Wildlife Service explains plan to eradicate invasive mice on Farallon Islands 

click to enlarge Farallon Islands
  • Courtesy photo
  • A proposal to poison the invasive mice on the Farallon Islands has led to questions about potential negative effects, including water pollution.

Using poison to exterminate animals is sure to rankle some San Franciscans, especially when the plan includes dropping it from a helicopter onto a wildlife refuge.

So when the U.S. Fish and Wildlife Service held a public meeting about its proposal to eradicate invasive mice on the Farallon Islands, it faced pointed questions about the possibility of water contamination and secondhand poisoning, among other concerns.

The federal agency that oversees the land within the Farallon National Wildlife Refuge — specifically, the archipelago about 30 miles west of San Francisco — has released a draft environmental study that essentially lays out two options for the unwanted rodents: kill them with poison or leave them.

The mice, which are thought to have reached the island as seafaring stowaways in the early 1900s when people sailed there to hunt seals and gather eggs from seabirds’ nests, are one of the last invasive species on the southern islands and have direct and indirect effects on the native birds, invertebrates and plants, refuge officials say.

Refuge Manager Gerry McChesney explained to several dozen people at a community meeting Thursday evening that burrowing owls have stuck around the islands past their normal migration times since there are mice on which they can feed. But once the mice population plummets naturally in the fall, as it does annually, the owls turn to feeding on Ashy Storm-petrels, a rare Pacific Ocean bird that primarily nests on the Farallons.

Thus, the mice eradication proposal is part of a move toward restoring the islands’ ecosystem. McChesney stressed the word “eradication.”

“We are really talking about an all-scale removal of mice from the island,” he said.

To kill the mice, refuge officials in cooperation with several nonprofit groups that do conservation work on the islands, have proposed a one-time use of the powerful rodenticides Brodifacoum-25D or Diphacinone-50.

But use of the poison has become a hot- button issue. Meeting attendees peppered wildlife service officials with questions about possible water contamination or poisoning of animals that might eat the dead mice.

McChesney addressed possible water contamination and said the bait would be spread by GPS-guided helicopters using baskets to precision drop the pellets. He said that while there is some risk of the bait getting into the water, the cereal pellets are not likely to be eaten by the area’s carnivorous fish, and the poison is insoluble.

As for secondary poisoning, the environmental report proposes a variety of possible remediation efforts, including capturing and holding birds of prey and shooing away Western Gulls during the poisoning period.

The comment period for the environmental study runs through Sept. 30, at which time the agency will incorporate any comments into a final report, which would have to address all of the concerns, McChesney said.

“If we don’t find we can minimize risks, this project will not go forward,” he said.

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