Plan approved to poison invasive mice plaguing Farallon Islands

In a 5-3 vote Thursday, the California Coastal Commission approved a federal plan to drop cereal bait with rodenticide onto the South Farallon Islands to kill an invasive mouse population.

The 211-acre Farallon Islands off the coast of San Francisco have served as an essential breeding site for seabirds, including half of the fragile Ashy storm petrel population. But it’s also been plagued with an invasive species of mice since the 1800s.

After years of debate on the best practices to eradicate the ever-growing species, the U.S. Fish and Wildlife Service plans to drop one and a half tons worth of brodifacoum, a second-generation anticoagulant rodenticide, via helicopter in two separate trips spread 10 to 21 days apart.

The proposal calls for the drop to occur between the months of October and December, as more rainfall will wash away the pellets quicker and there is a lower amount of wildlife animals during the winter months, the commission staff report reads.

People will disperse the island’s western gull population before the dropping and until all bait pellets are washed away, which would take about five weeks. Other conservation efforts for native populations listed in the proposal include capturing and relocating burrowing owls, peregrine falcons and arboreal salamanders, collecting poisoned mouse carcasses and implementing a spill contingency plan.

The Fish and Wildlife Service project description says:

The fragility of the native species at risk from invasive house mice on the Farallones and their vulnerability to other human-induced threats such as climate change present us with an imperative to act expeditiously.”

Upon approval, the commission also required the wildlife service to draft a water quality plan with the National Marine Fisheries Service to reduce the chances of bait washing into the ocean. Coastal Commission spokesperson Noaki Schwartz said there is no definitive date for when this plan will be presented to the commission.

Roger Harris of Marin Audubon Society considers this approval to be a “victory for science over ignorance”:

The plan, like some 700 other successful introduced rodent eradications on islands worldwide, is designed to promote biodiversity and to heal the past harm that humans inadvertently caused by introducing the mice.”

The Marin Audubon Society, along with a series of other environmental groups like the Marin Conservation League, the Golden Gate Audubon Society and Napa Solano Audubon, expressed support for the initiative. Many say that though rodenticide is not an ideal approach, non-lethal options have not proven successful in removing invasive mice species.

The Marin Conservation League wrote in a letter:

If there were a feasible non-lethal means of removing the introduced mice, surely the many highly trained scientists who have worked on this project for decades and have dedicated their lives to the restoration of the Farallones would have embraced it. … They have no interest in promoting rodenticide use for its own sake. And, in fact, they have been among the state leaders in opposition to the chronic use of pesticide on the mainland.”

But other environmentalists say the end doesn’t justify the means. The poison could still find its way into the food chain and hurt non-target species, and maybe even make its way into human consumption.

Frank Egger of North Coast Rivers Alliance in a statement:

Brodifacoum will end up in the public’s food chain via Chinook Salmon & Dungeness Crab.”

Lisa Levinson of In Defense of Animals said in a statement:

An untold number of animals large and small will be impacted by this decision for years to come. Once brodifacoum inevitably becomes part of the food web, it will kill exponentially more animals than it could possibly save.”

Richard Charter of the Coastal Coordination program of the nonprofit Ocean Foundation said there are still a lot of unresolved questions the commissioners brought up, and it was still approved in the narrowest of margins. He said the commission gave a blank check despite not seeing a concrete operations plan.

Charter said:

For those of us have been working on this issue for several years, it was incredible to watch the Coastal Commission. Really, each commissioner gave a big speech about how much they oppose pesticides and said well, we still have to get rid of the mice. … So five of them voted for it.”

Charter said alternative, non-toxic methods need to be explored more for the sake of the environment, and specifically mentioned contraceptive methods to control the mice population:

Apparently after waiting 100-plus years before doing something, because the mice have been there a long time, Fish and Wildlife can’t wait another year or two.”

The Fish and Wildlife Service considered contraceptives an impractical method that has not proven to be effective in its analysis of theoretical alternatives.

The agency specifically labeled three major hurdles in contraceptive methods that prevent it from being considered as an effective solution: First, contraceptives have only been used as a way to control a population, and not as a way to eradicate an entire population, secondly, contraceptive products for house mice are still a few years away until development, and thirdly, products may require multiple doses, which would be hard to track on such a large scale.

The agency’s report said:

At this time, [contraceptives] are not a feasible, less environmentally damaging alternative, and it does not appear that they will present a feasible alternative in the near future.”

Charter said this still should not be an “either-or” situation, either letting out poison or letting mice run amok, and the bottom line is that harming non-target species populations with slow-killing poison should not be rationalized.

Charter said:

They don’t know if it’ll kill all the mice for sure. Then they’ll be back in a year to try again.”

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