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Swimming by the thousands up the Eel River this year, Pacific lamprey are literally climbing the wall of a dam near Potter Valley in Mendocino County.

Driven by the biological imperative to spawn in the river’s gravel beds, the snake-shaped, prehistoric fish — commonly mistaken for eels — have almost no chance of scaling the 63-foot high Cape Horn Dam.

For decades, their best option has been a fish ladder that flanks the dam, but even it halts the migratory journey for most lamprey, a largely ignored ocean-going species that shares the stream with federally protected chinook salmon and steelhead trout. Those that do clear the passage, by inching their way up the concrete walls, take up to five weeks to do so.

“They go crazy at night just trying to find a way up,” said Scott Harris, a Department of Fish and Wildlife biologist who runs the Van Arsdale Fisheries Station next to the dam.

The surge of lamprey numbers at the dam this year is a mystery, but wildlife watchers welcome the spectacle as a possible sign of a rebound in the population that mistakenly gave the Eel River its name in the 19th century.

Across California and the West Coast, where lamprey are a native species in many rivers flowing to the sea, there is scant data on their numbers, but biologists agree they were once far more prevalent.

Peter Moyle, the eminent UC Davis biologist who has studied California freshwater fish for more than 40 years, said the apparent abundance of lamprey is “good news,” considering the history of their shrinking range in California and, in the Eel River, a habitat bashed by decades logging, mining, grazing, urban development and major floods in 1955 and 1964.

Moyle said the lamprey surge may be the result of high survival rates of larvae, which spend five to seven years as tiny filter feeders buried in sand or mud consuming algae and bacteria in fresh water, or adequate prey in the ocean, where Pacific lamprey spend two to three years as adults.

“But this is pure speculation,” Moyle said in an email. “We don’t actually know what makes lamprey populations tick.”

Some experts attribute the surge to higher river flows following a historically wet winter.

The Pacific lamprey is a primitive creature, largely unchanged from ancestors that date back 360 million years. But it is a weak swimmer ill-suited to overcoming the man-made obstacles now common in much of its home range, which stretches from Japan along the coasts of Alaska, Canada and as far south in California as San Luis Obispo County.

To ascend the fish ladders installed to help imperiled salmon and steelhead, for instance, lamprey must attach their round sucker mouths to the walls — or any wet surface — kink their sinuous bodies and then suddenly straighten up, enabling them to reattach their mouths about 2 inches higher up.

The climbing behavior, documented in detail by scientists, is unique to Pacific lamprey.

At Van Arsdale, biologists have given lamprey a novel and decidedly low-tech boost via a pair of 4-inch diameter flexible plastic tubes laid 300 feet from the bottom of the fish ladder to the top of Cape Horn Dam, cutting lamprey travel time to as little as 45 minutes.

“Sure enough, man, they go up the tubes like nobody’s business,” Harris said.

Standing on the fish ladder’s steel walkway overlooking a deep pool at the base of Cape Horn Dam, Harris declared: “Trust me, there’s a lot of lamprey in there.”

Behind him, a white plume of water spouted from the dam’s central gate, and below it a dozen or so doughty lamprey clung to the concrete.

Fish biologists Damon Goodman of the U.S. Fish and Wildlife Service and Stewart Reid, a consultant to the federal agency, came up with the tube system last year, spending less than $3,000 on materials. The do-it-yourself project motto, Harris said, was “KISS: Keep it simple stupid.”

The setup includes one high-tech feature, a “video box” consisting of a semi-transparent section of tubing and a camera operating on surveillance system software that records whenever a lamprey comes by.

Seated at a computer in the kitchen of the research station’s cottage, Harris pulled up black-and-white images of lamprey in the tubes that make Van Arsdale the only year-round, 24/7 lamprey counting site on the West Coast.

The system counted more than 5,000 lamprey in three days from July 1-3, he said.

A total since the lamprey spawning run began in April is not available because it requires an exhaustive count by technicians reviewing the video in which multiple lamprey often appear on the screen at one time.

Counts of the steelhead and salmon migrations this year at Van Arsdale numbered in the hundreds.

Everyone connected with lamprey — including Native American tribes, environmentalists and scientists — considers this a boom year for the fish. Harris noted that last year’s lamprey spawning run was also strong and a similar trend occurred in 2012 with a major water release from Lake Pillsbury to assist chinook salmon.

In appearance, the slimy, scaleless brown fish with two beady black eyes and seven round gill pores on each side of their head are off-putting.

They are classified as fish, rather than eels, because they share a similar life cycle to salmon and steelhead, spawning in fresh water and migrating to the ocean where they become adults.

Lamprey have been part of the human diet since at least Roman times, and remain a delicacy today in parts of Europe.

Frontiersman Josiah Gregg misnamed the Eel River after Native Americans gave lamprey to his famished exploratory party at the river in December 1850.

Two months later, Gregg died from a fall off his horse at Clear Lake because of starvation.

Inspired by Goodman’s earlier work on lamprey passage projects, Caltrans engineer Kristine Pepper spearheaded a separate project to aid lamprey passage along a tributary to the South Fork of the Eel where it flows under Highway 101 near Leggett. The nearly 800-foot culvert now has a series of rounded weirs for lamprey as well as a conventional fish ladder contour for salmon and steelhead.

“It’s the first time (lamprey) have really been on our radar,” Pepper said.

If the experiment proves successful, Caltrans will incorporate it in future projects, she said.

Members of the Wiyot Tribe, who continue their ancestors’ practice of fishing for Eel River lamprey, were the first to note this year’s heavy spawning run, said Tim Nelson, natural resources director for the tribe, which occupies an 88-acre reservation at Loleta in Humboldt County.

Tribal anglers, who scoop lamprey from the river using a pole and crooked hook, were “just blown away by how many they were catching,” Nelson said. A couple of men reported catching more than 50. The legal take-home limit is 5.

Lamprey have long been a “traditional food source” for the Wiyot, he said.

Scott Greacen, executive director of Friends of the Eel River, an Arcata-based conservation group, said he has heard anecdotal accounts of a surge in lamprey but suggested it may be “partly because we are looking.”

The apparent upswing is heartening because it “shows the potential for life to rebound” following a prolonged drought, Greacen said.

The group he leads is advocating removal of the Cape Horn Dam and another in the Eel watershed to open up an additional 287 miles of potential spawning habitat for lamprey, salmon and steelhead.

At the Van Arsdale fish ladder, bald eagles, raccoons and bears feast on the mass of lamprey that are easy pickings, Harris said.

His only concern, he said, is that a marauding bear might turn its paws on the lamprey project and tear open the video box.

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com. On Twitter @guykovner.

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