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

Harbor porpoises (Phocoena phocoena) were rarely seen in San Francisco Bay prior to 2008 despite numerous marine mammal search efforts beginning in the 1970s. The species inhabited the bay historically before they were functionally extirpated by the early 1940s. Their disappearance correlated with increased anthropogenic disturbances such as dredging, shoreline construction, World War II military defenses, and environmental impacts from industrialization. After observing porpoises throughout the central San Francisco Bay from 2008 to 2010, we documented the porpoises' occurrence by means of a visual count from the Golden Gate Bridge. From 2011 to 2014, we spent 288 h counting porpoises from that unique platform, resulting in 2,698 porpoise group sightings recorded in all months of the year. Group size averaged 2.15, and the maximum group size was 16 animals. Calves comprised 10% of all porpoises sighted and were also seen throughout the year. Porpoises were observed on 96% of 169 survey days, and an average of 34.37 +/- 29.19 were seen per day. Porpoises can now be seen in the central bay daily throughout the year. Stranding records for the bay reflected the general pattern of the porpoises' decades-long absence and their reappearance. Potential reasons for the porpoises' return include decreased water and noise pollution, improved water quality, and increased marine productivity that created conditions in the bay that were favorable for porpoises.