Feds Punt on Plan to Control Island Mouse Problem With Poison

South Farallon Island, located west of San Francisco, California. (Jan Roletto / NOAA)

(CN) – The ashy storm petrel is in trouble.

A small seabird endemic to the islands off coastal California and Mexico, the petrel is one of the longest-lived of the avian species, with adults often making it to 31 years.

But its long lifespan belies problems in the overall population, as predation from gulls, owls, feral cats and other mammals have cut into the population-level health of the species.

About half of the population of the small gray bird – which can fit into the palm of a hand – nests in the Farallon Islands, a group of rocky uninhabited islands about 30 miles west of the Golden Gate Bridge in San Francisco.

The Farallon Islands are a national wildlife refuge offering important protection for a huge and diverse seabird colony that includes the ashy storm petrel. But recently a long-standing problem at the South Farallon Islands has gotten so bad, scientists and wildlife managers fret the petrel population will recede further if nothing is done.

House mice, introduced to the island in the 1800s, have flourished to the point where there are 490 of them per acre. Beyond their status as general pests, the mice have been seen preying on petrel eggs. The bird lays one egg per year, meaning any such loss is keenly felt by the vulnerable population.

Meanwhile, a number of barred owls – which are not native to the island – have also taken up residence given the abundant supply of mice. But the owls have also become adept at capturing and eating the much smaller petrel.

The U.S. Fish and Wildlife Service recently estimated that if the mice are not eradicated from the islands, the ashy storm petrel population will dwindle by as much as 63% in coming years.

“The mouse impacts to the ecosystem need to be eliminated,” said Gerry McChesney, manager of the Farallon Islands National Wildlife Refuge.

McChesney talked about eradicating mice from the island during a meeting of the California Coastal Commission on Wednesday, since he needs approval from the commission to move forward with Fish and Wildlife’s preferred method of eradication: poison.

Specifically, the agency proposed using a GPS-guided helicopter to drop about 2,900 pounds of a substance laced with the rodenticide brodifacoum while hand-placing other baits in caves and nooks across the island in hopes of totally eliminating the mice.

“We aim to take care of the entire problem in a short period of time,” McChesney said.

Opponents at the meeting all agreed mice eradication is necessary to protect endangered birds and other threatened species, but said such a widespread use of a highly toxic substance in such a sensitive area is misguided.

“It’s more appropriate to use a more moderate, more thoughtful, more environmentally responsible way to get rid of the rodents,” said Alison Hermance, director of communications for Wildcare.

Other opponents pointed to episodes in the recent past where the unintended consequences of rodent eradication resulted in the massive deaths of nontargeted species.

For instance, on Hawadax Island in the Aleutian archipelago off the coast of Alaska, one rodent eradication restoration project killed hundreds of gulls and as many as 46 bald eagles.

“This plan could cause damage to the biological systems of the sanctuary if we take this gamble,” said Richard Charter at the meeting.

Opponents asked Fish and Wildlife to consider alternatives, including contraceptive chemicals currently being developed or hazing and even removing the barred owl population.

McChesney said his agency focused on “tried and true” eradication methods, noting successes with similar approaches on island ecosystems across the globe. The manager said removing the owls, to the extent it is possible, would only solve a minor problem related to the mice while leaving the overall ecosystem of the island out of balance.

However, Coastal Commission members did not find McChesney entirely persuasive, demanding more specifics about how, where and for how long the poison would be applied.

“I cannot find consistency in this instance,” said Commissioner Sara Aminzadeh, explaining why she is prepared to vote against the poison program. “There are a number of outstanding impacts including potential harm to the public.”

Before the commission could vote, Fish and Wildlife – reading the writing on the wall – withdrew its application and requested more time to address specific concerns aired by commissioners during the meeting.

Lauren Garske-Garcia, an independent island ecologist enlisted by the Coastal Commission, said time is of the essence.

“Taking no action will mean a 63% reduction in the population of ashy storm petrel,” she said. “I think that outweighs whatever short-term impact of the rodenticide being introduced.”

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