Marc A. Webber

Return of Harbor Porpoises (Phocoena phocoena) to San Francisco Bay

Introduction Harbor porpoises (Phocoena phocoena) were Herein, we report an instance that counters the rarely seen in San Francisco Bay prior to 2008 global trend of biodiversity loss in estuaries despite numerous marine mammal search efforts (Wolff, 2000; Lotze et al., 2006): harbor porbeginning in the 1970s. The species inhabited poises (Phocoena phocoena) have recently reocthe bay historically before they were functionally cupied central San Francisco Bay (SF Bay) after extirpated by the early 1940s. Their disappearance an absence of more than 60 y. This species has correlated with increased anthropogenic disturbeen known to recover habitat in other parts of bances such as dredging, shoreline construction, its range, including the North Sea (Camphuysen, World War II military defenses, and environmen2004), Germany’s Weser River (Wenger & tal impacts from industrialization. After observing Koschinski, 2012), and farther north along the porpoises throughout the central San Francisco U.S. Pacific coast in Puget Sound where they Bay from 2008 to 2010, we documented the porreturned to habitat abandoned decades earlier poises’ occurrence by means of a visual count (Evenson et al., 2016; Jefferson et al., 2016). from the Golden Gate Bridge. From 2011 to Harbor porpoises are one of the smallest toothed 2014, we spent 288 h counting porpoises from whales, reaching adult lengths of 1.5 to 2.0 m and that unique platform, resulting in 2,698 porpoise weights of 50 to 70 kg (Gaskin et al., 1974; Jefferson group sightings recorded in all months of the et al., 2015). They inhabit the cool coastal waters of year. Group size averaged 2.15, and the maximum the northern hemisphere and are common along the group size was 16 animals. Calves comprised central and northern California coast (Allen et al., 10% of all porpoises sighted and were also seen 2011). They tend to be inconspicuous, avoiding throughout the year. Porpoises were observed boats, keeping a low surface profile, and occurring on 96% of 169 survey days, and an average of in small groups (Jefferson et al., 2015). The species 34.37 ± 29.19 were seen per day. Porpoises can is listed as of “Least Concern” by the International now be seen in the central bay daily throughout Union for Conservation of Nature (Hammond et al., the year. Stranding records for the bay reflected 2008) and is not listed under the U.S. Endangered the general pattern of the porpoises’ decades-long Species Act. Harbor porpoises in the eastern North absence and their reappearance. Potential reasons Pacific (P. p. vomerina) are divided into nine manfor the porpoises’ return include decreased water agement stocks. The nonmigratory San Francisco– and noise pollution, improved water quality, and Russian River stock, which includes the Gulf of the increased marine productivity that created condiFarallones adjacent to SF Bay, is estimated to have tions in the bay that were favorable for porpoises. a stable population with no apparent trend of 9,886 animals (CV = 0.51) and an average group size of