Feds’ plan to eradicate invasive mice on Farallon Islands divides conservationists

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The U.S. Fish and Wildlife Service may blanket the South Farallon Islands with poison bait in an effort to eradicate invasive house mice, despite concerns among some environmentalists about accidental spills and potential effects on birds, fish and other wildlife in the waters of the Greater Farallones National Marine Sanctuary.

If the plan goes forward, 2,900 pounds of cereal-grain bait containing a total 1.16 ounces of a rodenticide called brodifacoum will be distributed across 121 acres of the southern islands, mostly using special bait-spreading buckets slung from a low-flying helicopter. Some hard-to-reach areas would be baited by hand or using refillable bait stations to lure mice into places like caves or man-made structures, as well.

The proposed operation is still at least 19 months off and requires additional authorizations before the final go-ahead is granted to managers of the Farallon National Wildlife Refuge, who are spearheading the effort.

The California Coastal Commission also will review the plan for consistency with its regulations. That hearing is expected to occur during the commission’s June 10-12 meeting in Rohnert Park.

The proposal has been under development for several years, inspiring controversy with each recirculation of supporting environmental documents. Like some other efforts to control nonnative animal species in remote, wilderness areas of the United States, it has managed to pit conservation interests against one another.

Point Blue Conservation Science, whose biologists have monitored Farallon Island populations for the past five decades, has supported the science and analysis for the plan, which has backing from many other conservation and bird groups as well, Refuge Manager Gerry McChesney said.

“We are a research and science-based organization,” said Zach Warnow, director of communications for Point Blue, “and what gives us the confidence is the strength of the science that is behind the proposal. This has been studied quite intensively for many years now.”

But for others, people like Richard Charter and Cea Higgins, Bodega Bay residents who have devoted decades of their professional lives to safeguarding the shoreline and coastal waters, the prospect of deliberately dropping toxic pellets anywhere near protected waters teeming with wildlife is anathema.

Their concerns include the possibility of bait pellets rolling down the islands’ steep slopes and into the water, western seagulls picking up stray tidbits and the potential harm for seals and sea lions exposed to the loud noises planned to keep seagulls away from the island.

The potential for secondary poisoning is especially worrisome, they say, although backers of the project say the tiny amount of toxin that would be spread among the nearly 1½ tons of bait is not water soluble, and would sink to the ocean floor and bind there, if it got into the water at all.

“I feel that there’s a lot of unanswered questions,” Higgins, executive director of Coastwalk/California Coastal Trail Association, said in an interview. “And it’s been bothersome to me that the public has been dismissed as emotional when they have questions, when they have concerns.

“People are trying to step away from an emotional reaction, but when they get told, ‘Gosh, there’s no impacts — this is perfect,’ they’re being dismissed, and so that creates mistrust,” she said.

The federal wildlife service has been working for years to develop a plan to address the overwhelming mouse population on the southernmost Farrallon Islands, a cluster of steep, rugged islands about 30 miles west of San Francisco that support a wide variety of wildlife.

The rocky outcropping holds the largest seabird breeding colony in the lower 48 states, supporting 13 species and about 300,000 birds that nest on the islands each year, as well as five species of pinniped that haul out there.

Once humans began to occupy the Southeast Island in the 19th century, however, they began introducing nonnative species, including cats, rabbits and mice. The mice remain, and in such abundance that during peak season they number as high as 490 animals per acre, Fish and Wildlife officials said.

“You cannot walk around the island and not see mice scurrying all over the place,” McChesney said.

Competition for food is so fierce, he said, they even cannibalize each other at times.

The mice are omnivores and feed on indigenous creatures found nowhere else — like camel crickets and arboreal salamanders — as well as compete with them for food. They also consume native seeds and plant parts, suppressing indigenous plants and promoting hardier, introduced species, Fish and Wildlife personnel say.

But the biggest problem is mouse population’s impact on migratory burrowing owls, which stop over from the mainland in the autumn to feed on the oversupply of rodents at their peak population and, over time, have learned to ignore their natural inclination to leave for wintering grounds, McChesney said.

Though the owls are relatively few in number — monitors who have access to about a third of the island have found about eight to 10 owls at a time — the birds gorge on mice until the tiny creatures’ population crashes in January and February.

Then the owls turn to a tiny, rare seabird called the ashy storm-petrel, which begins arriving to nest at that time. While not yet listed under the U.S. Endangered Species Act, the ashy storm-petrel is considered “a species of special concern” by the state of California. It holds similar designations from other groups, McChesney said.

Half of the world’s ashy storm-petrels breed on the South Farallon Islands, so their survival there is key to the longevity of the species. One study from 1972 to ’92 showed a one-third decline in their population, in part the result of burrowing owls, McChesney said.

Forty-nine different alternatives for controlling the mice have been evaluated since the Fish and Wildlife Service began its formal effort to find a solution, all but one eliminated before Brodifacoum-25 Conservation was selected as the preferred solution.

McChesney said he is the first one to concede that the project’s broad outlines sound a little “crazy” to many people at first, telling members of the Greater Farallones Marine Sanctuary Advisory Council during an informational presentation this week, “I understand that. I had some of those same feelings when I started looking into it.”

But the process has been used repeatedly to successfully rid island ecosystems of human-introduced rats and mice as a means of restoring ecological balance at other sites around the globe, he said.

“These are not novel techniques,” McChesney told the advisory council. “These are not experimental.”

Efforts to minimize impacts on other species include scheduling the operation in late fall, to avoid periods when breeding is underway and migratory wildlife is abundant. Hazing trials have been conducted to ensure seagulls will be responsive to efforts to keep them off the island during the work. There are plans to capture predatory birds that come around and collect any mouse carcasses, though mice would typically go underground to die.

There will be contingency plans for how to respond if something goes wrong, like when a truck carrying 18 tons of rat bait pellets containing brodifacoum careened off the side of a coastal road on New Zealand’s South Island in 2001, spilling some of its cargo into the water.

But many of those details won’t be finalized until operational partners are contracted and the final decision to go forward with the plan is made, which is testing the patience of those who are still awaiting assurances.

“ ‘Don’t worry. Trust me.’ — It’s not enough,” Dominique Richard, chairman of the Sanctuary Advisory Council, said in an interview. “There’s been enough evidence of spillover and collateral damage in however many operations they have made to know that there is, I wouldn’t say a large likelihood, but large enough to be concerned about it.”

You can reach Staff Writer Mary Callahan at 707-521-5249 or On Twitter @MaryCallahanB.

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