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.