U.S. federal wildlife officials were urged Wednesday to withdraw a proposal to drop nearly 1.4 tonnes of rat poison on remote islands off the coast of California to kill a mice infestation until it addresses questions on the impact to wildlife.

The California Coastal Commission heard public comment on the U.S. Fish and Wildlife Service plan, which has drawn criticism from local conservation groups. The commission is seeking to determine whether the plan complies with state coastal management rules.

The U.S. Fish and Wildlife Service said in a report presented to the commission in March that a massive house mice population is threatening the whole ecosystem on the rugged Farallon Islands, 44 kilometres off the coast of San Francisco.

The archipelago is home to the largest seabird breeding colony in the contiguous United States, with approximately 300,000 to 350,000 birds of 13 species, including the rare ashy storm petrels. The islands are also used by marine mammal species for resting and breeding, and by migratory birds.

‘100 per cent eradication’

Federal wildlife officials proposed using helicopters to dump 1,315 kilograms of cereal grain pellets laced with brodifacoum, which causes rodents to bleed to death and is banned in California.

Officials acknowledged the plan will kill some seagulls and other species, but argue that the benefits of eliminating the invasive species will heal the whole ecosystem.

“The only way to protect these species and allow the ecosystem to recover is 100 per cent eradication of the mice,” said Pete Warzybok, a biologist who has worked on the Farallon Islands for more than 20 years.

“Anything else is simply a stopgap measure that will not adequately address the problem.”

A herd of elephant seals lounge in a slumbering pile in a sandy cove on Farallon Island, Calif. Critics fear the rat poison that federal officials want to drop on the island would harm animals other than mice, which have infested the island. (Eric Risberg/Associated Press)

Critics argued the poison will not only kill the mice, first introduced by ships that stopped in the islands 200 years ago, but also wildlife on the island and scavengers that would feed on the carcasses of the poisoned animals.

“These poisons are deadly, they persist in the environment for hundreds of days and they do kill animals,” said Alison Hermance, the spokesperson for the conservation group WildCare.

“The situation on the Farallon Islands has existed for decades. It does not need to be solved overnight with a massive poison drop.”

The California Coastal Commission has no power to veto the plan but before federal officials can proceed, their plan needs approval from the various state and federal agencies.

After a nearly two-hour hearing, commissioners said they still have questions on the impact to seabirds and other species.

“We haven’t been convinced that this is the best and only way to go,” commission chair Dayna Bochco said.

The commissioners asked federal officials to withdraw the proposal and resubmit it after their questions have been answered.

The project would be implemented between November and December, when the mouse population is declining and stressed for food, and would occur no sooner than late 2020.