San Francisco’s volunteer seaweed fighters | KALW

San Francisco’s volunteer seaweed fighters

Nov 8, 2012

Carliane Johnson is about to dive into a narrow channel of water, just about five or six feet between the dock and the seawall. She won’t have to dive deep to find what she’s looking for.

“The population can explode all of a sudden, and we’ll be pulling a lot,” she says. “Some can be six or seven feet long. It’s a lot of plant material, and I just bring it to the docks, someone grabs it, and I go back and look for more.”

Up on the dock, 14-year-old Gemma DeCarvahlo, her mother Marianne Kavanagh, and her cousin, Brianna Kavanagh, walk the docks with a trash bag and a clipboard looking for the invasive algae.

“Once we got a really big piece of it,” says DeCarvahlo. “And it was just fun to pull it out.  It was huge.  It felt really good because you’re like, ‘Yes!  It’s gone!’”

“It’s a lot of work to pull up hundreds of individuals by hand,” says Kavanagh. 

If she and her daughter didn’t come out at least once a month to locate and remove the invading algae, they say the San Francisco Marina might look like a jungle. 

“I would say if we skipped two months – we did that once – we were finding five and six foot individuals out here. It grows very quickly,” she says.

Picture a slimy seaweed thicket – each plant a dark gold-brown, with a strong spine – wrapping around ropes, chains, and other sea life. This is Undaria, an invasive kelp that has a small but persistent hold in just a few corners of the Bay. Kavanagh says it’s in all the marinas in the Bay Area.

Chela Zabin, an ecologist at the Smithsonian Institute, and some of her colleagues, first spotted the kelp back in 2009.

“So we were at San Francisco Marina, doing this survey,” says Zabin. “And I looked down and saw an enormous alga growing there that was over six feet long. I pulled it off of the dock, and pulled it out, and was pretty sure it was Undaria. It had not been recorded from north of Monterey previously.”

Zabin says the kelp probably came in on small boats – it’s native to some areas in Asia. It’s been a problem in Southern California for a while, but Zabin says it took Northern California by surprise.

“We pulled out 80 pounds of it that day.  And so that sort of launched the effort to see if we could get on it quickly and remove it,” explains Zabin.

Since then, Zabin and some graduate students have been keeping track of all the field data and monitoring whether the kelp is spreading or shrinking. Kavanagh and her daughter are some of their most dedicated field volunteers. Zabin says these days she leaves most of the organizing to them.

For two years, volunteers have been pulling all this algae out by hand, like weeding a garden. You’re right if you think that doesn’t exactly sound like a sustainable strategy. But unfortunately, Zabin says, there’s no funding to do it any other way. If no one were monitoring it, the kelp would likely spread, changing the Bay’s ecosystem for good.

The kelp’s growth could have economic consequences as well.

“So there are two ways that it poses problems for our pocketbooks as human beings,” says Zabin. “One is, it’s just another species that grows on boats, and you really don’t want to be sailing with six-foot-long kelp attached to your boat.”

The kelp is also a threat to local oyster and mussel farmers. Zabin says Undaria probably won’t go away, so now that it’s here, we should focus on controlling it. 

The good thing, though, is that removing the kelp by hand actually does work. The more frequently they remove it, the less it seems to come back. And just being out there means every month, boaters and passerby get a reminder that it’s still a problem.

Zabin says students, community volunteers, and others from the Smithsonian all come out sometimes to help. Sometimes there are big student groups, sometimes just groups of friends.

In another part of the Marina, down by the Hyde St. pier, the staff of Aquarium of the Bay does its own removal. But for the most part, the kelp hunters are the ones here today: mom, the girls, and a lot of garbage bags. Brianna Kavanagh is holding a 50-centimeter piece of Undaria. 

“It’s developing a reproductive structure, so we’ll put ‘developing repro.,’ says Kavanagh, marking a list for the ecologists to see. 

“The first time we came there wasn’t that many,” Kavanagh recalls, “but then it increased. And once we came here, we found these huge ones. We had these huge bags full of them that we could, like, barely carry. But it’s been getting a lot better.”

And with that, she finishes writing on her clipboard, and moves on down the dock.

This story was co-produced by Alyssa Kapnik. If you want to help clear kelp from the bay, contact or visit