131 eels went through a compact hydropower turbine built for canals, and every single one swam out alive

The eel came out of the metal tube still moving, long and silver, like nothing had happened to it at all.

It had just been pushed through a spinning turbine.

That turbine was built to make electricity, not to be gentle.

In a recirculating rig built to study exactly this question, gentle turned out to be exactly what it was.

Biologists were about to find out just how gentle, and the mechanism behind it starts with the shape of the blades themselves.

Blades spaced wide enough for a body to bend through

The machine at the center of the test is called the Restoration Hydro Turbine, and its design runs backward from almost every turbine before it. Conventional hydro runners spin fast and thin, built to squeeze maximum power from narrow gaps that leave little room for anything soft to pass through. This one does the opposite, with thick rounded blades and wide, open spacing that a body can slip between rather than collide with.

The blades also carry a forward slant from hub to tip, a shape meant to move fish away from the leading edge instead of into it. Most turbine injuries come from blunt strikes, pinching between moving and stationary parts, or sudden pressure change.

Thicker leading edges cut the chance of a severe strike. The wider blade spacing gives a long, flexible body like an eel’s room to bend through intact, so the turbine was never trying to outrun the fish, just to let it pass.

Two test sites, two very different fish

The eel trial itself ran not on a river but at the developer’s own hydraulic and fish testing facility in Alameda, California, using a smaller version of the turbine built for controlled study rather than commercial power. American eels were released into the intake and recovered downstream, over and over, while biologists checked each one for injury and then held it for observation.

A separate strand of testing moved to an installed turbine at a working hydropower site on an irrigation canal in Jefferson County, Oregon. There the question was size rather than species, whether adult rainbow trout, far larger than the juveniles most turbine trials focus on, could pass through unharmed.

Nobody expected the eels to be the easy part. Their narrow body shape and strong migratory instincts make adult eels hard to keep out of turbines in the first place, and conventional designs can injure them badly.

131 eels passed, 43 held back, zero losses in either group

Engineers from Natel Energy worked with researchers from the Pacific Northwest National Laboratory on the releases and recapture counts, and the result was published in Transactions of the American Fisheries Society. A 55 centimeter runner, about 22 inches across, was operated under 33 feet of hydraulic head at 667 revolutions per minute, with 131 eels passed through and 43 used as controls, measuring 13 to 26 inches long. Some of the eels were longer than the turbine was wide.

Every eel in both groups was alive after a 48 hour holding period, with no major internal or external injuries. A subset of 37 was also X rayed for internal damage. The authors call that a substantial improvement over eel survival through conventional Kaplan and Francis turbines, which may range from 40 percent to 95 percent.

High speed video captured passage through the runner region for 89 percent of the eels that went through, which the developer describes as the first footage of eels moving through a turbine. The American eel is listed as Endangered on the IUCN Red List, which is why the trial carried weight beyond its small scale.

The Oregon canal site turned to size instead. A test of 186 large rainbow trout measuring up to 20.9 inches found no meaningful difference between fish passed through the roughly 6 foot turbine and the control group, with immediate survival of 100 percent in the first trial period and 98.6 percent in the second.

Past 20 feet of blade width, the fix gets harder to prove

None of this means every fish everywhere now swims through hydropower unharmed. The turbine is a small diameter machine, built for canals, irrigation drops and small rivers rather than the towering runners inside major dams. By the developer’s own comparison, at the few plants over 50 megawatts that aim for fish friendly design, turbine diameters run past 20 feet, while safe passage has mainly been shown for fish around 8 inches long.

So the eel numbers do not transfer automatically to the Snake or Columbia rivers, where other engineers are chasing the same goal with different machines entirely, not unlike how wider flight corridors and live radar monitoring are being tested offshore to protect birds and bats around a very different kind of turbine.

At Ice Harbor Dam on the Snake River, a redesigned turbine built with Chinook salmon in mind recorded a 98.25 percent direct survival rate in live fish testing, a different fix for the same underlying problem at a much larger scale. Each design has to earn its numbers separately, river by river.

A small fix that could matter beyond one test rig

Even so, the results matter beyond their own small scale. Hydropower has long carried a reputation as one of the more disruptive forms of clean energy for migratory fish, and any design that chips away at that tension without cutting power output is worth watching closely.

Eliminating the need for costly fish screens or bypass structures, if it holds up at larger sites, could change how operators weigh new hydro projects against ecological cost, much like how a solar park near Berlin built wildlife refuges alongside its power output instead of treating habitat as an afterthought.

There is still ground between a test facility in Alameda and a dam on a major salmon river, and nobody is pretending otherwise. But watching an eel slide out of a spinning turbine and simply swim away is the kind of small, countable proof that this old tension between power and fish might not be permanent.