonly sighted in some formerly abandoned portions of their range. Harbor porpoise were the most common cetacean in the inland marine waters of Washington State, including the Puget Sound, in the 1940s. They were virtually extirpated from the Sound by the 1960s, with only occasional sightings through the 1990s reported by the public as well as opportunistic observations by researchers. An increase in sightings were reported in the early 2000s, however, more quantitative data were necessary to document and explain the timing of their increase, extent of their expansion throughout the Puget Sound and if a recovery was indeed in progress. The annual winter aerial seabird surveys, conducted by Washington Department of Fish and Wildlife (WDFW) from 1995 to 2014 recorded all sightings of both marine birds and mammals. Analyses of these survey data reveal an overall increasing trend in harbor porpoise density in Washington's inland waters, with a progressive expansion of their range into the Puget Sound. It is not known for certain what the reasons were for the decline and subsequent increase in the harbor porpoise population, though areas throughout the Puget Sound basin included many common anthropogenic threats, such as gillnet fisheries and major pollution sources, which have been more strictly regulated in recent decades. The survey also noted a concurrent reduction in Dall's porpoise (Phocenoidies dalli) densities, which may be related to the increased presence of harbor porpoise and the interspecific competition between these two species. According to reports from the 1940s, before the near extirpation of harbor porpoise from the Sound, Dall's porpoise were not found in these waters. It is possible that Dall's porpoise had filled the niche vacated by the harbor porpoise when their numbers declined. We document a progressive return of harbor porpoise into waters they previously occupied, as diminishing threats allowed an increasing population to expand their range.