The plan to eradicate house mice from the South Farallon Islands met resistance last week, when the United States Fish and Wildlife Service failed to convince the California Coastal Commission that dropping rodenticide from a helicopter is the best and only way to go.

Advocates and opponents of the eradication plan made their cases for over two hours last Wednesday in advance of a vote by coastal commissioners. But instead of going to a vote, the commission chair advised the wildlife service to withdraw its request for approval and return at a later meeting after detailing further mitigation measures and exploring alternatives.

The coastal commission received over 600 comments in opposition to the plan to drop 1.45 tons of poison pellets—carrying a total of 1.16 ounces of the anticoagulant rodenticide Brodifacoum—on the islands, which are located 20 miles south of Point Reyes.

Still, the wildlife service, which maintains that rodenticide is the only tried-and-true method for clearing islands of invasive mice, said it will not explore other methods. Spokesman Doug Cordell told the Light last Friday that the service will instead take the time to respond to questions that were raised about the logistics and effects of dropping the poison on the unique seabird breeding colony. 

Some of that information is already contained in the project’s final environmental impact statement, he said, while other details will have to be fleshed out. 

“We withdrew our application to give us time to answer some of the concerns that were raised, specifically about details of implementation,” Mr. Cordell said. “We are not going to be doing additional studies in terms of looking at other methods.”

Yet opponents of the plan—along with several coastal commissioners—want more consideration to be given to non-toxic alternatives, such as contraceptive bait that would render the mice infertile. Contraceptives have not yet been used for an island eradication despite scientific progress in recent years, according to Gerry McChesney, manager of the Farallon Islands National Wildlife Refuge. 

“When we first looked at that in our planning process, the idea of contraception at that point was completely theoretical. It has come along a bit since then,” he told commissioners at the July 10 meeting. “We do not consider the Farallones to be a place for experimental techniques.” 

In the past 12 years, 28 of 32 rodent eradication attempts using rodenticide worldwide have been successful, Mr. McChesney said.

But commissioners were interested in the idea of exploring and researching contraceptive methods for the project. “It seems a much more rational way of thinking to me than spreading poison all over an island,” commissioner Carole Groom said.

Last week’s meeting was the first time the coastal commissioners were presented with the plan’s final environmental impact statement, released in March. Mr. McChesney summarized the 300-page report, outlining the ecological value of eradicating the mice, which were inadvertently brought to the islands by ship around the time of the Gold Rush. 

“The ashy and leach’s storm petrels, our endemic salamander, the endemic cricket, and our native plant community should all benefit from this project,” he said. 

Scientists and representatives from environmental groups also expressed their support for the project, which the wildlife service says targets one of the highest-density populations of house mice in the world.

Pete Warzybok, a biologist for Point Blue who has lived and worked on the Farallones for 20 years, said his group’s research “clearly demonstrates that owl predation, which is facilitated by the presence of the mice, negatively impacts storm petrel survival. Delaying the removal of mice presents a very real risk,” he said.

Hannah Nevins, the seabird program director for American Bird Conservancy, also weighed in on the owls, whose time on the island is prolonged into the winter due to the presence of mice as a food source, the wildlife service says. “I witnessed a barn owl predation event where 40 carcasses of ashy storm petrels were found under one owl roosting area, so this is a significant impact to a species that only lays one egg a year,” she said. 

Alongside the benefits to the ecosystem, Mr. McChesney acknowledged that there will be short-term adverse impacts. “We do expect some individuals, especially gulls, to ingest the bait,” he said.

Mr. McChesney touched on the mitigation measures the wildlife service has planned to minimize harm: keeping gulls away, capturing birds of prey and salamanders, precision bait delivery, seasonal timing, carcass removal and monitoring of beaches. “I wish I had time to go through every one of these in detail,” he said.

After his summary, opponents rebutted. “The known damages and unmitigated risks simply exceed the hoped-for claims of possible benefits,” said Richard Charter, a senior fellow for the Ocean Foundation, who has criticized the plan since it was first outlined in the 2013 draft environmental impact statement. He highlighted past eradication mishaps, such as on Rat Island in Alaska, where 460 birds, including 46 bald eagles, died. 

(Despite the unplanned deaths, the organization that performed the Rat Island eradication, Island Conservation, does not consider that project a failure because all of the island’s rats were killed.)

Representatives of WildCare echoed the concerns of opponents that the wildlife service didn’t give enough weight to non-toxic alternatives for the Farallones. “We see on a regular basis in our wildlife hospital the incredible danger and the terrible effects of anticoagulant rodenticides on wildlife,” said Alison Hermance, the group’s communications director. 

Over three-quarters of patients at WildCare’s hospital tested positive for Brodifacoum at their hospital, Ms. Hermance said, which is why she wants the service to find a slower, more moderate way to eliminate the mice.

“They pretty much did the research towards the way they wanted,” Kelle Kacmarcik, the director of wildlife solutions at WildCare, told the Light after the hearing. (The wildlife service draws a distinction between the use of rodenticide for island eradications and its prolific use on the mainland, which it acknowledges is a problem.)

Beyond concerns about how thoroughly alternatives were researched, coastal commissioners also inquired about contingency and monitoring plans.

Commissioner Linda Escalante asked Mr. McChesney to define the circumstances that would trigger the early removal of bait. 

“That’s a good question, and it’s something we have not come up with an answer for,” he said, explaining that the project does not yet have a final operational plan that would specify the threshold for cancelling a second bait drop and removing bait from the islands by hand.

Commissioner Sara Aminzadeh pressed further. “When we are dealing with poison, we need a more comprehensive plan,” she said, disagreeing with the coastal commission’s staff report that found the plan was consistent with the Coastal Act.

“It’s like dropping a nuclear bomb on this island, as far as I can see,” commissioner Roberto Uranga remarked.

No action was required by the coastal commission at the meeting, since the wildlife service decided to withdraw its request for a consistency determination. Although that determination isn’t necessary for the plan to proceed, Mr. Cordell said the federal government likely would not move forward without the state’s

Nor did he rule out the drop happening in 2020; however, he said, that close date was always unlikely and now depends on the coastal commission’s schedule and the project’s permitting process.

The plan requires permits from three state agencies—the California Fish and Wildlife Service, the State Water Resources Control Board and the California Environmental Protection Agency—and two federal agencies, the National Oceanic and Atmospheric Administration and the Bureau of Alcohol, Tobacco, Firearms and Explosives (for the pyrotechnics used to scare gulls away). 

Before the service applies for those permits, it will issue a record of decision that describes the alternatives the agency considered and discusses its plans for mitigation and monitoring. 

Mr. Cordell did not have an estimate of when that record will be published.