Effort to curb invasive crabs in Seadrift backfired

David Briggs
The number of green crabs in Seadrift Lagoon skyrocketed in 2014 thanks to the creatures’ cannibalism. Now researchers, who have been managing the invasive population for almost a decade, are leaving enough adult crabs to keep the younger ones in check.  
08/31/2017

In recent years, the mile-long, man-made lagoon in the gated community of Seadrift has seen one of the worst infestations of invasive European green crabs on the West Coast. With no natural predators to keep the population in check, scientists have been using the site to experiment with management strategies to contain the notorious invaders. But efforts backfired in 2014, resulting in the population reaching historic highs before the team adapted its strategy. 

Researchers from the University of California, Davis, the Greater Farallones National Marine Sanctuary, the Smithsonian Environmental Research Center and elsewhere have been working in Seadrift since 2008 to stabilize the numbers of green crabs, which are native to the northeast Atlantic Ocean and Baltic Sea. 

At the same time, researchers have monitored populations of green crabs in Bolinas Bay, Tomales Bay, San Francisco Bay, Bodega Harbor and Monterey Bay in order compare the effects of their work in Seadrift with the ecological trends in the region.

But after a few years of successfully reducing the lagoon’s crab population, a shock came in 2014, when populations skyrocketed from around 8,000 to around 300,000 individuals.

“No one had seen that happen before,” Ted Grosholz, a professor of environmental science and policy at Davis who spearheaded the research, said. “We learned an important management lesson here for handling invasives.” Green crabs are cannibalistic, and researchers had been eliminating adult crabs without considering that they were responsible for consuming many of the younger crabs. This, Mr. Grosholz believes, led to the population explosion.

Now, scientists leave some of the adults, keeping the population at “a low abundance, or a dull roar,” as opposed to completely eliminating it, Mr. Grosholz said.

Funding for the project from the National Science Foundation has waned since 2010, leaving the project mostly up to volunteers beginning in 2012 and 2013 – something that may have also contributed to the population explosion. For the past three years though, the Greater Farallones Association and the Seadrift Homeowners Association have contributed enough additional funding to keep efforts afloat.

Kate Bimrose, the Bolinas Lagoon restoration coordinator for the Greater Farallones Association who coordinated fundraising and volunteer recruitment for the project, explained that every year, from June through September, a team of 10 to 15 goes out every other week to remove crabs.

They drop 90 traps at six different sites, often baiting them on a Monday and returning the next day to pull out the crabs. “We count all the crabs we remove, measure them, sex them, identify basic health characteristics such as any damages, then pack them up to be frozen,” Ms. Bimrose said.

Bolinas farmer owner and operator of Gospel Flat Farm, Mickey Murch, has received the frozen crabs since the project’s inception for use in his farm ecosystem.

“I mix them with goat manure to make a high strength compost,” said Mr. Murch, who has been raising livestock and growing vegetables on around 10 acres since 2004. “They are really stinky and would attract interest from animals like racoons, and so I bury them about six inches deep. When you come back just a week later, they are hollow—but you have to bury them!”

Mr. Murch, who said he is always looking to diversify what he puts into his compost to increase soil fertility at Gospel Flat, said the crabs are a great resource. During the eradication season, he gets about 80 pounds every other week. 

The crabs have also gone to Slide Ranch, an educational center in Muir Beach.

Green crabs are listed by the International Union for Conservation of Nature as one of the 100 worst invasive species in the world. Their range now stretches from their native waters of northern Europe and northern Africa to Australia, Russia, South Africa, Argentina and both coasts of the United States. On the East Coast, they cost the shellfish industry an estimated $22 million each year.

The first individuals to make it to the West Coast probably arrived in 1989 as stowaways within lobster or bait-worm shipments to the Bay Area, and populations now dot the Pacific coast all the way up to British Columbia.

Mr. Grosholz said that so far, green crabs in the area have not been the problem that they are on the East Coast. Yet for a brief time, they were a big issue for shellfish growers in Tomales Bay, when they first showed up around 1996. That year, Hog Island Oyster Company lost about 40 percent of its Manila clam production, Mr. Grosholz reported. But the numbers in Tomales Bay have since dropped off.  

“Green crabs definitely do eat clams and they can eat oysters too, when they are young and smaller than the crabs themselves,” Eric Schlagenhauf, Hog Island’s director of production and farm operation, said. “There are also other predators and we know how to manage around [the green crabs]. We plant clams in the spring and summer so there’s some time before there’s a new growth set of crabs, and we also put the clams under protective mesh.” 

Jeff Loomans, president of the Seadrift Association and a member of the board for the Greater Farallones Association, said residents in Stinson and Seadrift are very concerned about the crabs. 

“We can always tell when the populations are high because there are noticeable impacts on the clams and other invertebrates, and birds,” he said. Residents noticed how native crabs returned to docks and clams reappeared in the sandy parts of the lagoon when the green crab population dipped down again after 2014, he said. 

In the next few months, Mr. Grosholz and his team expect to publish the results of their work in Seadrift, supported by genetic data, in a peer-reviewed journal. The results will show how their management strategies backfired, comparing the population surge to the lack of change in populations in nearby ecosystems.

Information gleaned from the project will also inform management projects planned for other coastal ecosystems. Just last year, green crabs popped up in Puget Sound for the first time.

“We’ve accomplished what we set out to do, because now we have a kind of yardstick to determine how much effort, time and money it takes to manage a given population down to a more sustainable level,” Mr. Grosholz said.

He also noted that prevention may be the best tactic. “Early detection and rapid response is the key,” he said. “It’s a good idea to survey the major ports annually to see what new species may be there: that’s a much easier approach than waiting for something to get established.”