In the Seadrift Lagoon, green crabs will be limited, not eliminated.   David Briggs

Scientists have given up hope on fully eradicating the invasive European green crabs that dominate the Seadrift Lagoon, after a research effort there went awry. They are now recommending keeping the population at a manageable threshold. 

The attempt to eliminate the species, which is ranked among the 100 worst invasives worldwide, in the lagoon was nearing success until 2014, when the population exploded by a magnitude of 30. The explanation? Scientists had focused on taking the adult crabs, but because they are cannibalistic and consume their young, their absence inadvertently allowed the population to proliferate. 

“A failure in science often leads to unexpected directions,” said Dr. Ted Grosholz, a professor and ecologist with the University of California, Davis. “We slapped our foreheads at the time, but with thought and understanding, it’s told us a lot about what we shouldn’t be doing and provided a way forward for us. The world should get less focused on total eradication and work toward functional eradication.”

A study published in the Proceedings of the National Academy of Sciences in March depicted the dynamics of the Seadrift event, providing genetic data that proved the management strategy was at fault and that the crabs had not come from somewhere else. The mile-long, manmade lagoon connects to the adjacent Bolinas Lagoon by a culvert that is opened once or twice a year. 

Dr. Grosholz and his partners are recommending a new, much less aggressive management approach they call “functional eradication.” In practice this means that the project’s volunteers, who are organized by the Greater Farallones Association, continue to trap crabs in the Seadrift Lagoon with the aim of keeping the population below 40 percent capacity. (All the crabs they catch are taken to Gospel Flat Farm for compost.)

Dr. Grosholz and his team have also monitored the populations of green crabs in Bolinas, Tomales and San Francisco Bays, Bodega Harbor and Monterey Bay in order to compare the effects of their work in Seadrift with the ecological trends in the region.

Green crabs, which consume shellfish and compete with birds and fish for food resources, are listed by the International Union for Conservation of Nature as one of most destructive invasive species in the world. In the U.S., they cost the shellfish industry an estimated $20 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. 

For a brief time, they were a big issue for shellfish growers in Tomales Bay, after they showed up around 1996. That year, Hog Island Oyster Company lost about 40 percent of its Manila clam production, Dr. Grosholz reported. But the numbers in Tomales Bay have since dropped off due to improved management strategies like the use of protective mesh over clams and a revised planting schedule, and clams only account for around 5 percent of Hog Island’s operation. 

“We have seen the crabs come and go over the years, and some years have been worse than others. They are around and they are here to stay,” said co-owner John Finger.

The green crab research has wide-ranging impacts. Two additional, recently published studies by Dr. Grosholz and others develop the concept of functional eradication for marine invasive species and recommend citizen science as a viable means of implementation. Dr. Grosholz is advising marine managers up and down the coast based on his findings for green crab management. 

“While most managers in North America now focus on ongoing containment and suppression interventions, they often lack quantitative guidance from which to set targets and evaluate success,” a study published in Frontiers in Ecology and the Environment in October stated. “We propose practical guidelines for identifying management targets for invasions for which eradication is unfeasible, based on achieving ‘functional’ eradication—defined as suppressing invader populations below levels that cause unacceptable ecological effects—within high priority locations.”