Harbor porpoise (Phocoena phocoena) recovery in the inland waters of Washington: estimates of density and abundance from aerial surveys, 2013–2015

Abstract

The harbor porpoise (Phocoena phocoena (L., 1758)) used to be common in Puget Sound, Washington, but virtually disappeared from these waters by the 1970s. We conducted systematic aerial line-transect surveys (17 237 km total effort) for harbor porpoises, with the goal of estimating density and abundance in the inland waters of Washington State. Surveys in Puget Sound occurred throughout the year from 2013 to 2015, and in the Strait of Juan de Fuca and the San Juan Islands (and some adjacent Canadian waters) in April 2015. We used a high-wing, twin-engine Partenavia airplane and four observers (one on each side of the plane, one looking through a belly port, and one recording data). A total of 1063 harbor porpoise groups were sighted. Density and abundance were estimated using conventional distance sampling methods. Analyses were limited to 447 harbor porpoise groups observed during 5708 km of effort during good sighting conditions suitable for line-transect analysis. Harbor porpoises occurred in all regions of the study area, with highest densities around the San Juan Islands and in northern Puget Sound. Overall, estimated abundance for the Washington Inland Waters stock was 11 233 porpoises (CV = 37%, 95% CI = 9 616 – 13 120). This project clearly demonstrated that harbor porpoises have reoccupied waters of Puget Sound and are present there in all seasons. However, the specific reasons for their initial decline and subsequent recovery remain uncertain.

Increased presence of mammal-eating killer whales in the Salish Sea with implications for predator-prey dynamics

“…There is also some evidence that different groups of transient killer whales specialize on different types of prey and/or different foraging techniques (e.g., Baird & Dill, 1995), so new matrilines may be moving into the Salish Sea to take advantage of other marine mammals. Harbor porpoise, for instance, have been steadily increasing in Washington inland waters over the last 15 years (Jefferson et al, 2016; S Pearson, WDFW, pers. comm., 2018) This could help explain the unprecedented in sightings in 2017.…”

Return of the Salish Sea Harbor Porpoise, Phocoena phocoena: Knowledge Gaps, Current Research, and What We Need to Do to Protect Their Future

“…Despite the continued absence of harbor porpoise in the Canadian nearshore waters of the Victoria region, by the early 2000s opportunistic sightings, strandings and fisheries bycatch data indicated that the harbor porpoise had returned to the more southerly Salish Sea waters and the most recent United States assessments confirm this return and numerical recovery (Evenson et al, 2016;Jefferson et al, 2016). This is reflected in the stranding records as well.…”

“…Along the west coast of the United States and Canada, genetic studies reveal that harbor porpoises are not panmictic and movement patterns along the west coast of North America are sufficiently restricted that genetic differences have evolved, currently resulting in eight different recognized stocks between California and Alaska (see Carretta et al, 2016). The harbor porpoises that inhabit the Salish Sea are recognized by the United States National Oceanic and Atmospheric Administration (NOAA) Fisheries as a single stock (Washington Inland Water Stock, Carretta et al, 2016) and their range includes the United States waters of the Juan de Fuca Strait, San Juan Islands and Puget Sound (Jefferson et al, 2016). On Canada's west coast, there is no recognized stock structure, with the species assessed as Pacific harbor porpoise for the entire province of British Columbia.…”

“…Assessment of abundance trends or trajectories over the last half century is hindered by this lack of quantitative information and the jurisdiction of two nations. Human boundaries are likely not significant to mobile animals like harbor porpoises, and it is recognized that animals in United States waters may also use waters of at least southern BC, though the extent of cross-border movements remains unknown (Jefferson et al, 2016). While the socio-political boundaries are likely not significant, human actions are.…”

“…A relatively high number of strandings occurred in 2006-2007 prompting the declaration of an unusual mortality event; however later analyses showed that the increase in strandings was actually due to a growing harbor porpoise population, expansion of their range into previously sparsely populated areas and increased reporting and response of the stranding network (Huggins et al, 2015). The latest harbor porpoise abundance estimate (Table 1) for the United States waters of the Salish Sea is 11,233 harbor porpoises (in contrast there were only 3,509 harbor porpoises in 1996, Calambokidis et al, 1997), with the highest densities in the San Juan Island area with 8,103 harbor porpoises (Jefferson et al, 2016). Less effort was expended in evaluating British Columbian waters due to primary survey goals and funding limitations.…”