SF Natural History Series

It was 2008 when they began to see them: Harbor Porpoises slipping into the bay on high tides. They had not been seen there since the 40s. During WWII, a large net strung across the bay to keep enemy submarines out, also kept any other sizable fish out. Pollution in the bay may have also been a part of keeping them away– the Bay was more precious as a dumping ground than a natural resource.

Happily, we know the Bay is doing better than it has for a long time, and this is yet another sign. It turns out that the Golden Gate Bridge is also an excellent observation platform. From the bridge and by boat, Golden Gate Cetacean Research, has been taking thousands of pictures to try an document this somewhat shy and elusive species. In logging all the hours taking those photos, Bill Keener and his colleagues, have learned more about Harbor Porpoises than any previous research anywhere else.

There is of course the things that are generally known. Porpoises  are the smallest member of the whale family. The harbor porpoise is one of 6 species of porpoise — up to around 5 feet long, a 150 pounds, and living for 10-12 years. Dolphins and porpoises were once though of as the same thing, but porpoises have a shorter beaks, and flattened spad-shaped teeth. Porpoises are also prefer cold waters, whereas dolphins can more generally be found in warm waters. They eat different fish, and porpoises do not have the same intelligence.

The whales in general are only very, very distantly related to pinnipeds: seals and sea-lions. Cetaceans originate from hoofed animals, whereas pinnipeds are more closely related to dogs.

Now the bridge and the bay (they have a permit to approach the animals by boat as well) are allowing Keener and his colleagues to learn a lot more. For the first time they’ve been able to identify individuals. From skin tones,  markings, and scars they’ve  identified almost 250 individuals. This takes a lot of work, not just taking all the photos (nearly 1000 a week), but reviewing them and matching them up to other photos, known and unknown individuals.

Nearly 25% have scars of one sort or another. Many look to have been made by fishing gear, a few by sharks, prop scars, but others mysterious. They’ve seen porpoises with skin lesions and copepod parasites hanging from them.

The porpoises come into the bay to take advantage of the bounty of the bay created by strong tidal fluctuations, where the river meets the sea, and the mixing of water columns where water lower down hits the cavallo spire just inside the bay, and brings lots of goodies up. The harbor porpoises come in to hunt anchovies, smelt, and squid, going under water for 60 seconds or so before coming back up for air (they can probably go under water for 5-10 minutes, but a minute is the norm). They don’t spend much time at the surface however. Though they can be seen to hunt by swimming on their side and spinning. Like dolphins they hunt with sonar, and an underwater microphone is being added to help track them.

The gold mine that the GGB is for studying porpoises though has been in seeing never before seen social behavior. Porpoises weren’t really believed to have any, but it is clear they gather in small groups of 5-7. But even better than that has been witnessing porpoise mating behavior. It is a quick affair. Porpoises have the big testes, 4% larger than their brain, the same size as the 50′ fin whale. The strategy is mate fast and mate often. Males swim up from behind, on the left side (this has been a mystery — it is speculated that it has to do with the asymmetry of their sonar) and often explode out of the water in the attempt to mate, their sex often on display. Female porpoises seem to take it as it comes, no courtship seemingly required, and there is no fighting between the males.

Gestation is almost 11 months long, and it seems that female porpoises might be pregnant or lactating their whole adult life. This is one of the questions the researchers have and hope to answer with patient observation.

To see them, head for the Golden Gate at a strong high tide… the researchers have seen as many as 115 passing under the bridge. This happens all year long. You can see them off cavallo point after the tide begins to ebb (where the photo above comes from).

You might also see dolphins. Dolphins have been coming into the bay since 1982 when unusually warm waters led them north. There are about 450 bottlenose dolphins who are known as a coastal sub species who like our rocky shores. Then tend to come in hugging the coast and baker beach, swimming in along Crissy field, likely looking for salmon, two or three times a week in the spring.