SAN FRANCISCO (AP) — The only way to eradicate the nonnative mice infesting the environmentally sensitive Farallon Islands is to shower the islands with powerful rodenticides by helicopter, a new federal study concludes.

The U.S. Fish & Wildlife Service released a draft environmental impact report Friday analyzing alternatives for killing the mice that exist at densities of 500 per acre.

The plan analyzed nearly 50 alternatives for dealing with the mice, including releasing feral cats and snakes. In the end, the study concluded that the best alternative would involve air drops of poisoned food pellets.

The rodents most likely were brought to the islands on seal hunting trips a century ago, but now have colonized in a density unseen anywhere else in the world, researchers said. In some places they are so many mice that it looks like the ground is moving.

The mice pose  serious threat to a number of plants and animals that live on the islands naturally. However, ridding the islands of the pests has proven to be a lightning rod issue for the U.S. Fish & Wildlife Service. The service had delayed releasing its plan as viable alternatives to dropping poison were sought.

The agency’s eradication plan, if ultimately approved, would involve dropping diphacinon for 15 weeks, then the more powerful brodifacoum, which would last for five weeks before dissipating.

Sale of products made with brodifacoum was banned this year by the U.S. Environmental Protection Agency because of the toxicity to humans and wildlife, but it has been used on other islands for similar projects with impressive long-term results, despite some immediate collateral damage. The agent has a “very high” toxicity to mammals and birds, according to the EPA.

The mice attract hungry migratory burrowing owls, which make homes in the islands’ roughhewn cliffs. Once the owls move in, they eat a rare gray seabird called the Ashy Storm-petrel, which breeds on the islands and whose numbers are in decline.

While the cute Storm-petrel is the face of the service’s campaign to eradicate the Farallon mice, there are other creatures that might benefit from mouse eradication. The rodents eat food shared by the Farallon arboreal salamander, a bug-eyed, spotted creature unique to the island. The mice also feed on a species of cave cricket found only on the Farallons.

But critics worry that introducing poison to the islands will harm too many other, interconnected species — like the owls and other birds that may eat the poisoned mice.

“I don’t want to see the suffering of countless other species, especially when we don’t know what the long-term impact will be,” Maggie Sergio, an environmentalist formerly with Wildcare, told the San Francisco Chronicle. Wildcare is a nonprofit animal welfare organization that opposes the use of anticoagulant rodenticides because they pose a danger to other species.

The Farallons are a group of four small islands 27 miles west of San Francisco that constitute the Farallon National Wildlife Refuge. The seabird breeding colony there is among the top three largest in the U.S.

The report’s release triggers a 45-day public comment period. The Fish and Wildlife Service will make a final decision by the end of the year.