Disappearance and return of harbor porpoise to Puget Sound: 20 year pattern revealed from winter aerial surveys

Harbor porpoise were a common year-round resident in the Puget Sound in the 1940s, but by the 1970s, they had disappeared from the Sound, and their numbers were greatly reduced in the Straits of Georgia and Juan de Fuca, and around the San Juan Islands. Some survey efforts were conducted to verify that there were no harbor porpoise within the Puget Sound, but prior to 2013, there were no dedicated small cetacean surveys providing complete coverage of Washington’s inland waters, including the Puget Sound. Beginning in the early 2000s, there were increasing reports of harbor porpoise sightings by the public, and by researchers involved in other activities on the water. Unfortunately, there wasn’t funding during this period for the National Marine Fisheries Service (NMFS) to conduct aerial surveys of the inland Washington harbor porpoise stock, leaving many questions about the increase in sighting reports unanswered. Washington Department of Fish & Wildlife (WDFW) conducts annual winter aerial marine bird surveys that cover all of the Washington inner marine waters from southern Puget Sound to the Canadian border, out to the west entrance of the Strait of Juan de Fuca. These surveys have been conducted every year (except 2007) since 1994, and record all marine mammal observations in addition to the marine bird observations. While it is not designed as a dedicated marine mammal survey, because of the consistent survey methodology throughout the survey area, and that all marine mammal observations were recorded, this 20-year dataset provides a record of increasing harbor porpoise numbers in the northern portion of the survey area in the early years, followed by their expansion into the waters of the Puget Sound. The rate of increase exceeds the maximum potential local recruitment rate, not only within the Puget Sound, but throughout the inland marine waters, suggesting immigration from outside the study area is supplementing recruitment. During this same period, reports of Dall’s porpoise had been decreasing, concurrent to the increase in harbor porpoise sightings. These surveys confirmed a downward trend in Dall’s porpoise numbers, with none sighted in 2014.

Citation:

Evenson, J.R., D. Anderson, B.L. Murphie, T.A. Cyra, and J. Calambokidis. 2016. Disappearance and return of harbor porpoise to Puget Sound: 20 year pattern revealed from winter aerial surveys. Technical Report. Washington Department of Fish and Wildlife, Wildlife Program and Cascadia Research Collective, Olympia, WA.