Mice Capades

Potential use of rodenticides on the mice of the Farallon Islands divides environmentalists

From 1980 through 2003, Peter Pyle worked at the Farallon Islands off the coast of Marin. The veteran bird researcher counted seabirds, observed them feeding their young during nesting time, and many times witnessed and recorded great white sharks attacking and killing pinnipeds in the ocean waters surrounding the legendary islands. Pyle worked for the Point Reyes Bird Observatory at the time, and with a rotating team of assistant scientists, he lived in a small house on Southeast Farallon Island, the 357-foot-high crag visible to landlubbers from 30 miles away but off-limits to the general public. They hiked about the rocky shores, received grocery deliveries twice a month, and often fished for lingcod from a small skiff in the hours before dinnertime.

Today, Pyle, now working with the Institute for Bird Populations in Point Reyes, remembers his many seasons at the islands with a strange blend of sweet nostalgia and dread that makes the skin crawl—for the islands, now as then, are crawling with house mice. The animals are non-native, introduced accidentally more than a century ago by boaters, and every summer and fall their population explodes to grotesque numbers on two of the islands—namely, Southeast Farallon Island and an abutting crag called West End that becomes separated from the bigger island at high tide.

“They’re just crawling around everywhere,” says Pyle, who was working with the Point Reyes Bird Observatory during his years of island research. “It’s like some invasion-of-the-rats movie.”

The resident scientists, he says, sometimes kept a small compost heap in back of the house where hundreds of mice could be seen at a glance. Walking about on the rocky landscape, mice peeked out from nearly every crack and burrow. Nights in the old Victorian house were especially unsettling, he recalls. The rodents swarmed though the old dwelling. They skittered about on the counters, knocked over dishes, defecated on the dinner table and tousled sleepers’ hair. Many individuals, Pyle says, have made attempts at controlling the animals using snap traps. Killing 50 a night can be easy, but it’s a futile effort on an island whose mouse population in high season may reach 60,000 to 100,000.

The main problem associated with the Farallon Islands’ mice is a complex of ecological imbalances. For one, the mice prey on two native species that live nowhere else: the camel cricket and the arboreal salamander. The rodents’ presence has also attracted a population of burrowing owls, predators that previously only used the island for brief migratory stopovers but who now, due to the abundance of mice, remain for long periods.

When the mouse population suddenly plummets early each winter, the owls abruptly find themselves with almost nothing to eat. This turns their attention to native birds, in particular the ashy storm-petrel, a rare species that nests on the islands every winter and spring. The owls, according to experts, are slowly whittling away the petrels’ population. But the owls prefer mice, and if only the rodents could be eliminated, the owls, too, might go away.


For many ecologists associated with the islands, the solution to the matter seems clear: poison the rodents.

“Nobody is happy about maybe having to use poison,” Pyle says. “Nobody wants to do it, but when you weigh the costs against the benefits, it’s probably worth doing.”

The idea is more than an informal conversation topic. In October, the federal Fish and Wildlife Service released a 700-page environmental impact statement discussing the Farallones’ mice and dozens of ways to potentially address the matter. Doug Cordell, a spokesman with the service, says his agency considered a total of 49 different solutions to the infestation, including releasing cats onto the islands, using traps to curb their numbers and checking their fertility using medicine-laced bait. Most of these proposed actions have been dismissed, he says, leaving on the table just three. Two involve poisoning the rodents. The other would be to do nothing at all.

“We wouldn’t move forward with any option that posed more risk to the environment than benefits,” Cordell says. “Our job as an agency is to serve and protect wild lands and wildlife.”

Cordell stresses that the Fish and Wildlife Service currently has “no preferred alternative.”

Yet he describes the mice at the islands as “plague-like” in numbers, and he tells the Bohemian that successful rodent eradication would require removing every single individual mouse from a population. Traps, he says, would likely fail to substantially dent the mice’s numbers. Cats, too, would not catch every last one, and would certainly prey on the Farallones’ birds.

It may sound like an unlikely prospect—eradicating invasive rodents from a place where the ground appears to crawl with them. Yet this has been successfully achieved on many small islands worldwide. For instance, Anacapa Island, off of Santa Barbara, was successfully cleared of rats in 2001 using grain-based pellets laced with a powerful rodenticide called brodifacoum.

This is likely the poison that would be used at the Farallones. A tiny amount would be applied, according to Cordell. He says the pellets under consideration contain just 0.005 percent rodenticide—such a low density, Cordell says, that any bait pellets that drift into the ocean would dissolve and be rendered virtually harmless.

The pellets would not be aimlessly scattered either, according to Jaime Jahncke, a researcher with Point Blue Conservation Science, formerly the Point Reyes Bird Observatory. Jahncke, who backs the poisoning plan, says the pellets would be dropped from a low-flying helicopter and directed away from the tidal zone via a deflector at the mouth of the dispenser. This, he says, would minimize the number of pellets that reach the water.

Even if some pellets do dissolve into the tide pools, it may be unlikely that the marine environment would be effected. Jahncke points to an accidental spill in New Zealand in 2001 that put 15,000 pounds of poison pellets—containing almost a pound of brodifacoum—into a tidal marsh. The event, he says, had virtually no lingering measurable effects. Harvesting of shellfish for consumption was temporarily banned after the accident but was soon green-lighted again by officials.

“And that case involved a closed waterway and a humungous amount of poison placed directly into the water,” says Jahncke, who is also a member of the five-person Gulf of the Farallones National Marine Sanctuary Advisory Council.

By comparison, the proposed poison drop at the Farallones would involve no more than about two tons of pellets containing 1.5 ounces of brodifacoum. If officials opt for another rodenticide called diphacinone—less potent than brodifacoum—they will use about 16,000 pounds of pellets containing up to about a pound of the poison.

Still, opposition to the effort is strong. Jared Huffman has made statements questioning the wisdom of the plan, and the Marin County pest-management company WildCare Solutions is a firm opponent. The general public seems also to be leaning against the idea. Hundreds of written objections to the poisoning plan have been submitted to the Fish and Wildlife Service through its website since August.


Sean Van Sommeran, a shark researcher based in Santa Cruz, believes rodenticides applied at the Farallones could remain in the environment for long periods.

“They’re pretending this won’t have residual impacts,” says Van Sommeran, the founder and director of the Pelagic Shark Research Foundation. “It’s going to affect seabirds and marine mammals. It’s just going to be one more addition to the contaminants already in the water.” He believes the rodenticide could migrate through the food web and eventually contaminate large predators—like great white sharks, the core of Van Sommeran’s research—much the way that heavy metals find their way into sharks, swordfish and tuna.

There is little doubt that some birds—especially omnivorous western gulls—will eat the pellets and die. But Cordell says casualties could be minimized by scaring away the birds during the poisoning effort. Hazing methods—like using loud explosives and laser pointers to scatter flocks of gulls—have been tested already and proven effective at the islands. Owls, liable to suffer the consequences of eating poisoned mice, would need to be trapped and relocated during the eradication effort, Cordell says.

Eliminating the mice will benefit more than just petrels, says Brad Keitt of Island Conservation, a group based in Santa Cruz. “Removing invasive species has had incredible benefits to islands around the world,” he says. At the Farallones, Keitt says, “the driving issue is to restore the balance of the ecosystem.”

The Farallon Islands have seen non-native species come and go before. The islands were first visited by Russian sailors in the early 1800s, but it’s believed by scientists who have genetically examined the islands’ mice that the rodents were brought later in the century, from mainland American stock. Around the same time, rabbits were released on Southeast Farallon Island. Hundreds of them were still living there in 1971, as were several feral cats, when a scientist named David Ainley first set foot on the island.

“There was a lot of junk out there—sheds and garbage and things,” says Ainley, a Marin City resident who previously worked for the Point Reyes Bird Observatory and, through the 1970s, spent about half his life living on the island. “We got that all cleaned out.”

Ainley helped direct a focused trapping and shooting effort that successfully eliminated the rabbits. Three cats, he says, were captured and sent to the mainland. The mice, however, remained. In fact, removing the rabbits meant more food for the mice, especially the seeds of the many grasses that consequently thrived unchecked. The mouse population soared higher than ever.

“Poisoning is the only chance to get rid of the mice,” Ainley says. But mice, he says, are not easy animals to eradicate, both because they are small and easily able to remain unseen and because they reproduce prolifically. Southeast Farallon Island, at high tide, is roughly 60 acres, Ainley says. “There are infinite cracks and holes that they can hide in.”

Every winter, the Farallones’ mouse population plummets. Pyle explains that the first rains cause millions of small seeds scattered about the islands to germinate. This leaves the mice with nothing to eat. On top of that, winter rainfall tends to flood out their burrows, driving tens of thousands of starving mice into the cold open air.

“They come out of their holes and go wandering around eating each other,” Pyle says.

He feels that eradicating the mice would not just benefit birds but would eliminate immeasurable rodent suffering. So many mice starve each winter on the Farallones that for several months, from March to June, resident researchers don’t see a sign of the animals. Pyle guesses the mouse population bottoms out at perhaps 100 scrawny survivors in the early spring.

“Then the numbers start climbing, and by October it’s mayhem again,” he says.

Any poisoning effort would take advantage of this population cycle by hitting the mice while their numbers are down. The Fish and Wildlife Service has proposed the poisoning to take place in November of 2014, although the service is still considering its options and will release a final environmental impact statement this spring.