Should the Feds drop poison on the Farallon Islands to destroy invasive mice? Controversial plan goes to vote this week


For boaters approaching the steep, rocky coastline, the South Farallon Islands appear to be a reign of preening pelicans, barking sea lions, and other native seabirds and pinnipods.

But in the middle of this wilderness, 27 miles off the coast of San Francisco, is a long-ago human-introduced invader: European house mice. In such large numbers that they shake the ground, the rodents attract owls, which hunt a rare sea bird, the ashy petrel, and leave only piles of feathers behind. That has prompted wildlife officials to devise a plan to get rid of the mice by dropping about 3,000 pounds of poisoned bait on the islands, which promises minimal harm to wildlife.

The California Coastal Commission will vote Thursday on the latest version of the US Fish and Wildlife Service’s mouse eradication plan. If it approves the plan, it would then have to be given the go-ahead by the regional director of the US Fish and Wildlife Service and it would probably not be implemented until the fall of 2023 at the earliest.

“This is a gem of the Pacific,” said Gerry McChesney, manager of the Farallon Islands National Wildlife Refuge, the largest seabird nesting site in the lower 48 states with up to 350,000 birds and 13 species. “The mice were introduced by humans, and it is our responsibility to do everything in our power to correct this mistake – restore this ecosystem so that it is back in balance.”

Fish and Wildlife submitted a version of the Giftdrop Plan, which has been in the works since 2004, to the California Coastal Commission in 2019, and then withdrew it from scrutiny after the commission asked for additional details. The latest proposal from the federal agency to the Coastal Commission goes into greater detail on how the helicopter drop would take place and how the wildlife would be monitored afterwards. It is supported by dozen of conservation organizations, including the National Audubon Society and the Nature Conservancy.

“We wanted to make sure we were leaving no stone unturned,” said McChesney. “We know it’s a controversial project in the Bay Area.”

Critics – including other environmental groups like Sierra Club California and Friends of the Earth – say it is too risky to cover a conservation area with a strong rodenticide.

“This has never been an uncontroversial issue,” said Richard Charter, a senior fellow at the Ocean Foundation who helped coordinate the establishment of the Farallones National Marine Sanctuary gulf that surrounds the four groups of the Farallon Islands. “This is the worst possible persistent ecosystem toxin, in the wrong biological system, in the middle of the best-protected piece of ocean on earth.”

The US Fish and Wildlife Service argues that the benefits of the plan far outweigh the risks. The drop of poison would come in the fall outside of the breeding season, and staff would use intimidation practices to scare away seagulls. They point to the successful extermination of rodents on other pristine islands such as the Channel Islands of California and the Galapagos, and the fact that the invasive mice also prey on other species native to the islands, the Farallon tree salamander and the Farallon camel cricket.

The poisoned bait crumbles and has been shown to be undetectable in the soil at other restoration sites where it was used within months, Fish and Wildlife said.

“There are many reasons to support this. It is needed to preserve the Farallones wildlife and unique ecosystem, ”said Pete Warzybok, Farallon Islands program director at Point Blue Conservation Science, which conducts biological research on the islands. “The longer we don’t do anything to improve the situation, the more damage it will do.”

In autumn, their peak season, the mice can reach a population of 60,000, or around 500 per acre.

“Especially at night you can see them jumping around and digging up their caves,” says Warzybok, who has spent many months on the islands over the past 22 years. “It looks like the ground is moving.”

The mice, likely brought in by Russian fur traders in the early 19th century, attract burrowing owls to infest them until their populations decline in winter, and then move on to the ashy petrel, a species of particular concern in California. Half of that World’s population of 10,000 ashy petrels nests on the Farallones, a number that, if left untreated, will drop 63% in 20 years, according to a 2019 study by Point Blue. Recent observations by Point Blue biologists broadly confirm this population decline.

The US Fish and Wildlife Service’s eradication plan begins with an intimidation process to scare away the western gulls that spend the fall on the islands. In tests conducted by McChesney and others, one of the most effective methods was to hang the bodies of dead seagulls on stakes like portraits, keeping them away for days or weeks.

“In very simple terms, it drives you crazy,” McChesney said.

In the next phase, helicopters would thoroughly drop the grain pellets, which contain 0.0025% of the rodenticide Brodifacoum, over the islands. You would repeat the drop 10 to 21 days later.

Wildlife staff collected mouse carcasses to prevent them from being eaten by birds of prey. Since the poison takes a while to kill the mice, McChesney expects most of them to go to their burrows and die underground.

Some seagulls would also eat the pellets and die. The agency expects the death toll to be well below 1,050, which, according to the project’s environmental impact statement, is the highest amount the population could lose without harm. Wildlife personnel manually collected uneaten pellets after 10 days.

Critics like Charter say a lot more seagulls could be harmed and question the need to use Brodifacoum, an anticoagulant that causes internal bleeding that can drag on for weeks. Its use was restricted in California after it became widespread in predators such as mountain lions and spotted owls.

Instead, he and other opponents of the poison plan advocate waiting for a more humane alternative in the near future: a contraceptive for mice.

“It’s not a question of whether contraceptives will replace a poison, it’s a question of when,” said Loretta Mayer of the FYXX Foundation. The non-profit research organization is developing a contraceptive for mice that is already in commercial use in rats and could be ready for testing in one to two years, Mayer said.

Mayer also argued that contraceptives are more effective than poison, which must kill every final breeding pair in order to be effective.

The US Fish and Wildlife Service ruled out the use of such a contraceptive because it said it would not eliminate the population and would have to be repeated several times. McChesney also said that Brodifacoum has been used effectively in all but one of the 64 successful island mouse exterminations worldwide.

For Warzybok, who has spent so much time on the largest of the South Farallon Islands among fur and elephant seals, ash petrels, murmurs and puffins, the mouse poison plan is the only way forward despite the controversy.

“The first time I was out there, I fell in love with the majesty and diversity of the island,” he said. “It is very important to me to do the best for you.”

The chronicle’s author, Yoohyun Jung, contributed to this story

Tara Duggan is a contributor to the San Francisco Chronicle. Email: [email protected] Twitter: @taraduggan


Comments are closed.