By William Keener, and Larry Clinton, Sausalito Historical Society
Marine biologist Bill Keener began studying the of harbor porpoises to San Francisco Bay in 2008. In 2011, he wrote about his findings in BayNature Magazine. Here are some excerpts from that article: From Cavallo Point at Fort Baker you can often see porpoises swimming past in groups of two or three. Such sightings are all the more remarkable because for many decades porpoises weren’t seen inside the Bay. But we know they used to be here. Bones found in the Emeryville shellmound suggest the local Ohlone people consumed harbor porpoises in small quantities for some 2,000 years.
It’s unclear exactly when harbor porpoises abandoned the Bay, or why, but they seemed to have disappeared by the 1940s. Disturbance from ship traffic and environmental degradation likely played a role. The onset of World War II may have been the final stroke. To protect the harbor from submarine attacks, the navy stretched a steel net across the Bay from Sausalito to San Francisco. The net would have been a formidable obstacle to porpoises, an acoustic as well as physical barrier. The heavy mesh, straining against the currents, must have made an underwater racket. For animals that depend on their acute hearing to communicate and to locate prey, the noise might have been deterrent enough.
PHOTO FROM BELVEDERE-TIBRUON LANDMARK SOCIETY
Navy Lieutenant L.A. Wilson with a model of the WWII submarine net
Aerial surveys by the National Marine Fisheries Service do show a long-term trend of increasing abundance in harbor porpoises locally, helped by a mid-1980s ban on gill nets, which killed many porpoises as bycatch. The most recent estimate is that 9,000 harbor porpoises inhabit coastal waters between Pigeon Point in San Mateo County and Point Arena in Mendocino County.
Meanwhile, variations in the marine environment may have resulted in changes to the porpoises’ prey. Harbor porpoises along our coast are known to eat schooling marine fish, such as herring, anchovy, and jacksmelt, plus rockfish and squid. It’spossible that unusually low rainfall from 2007 to 2009 led to an influx of salt water into the Bay, which brought in fish species that attracted the porpoises. Also, thanks to modern sewage treatment systems and the regulation of industrial effluents, Bay water is less polluted than it was the last time the porpoises ventured inside the Golden Gate.
Marine scientists have recently correlated atmospheric conditions in the Pacific Ocean with winds and upwelling that can alter the habitat in estuaries, including San Francisco Bay. For the past several years, the Bay has been experiencing an ecological “regime shift” toward higher productivity of plankton and fish.
I was first tipped off to the porpoises’ return in 2008, when I got a call from San Francisco State University minke whale expert Jon Stern, who was surprised to see them from his boat off Sausalito. That spurred me to search the shorelines between the central Bay’s three main bridges. Everywhere I went, I found porpoises! In particular, deep trenches and steep peninsulas seemed to attract them, presumably because fish are concentrated there by strong tidal rips. Near Yellow Bluff, just north of Cavallo Point, up to a dozen porpoises at a time were congregating and diving, foraging for fish in ebbtide feeding sessions.
Porpoises are now seen regularly at locations much farther inside the Bay, such as Raccoon Strait, and near Angel, Alcatraz, and Treasure Island. Passengers on the commuter ferries have seen them as far south as the San Francisco-Oakland Bay Bridge.
Most of what we know about harbor porpoises is based on the examination of stranded animals — carcasses. With their regular appearance in the Bay, we now had a chance to learn how they live in their element. We have an extraordinary resource: The Golden Gate Bridge. From its deck, 220 feet above sea level, we can observe behavior nearly impossible to see from a boat: underwater feeding, chasing, and nursing, or riding the wake of a passing tanker. We have even seen porpoises mating, something never before observed in the wild.
The porpoises have a synchronized reproductive cycle, culminating in an early summer calving season quickly followed by a peak in mating behavior. Each spring the males’ hormones kick into overdrive. As with almost all of their behaviors, mating is done on the move. Copulation lasts just a few seconds, complete with a splash as the pair seems to fly apart. We know that gestation lasts 10 to 11 months and the single calves remain with their mothers for about a year.
By photographing their markings, we can tell individual porpoises apart based on unique characteristics, such as body scars and skin pigmentation patterns.
Our work to unravel the mystery of the porpoises’ return may also lead us to understand their social structure. It’s an open question whether they maintain long-term family bonds or form alliances, as other cetaceans do. One thing is certain: Observing these intriguing animals — as scientist or enthusiast — is a pleasure. We should all take the opportunity to get out and watch these porpoises once again making their home in our own briny “backyard,” San Francisco Bay.
Bay Area native Bill Keener has recently joined a new cetacean field research team at The Marine Mammal Center in Sausalito to advance the study of local whales, dolphins and porpoises.