The California Coastal Commission signed the death warrant for tens of thousands of house mice living on the South Farallon Islands. The controversial plan, years in the making, was approved by a 5–3 vote on Thursday, Dec. 16, after commissioners heard seven hours of compelling and passionate testimony from supporters and opponents.
While experts agreed the non-native, invasive critters are wreaking havoc on the environmentally sensitive island ecosystem, the means of execution—dropping 2,880 pounds of bait pellets laced with brodifacoum, an anticoagulant poison, onto the islands from a helicopter—pitted scientist against scientist.
The rich and fragile biodiversity of the South Farallon Islands is at stake, either from the effects of the house mice or the poison used to kill them. The islands are home to a variety of wildlife and plant species, as well as a breeding ground for seals and their relatives. The largest seabird-breeding colony in the contiguous United States and the world’s largest population of the rare ashy storm-petrel, a small seabird, inhabit the rocky isles. Migratory birds, bats and insects stopover on the archipelago during migration. Rare and endemic species found on the islands include the Farallon arboreal salamander and Farallon camel cricket.
Sailors brought the house mouse (Mus musculus) to the Farallons in the late 19th century. Since rabbits and cats were removed in the 1970s, the mice are the last remaining invasive species. With the population of the tiny rodents estimated at 500 mice per acre on the South Farallon Islands, the U.S. Fish and Wildlife Service, which manages the islands, maintains that distributing bait pellets treated with the rodenticide brodifacoum is the only effective eradication method.
November 2022, after bird-nesting and marine-mammal pupping seasons have ended, is the target date to spread the ton and a half of poison-laced pellets in two helicopter drops. The rodenticide-laced pellets will be dispersed by hand in any areas of the islands not reached by the helicopter drop. The goal is to eradicate every house mouse inhabiting the South Farallon Islands.
Opponents of the plan claimed the poison pellets have the potential to land in the ocean and move up the food chain to nontarget species. While acknowledging the mice need to go, they suggested using less-destructive poisons or waiting until a highly effective mouse contraceptive is developed.
“There is no mouse emergency,” Sarah Wan, founder of the Western Alliance for Nature and former member of the California Coastal Commission, said during her emotional testimony. “Dropping poison kills non-target species. And doing it in November, during the raptor migration, will affect the raptors on the entire West Coast.”
Eradicating the mice, which directly and indirectly impact the South Farallon Islands’ ecosystem, will help restore the natural balance, according to the U.S. Fish and Wildlife Service. The mice eat native insects, plants and seabirds, and compete with salamanders for food. Migrant burrowing owls visit the islands during their fall migration, which coincides with the peak of the mouse population. Typically, the owls, which prey on the mice, would leave the islands to continue on their migratory route; however, some remain due to the easy food source. When the mice population dwindles, the burrowing owls feed on the ashy storm-petrel, the Farallon camel cricket and other native insects.
WildCare, a nonprofit wildlife hospital in San Rafael, also provided testimony during the commission hearing. The agency is concerned the brodifacoum poison won’t remain on the island. Western gulls, which are plentiful on the Farallons, travel between the mainland and the islands. San Francisco is 27 miles from the archipelago, and Marin is just 20 miles away.
“A Fish and Wildlife Service letter stated 95% of the gulls at Fisherman’s Wharf are from the Farallons,” Alison Hermance, WildCare spokesperson, said during her testimony.
The U.S. Fish and Wildlife Service plan includes hazing the gulls, by using lights, statues and sounds, to keep them from staying on the islands while the poison is present and for weeks afterward. However, WildCare is not convinced the gull hazing will be effective, as the poison is delivered in “delicious cereal pellets,” according to Hermance.
WildCare fears dozens, hundreds or thousands of poisoned gulls could end up on the region’s beaches, as happened in Cape Cod, Hermance said in an interview with the Pacific Sun. If true, scavengers such as turkey vultures, red-tailed hawks and coyotes could prey on the highly toxic carcasses, spreading the poison up the food chain. Already, 76% of predatory animals tested by WildCare have anticoagulant rodenticides, such as brodifacoum, in their bodies. The agency believes they will be caring for a large number of animals impacted by the rodenticide drop next year.
Supporters of the plan, including Petaluma’s Point Blue Conservation Science and Island Conservation, an international organization based in Santa Cruz, said the benefits of eradicating the mice with poison pellets outweigh the potential risks, pointing to hundreds of studies.
The eradication of the non-native black rat from Anacapa Island was one success story offered up several times during the hearings. The island, located 12 miles off the coast of Ventura, was once overrun with the rats, which upset the delicate ecosystem. A similar method to the one planned for the Farallon Islands was used to kill the rats, and scientists quickly reported positive results for the Scripps’s murrelets, a rare seabird that nests on the island. Twenty years later, there are still no black rats on the island, and an amazing recovery of rare birds and other species has taken place.
At the end of the hearing, members of the California Coastal Commission took turns questioning Gerry McChesney, manager of the Farallon Islands National Wildlife Refuge. McChesney fielded some tough questions from Commission Vice Chair Dr. Caryl Hart, who is also the interim director of Sonoma County’s Agricultural Preservation and Open Space District.
Hart grilled McChesney about a photograph used during his presentation at the hearing, which showed a mouse eating a bird egg and elicited information he failed to reveal previously—the photo was not taken on the Farallon Islands, and there is no proof of the mice eating eggs on the Farallon Islands. Another concern of Hart’s was the probability of some pellets entering the water. Hart ultimately voted against the rodenticide drop.
Commissioner Katie Rice, who also serves on the Marin County Board of Supervisors, asked few questions of McChesney. In an interview with the Pacific Sun, Rice said she had already made up her mind to vote for the plan by then.
“I really leaned on the scientists, ecologists and the folks in the conservation movement and relied on their experience and knowledge about the restoration of other islands,” Rice said. “On balance, they all agreed about the importance of getting rid of the mice and allowing the species to recover. If we stand by and do nothing we’re causing way more harm.”
EDITOR’S NOTE: Paragraphs two and five have been updated to clarify that the reporter was referring to the weight of the poison-laced pellets, not pure poison.