By Thomas K. Pendergast

A controversial plan to exterminate the entire mouse population on the Farallon Islands this coming fall is moving forward, after the California Coastal Commission voted to green-light an invasive rodent eradication project last December despite environmental concerns. 

The U.S. Fish and Wildlife Service (USFWS) plans to dump grain bait pellets laced with the rodenticide Brodifacoum-25D Conservation from specially designed buckets slung underneath helicopters flying over the islands, hoping to create a miniature mass extinction event for the islands’ mice. 

An anticoagulant that causes animals ingesting it to bleed out until dead, the use of Brodifacoum-25D has been restricted on the mainland because it causes significant secondary poisoning in birds and mammals that ingest the rodents it kills.

Gerry McChesney of the USFWS manages the Farallon Islands National Wildlife Refuge and says the project has been in the planning phase for 16 years as they looked at other options.

McChesney told the Commission that the non-native mice were introduced to the south Farallon Islands by human visitors in the 19th Century and the full natural ecosystem cannot be restored until they all get evicted. When they are at “peak abundance,” he said, they occur in numbers of up to 500 mice per acre.

“The mice are so numerous it looks like the ground is literally moving with them and they eat just about everything too, including each other,” he said. “This has caused significant suffering to the native fauna and flora of the Farallons: suffering to the rare crickets that they prey on; suffering to the salamanders; suffering to the native plant community; suffering to the rare storm petrels, who get eaten alive by the mice and get killed by the owls that are attracted to the islands by the mice.”

McChesney said the mice are throwing the whole ecosystem out of whack. 

“What this project is all about is eliminating the ecosystem carnage caused by invasive mice and restoring natural ecosystem function,” he said. “There is only one way to remove the impacts of mice and restore the Farallon ecosystem and that’s a highly-targeted one-time application of a specially formulated and registered rodent bait called Brodifacoum-25D Conservation.

“We recognize that using rodenticides comes with risks, especially to non-target fish and wildlife. But, from studies and lessons learned from previous eradications, those risks are predictable and mitigateable.”

But there are plenty of skeptics who doubt the risks are completely predictable and, thus, not assured they can be mitigated. One critic is former U.S. Congressman Leon E. Panetta (D. 20th-District), a former U.S. secretary of defense, CIA director and White House chief of staff, who had a representative read a letter to the Commission for the record.

“Experimenting with a multi-species poison already prohibited from retail sale in California, even as our state legislature is clearly moving toward banning its application on state lands with few exceptions, simply does not make sense,” Panetta wrote. “To attempt to spread this compound at any time, in proximity to the Dungeness crab or salmon seasons only further courts economic disaster. 

“My own lifelong efforts toward protecting California’s amazingly productive ecosystems grew from my family background in the sustainable fishing community. We always knew that our lives and our livelihood depended on a healthy ocean and I have never forgotten that principle.” 

“We have spent well over a decade and a half researching this problem and the most appropriate solution. And through all of that, we have found that there’s only one clear option,” McChesney responded. “And the dozens of conservation and scientific organizations who have gone on record supporting this project agree with us. None of us take this project or this decision lightly and we know first-hand that the Farallon ecosystem is suffering from human-imposed impacts. 

“We have the means to restore this ecosystem and we are responsible for safeguarding and healing these islands,” he continued. “We are suggesting the use of Brodifacoum-25D Conservation, a product specifically approved by the EPA for island conservation. This is a tool that the California Ecosystem Protection Act of 2020 calls ‘critically important,’ not the worst ecosystem poison.” 

The Ashy Stormy Petrel is particularly vulnerable to the mice because the rodents attract burrowing owls to feed, but when the furry prey start to thin out, the owls turn to these birds for food. 

The USFWS wants to lay out the poison pellets in October and November, before the breeding season. They plan on trapping and relocating migrating raptors during this period so that they do not eat the poisoned mice. 

“There’s a narrow window here because there are so many species on these islands,” Cassidy Teufel of the USFWS told the Commission. “We’re dealing with marine mammals that do breed out there, large numbers of them, so it has to be done before that breeding season, before they’re pupping. There are also a variety of seabirds that come to the islands seasonally, so it has to be done before they’re at their maximum numbers. Gulls are obviously a big concern.”

He explained the plan to capture and relocate or hold the predatory raptors during this time.  

“We’re not talking about thousands of raptors coming to these islands. We’re talking what might pass through, several dozen at most,” Teufel said. “Typically with burrowing owls we’re talking between two and 12, with an average of six to eight. For the raptors, it’s mostly burrowing owls and peregrine falcons and a fairly low number.” 

But three of the commissioners did not buy into the plan, including Commissioner Roberto Uranga.  

“This problem did not start overnight but it seems that we want to get rid of it overnight and I have an issue with that. It took many years for the mice to get to the point where it has become an environmental disaster,” Uranga said. “How do you know that other options don’t work unless you try them? That’s what experimentation is all about….

“We have to look at a better way of dealing with this,” he said. “I’m convinced that there is a better way out there other than the current option that we’re considering now.” 

The Commission approved the plan in a 5-3 vote.