Abstract

Harbor porpoises (Phocoena phocoena) have returned to daily, year-round use of San Francisco Bay after an almost total extirpation of approximately 65 years. Harbor porpoises are upper-level trophic predators, and their re-occupation of historical habitat in the central bay may be an indicator of the health of the ecosystem. We have begun a multi-year assessment to document this population's size, distribution, and behavior. While the basic biology of the species is well known, relatively little work has been done on free-ranging animals. Our efforts are focused on the photo-identification of individuals, behavioral observations, and reproductive timing.

Photo-identification has not been successful with harbor porpoises in other locations because of their small size, timidity and lack of prominent markings. However, using a combination of platforms - bridge, shore, and boat - we have been able to photograph and catalog 575 animals, recognizable from scars and pigmentation patterns. Of these, approximately 20% have been resighted. The shortest interval between first and last sighting is 1 day, and the longest interval is 525 days. By tracking known females over time we may be able to determine their birth rate, previously possible only through examination of carcasses.

Mating is difficult to observe in free-ranging cetaceans, but we used the Golden Gate Bridge as a platform to observe sexual behavior, including intromission, heretofore unseen in wild harbor porpoises. Since April 2010 we photographed 40 events that appeared to be mating behavior (copulations or attempted copulations). These events shared the following features: the presumed male rapidly approached the presumed female from below and behind, contact was made on the female's left flank, and all or part of the male's body emerged from the water, likely due to the momentum of his charge at the female. In 12 of the events, females were mothers accompanied by calves.

Acknowledgements

We thank The Marine Mammal Center, the Gulf of the Farallones National Marine Sanctuary, Oceanic Society, the California Academy of Sciences and San Francisco State University's Romberg Tiburon Center for collaborative support and sighting and stranding information. Our research activities are permitted under NOAA Letter of Confirmation No. 15477.

*Presenting author