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Darkness falls along the shores of Florence Lake as Stan Orchard and his assistant Kevin Jancowski glide silently through a patch of water lilies in search of their prey, the voracious and invasive American bullfrog.

In a ritual that's repeated nightly from late April through mid-September, Mr. Orchard paddles a beat-up Zodiac through the shallow, murky waters while Mr. Jancowski beams a spotlight on the water's surface, searching for the telltale glint of frog eyes.

“There's one,” Mr. Orchard whispers. “They're transfixed by the light.”

Mr. Jancowski stands over the bow armed with the “frog zapper” – an electrically charged fishing net attached to a 2.5-metre pole. At the moment of truth, he slips the zapper into the water, hits the switch and immobilizes the unsuspecting amphibian with a jolt of electricity. The temporarily stunned bullfrog, measuring about 20 centimetres from head to toe, is plunked into a bucket as the duo sets about stalking another victim.

For Mr. Orchard, Florence Lake is a key battleground in his guerrilla war against the American bullfrog's relentless advance into Greater Victoria's watershed. Over the past three years, he has captured and killed close to 8,000 enemy troops, culled from more than two dozen local water bodies.

It has become both an occupation and an obsession.

Since launching his program, Mr. Orchard has subsisted on a bare-bones budget of about $60,000 a year, cobbled together from local grants, which pays for office space, equipment, research assistants, maintenance on his road-weary Dodge Caravan and his dedicated, 24-hour bullfrog hotline, 1-250-858-FROG.

Mr. Orchard wages his lonely war from a converted activity room on the ground floor of a rundown apartment building. Cluttered with computers, research manuals, microscopes, press clippings and a pair of huge freezers, the command centre smells vaguely of algae and frog guts.

Despite the sacrifices, Mr. Orchard says his efforts are absolutely crucial.

The American bullfrog is recognized internationally as one of the world's 100 most invasive species, he says. “They are a voracious predator. They have high reproductive potential and they are highly adaptable to different conditions. A single female can lay between 10,000 and tens of thousands of eggs. You can very quickly get into millions of eggs and a very high proportion of those eggs are going to survive.”

American bullfrogs feed primarily on other frogs, but have a diverse diet that includes crayfish, salamanders, snails, newts, garter snakes, baby turtles, small mammals and the occasional bird. Meanwhile, most potential predators find American bullfrog tadpoles distasteful, further increasing their survival rate.

It all amounts to a huge conservation problem, Mr. Orchard says. Once established in a pond, American bullfrog populations begin to eliminate native species “through predation and competition.” Then, when existing food sources have been wiped out, or close to it, the bullfrog population begins to feed on itself.

“It literally becomes a frog-eat-frog world,” he said. “They don't need the other species.”

Mr. Orchard first detected a thriving American bullfrog population at Elk Lake almost 20 years ago, as a research associate with the Royal B.C. Museum's amphibian and reptile program. Native to Ontario, Quebec and the northeastern United States, American bullfrogs were transported west and introduced to Colorado and California in the early 1900s by people looking to farm them for their meaty legs. After spending several years trying in vain to secure government funding to try to eradicate the invasive species, Mr. Orchard left for Australia, where he oversaw a $1.3-million budget with the World Wildlife Fund earmarked for amphibian conservation programs in that country.

He returned to Victoria in 2003, armed with a new tactic: He began testing electro-shock techniques on frogs, an idea he borrowed from a fellow B.C. biologist.

“I came back determined to show that, with some commitment and a little ingenuity, this was a fixable problem,” he said.

Mr. Orchard says he has now eliminated most of the adult American bullfrogs from the ponds he patrols, leaving him to focus mainly on the juvenile population this year.

But provincial authorities have shown little interest in spending money on bullfrog control.

Environment Ministry biologist Purnima Govindarajulu dismissed eradication as “extremely expensive” and “close to impossible.”

“Unless you've taken all the adults out, you're running very hard to stay in one place,” said Dr. Govindarajulu, who researched the American bullfrog while earning her PhD from the University of Victoria.

A percentage of juvenile males migrate to a new body of water every fall, a reality that would require continuing monitoring and management, she added. Indeed, as Mr. Orchard toils on the front lines in Greater Victoria, the American bullfrog continues to expand its range.

“We've seen a pretty major explosion, specifically over the last five years,” said Kate Miller, environmental manager for the Cowichan Valley Regional District, an area that stretches from Shawnigan Lake in the south to Ladysmith in the north. And in recent years, American bullfrogs have been spotted in Humpback Reservoir, which is located near Victoria's water supply. Although it is unclear what impact the bullfrogs would have if they spread to the water supply, the Capital Regional District has hired Mr. Orchard to patrol the reservoir as a precautionary measure.

Dr. Govindarajulu argues that American bullfrog populations are “mainly controlled by cannibalism” once they reach critical mass.

But Mr. Orchard said his research shows native species such as the Pacific tree frog and the red-legged frog are being devoured at an alarming rate.

“We've found as many as four Pacific tree frogs in the stomach of one American bullfrog,” he said. “It's ridiculous to suggest the problem will control itself.”

For now, Mr. Orchard will continue zapping bullfrogs and transporting them back to bullfrog control HQ.

Some will be discarded, some will be used for research, and others will be sent to the BC SPCA Wild Animal Rehabilitation Centre in Metchosin, which feeds the carcasses to raccoons and other animals.

“I hope they'll take some more soon,” Mr. Orchard said. “There's certainly no shortage. I'm running out of room in the freezer.”

Special to the Globe and Mail