Wearing a battery-powered electro-shocker on his back, Josh Newhard came up short as he waded knee-deep in Buffalo Creek to probe for any eels lurking under rocks or along its banks.

“Well, that’s something we haven’t seen before,” he said. Just ahead, a herd of cows standing in the central Pennsylvania creek stared back at Newhard and the other five U.S. Fish and Wildlife Service biologists with him.

The biologists were using their equipment to catch and survey eels that have been stocked in the creek in recent years. As with the rest of the Susquehanna River basin, the snakelike fish had been absent for most of the last century.

The cows wandered up the bank and out of the stream after several minutes, but served as living proof that Buffalo Creek, which winds through rolling farmland and woodlots, is far from pristine — something also evident by its slippery, algae-covered rocks.

But the eels don’t seem to mind. In fact, they appear to be thriving. Information collected by the biologists confirmed earlier indications that the fish were growing rapidly. Nor is Buffalo Creek alone: Data show that the hundreds of thousands of eels stocked in Pennsylvania since 2006 are surviving and spreading throughout the watershed.

Eels were once one of the most abundant fish in the Susquehanna and other East Coast rivers. More than a million pounds a year were harvested before the construction of Conowingo Dam in 1928–29 blocked their migration just 10 miles from the river’s mouth.

While eels can crawl around or over small dams and obstacles, the 94-foot high dam has been insurmountable, and totally blocked the river for nearly a century.

Though they generally receive less attention, the eels’ restoration trajectory on the river stands in stark contrast with that of American shad, which have been traversing Conowingo and three other hydroelectric dams on the lower river in dwindling numbers over the last decade.

Just 14,276 shad were hoisted over Conowingo by a multimillion-dollar fish lift this spring. And while that was a modest improvement over the previous two years, only 178 of those fish made it past all four dams. Poor shad passage sparked a new 50-year agreement between federal agencies and Exelon, owner of the Conowingo Dam, this spring that will spur tens of millions of dollars in investments to get more of the migratory fish upriver once the utility gets a new operating license.

But biologists have found it to be relatively inexpensive to collect juvenile eels below Conowingo Dam and truck them upriver beyond the dams.

From 2010 to 2013, they released 118,642 eels into Buffalo Creek. Most were caught just below Conowingo, though some were also collected in Maryland’s Coastal Bays.

“We only proposed about 40,000,” said Julie Devers, a USFWS fisheries biologist. “We didn’t have any idea how many eels we would be catching at Conowingo.”

Unlike large adult shad, which are trying to migrate up the river to spawn, eels spawn in the Sargasso Sea in the mid-Atlantic but spend most of their lives in coastal rivers. By the time they reach Conowingo, they are about 2 years old, though still only 4–5 inches long.

They don’t require giant multimillion-dollar fish lifts to get upstream, either. Biologists simply run a trickle of water down the rocky river slopes below the dam, and the eels follow the water, slithering several feet up a plastic conduit into holding tanks, where they await their ride upstream.

Also unlike shad, which are notoriously fragile to handle, eels are remarkably durable when being transported. The ones netted from Buffalo Creek were placed in buckets of water and treated with a sedative so the slippery fish could be measured. After that, they’re injected with pit tags beneath their skin, so they can be identified if caught again.

“It’s nice to work with a species that doesn’t die as soon as you look at it, like American shad or something,” Newhard said as he measured one lethargic eel.

As seen in earlier monitoring on the stream, eels caught on this late summer day were continuing to grow at a rate of about 1.8 inches a year, though females — which get larger — are averaging about 3.3 inches a year. Some of the eels are already 1.5 feet long, and a few were approaching 2 feet.

While shad migrate out of the river after spawning, these eels will remain in the river for 10 to 20 years before returning to the ocean to spawn, making it easier to rebuild their populations.

Since 2006, biologists have stocked nearly 840,000 eels in the Susquehanna River, and they continue to add more each year. Because most eels won’t begin to leave the river for years, they are continuing to increase the overall eel population above the dams.

Besides those going into Buffalo Creek, 122,000 were released in Pine Creek in northern Pennsylvania, where their progress is also being closely monitored. The rest were released at a handful of other sites.

Now, eels are starting to turn up throughout the watershed. They have been recaptured near Hornell, NY, on a Susquehanna tributary. They have also been captured at a hydroelectric dam near Colliersville, NY, just a few miles south of Cooperstown, where the river begins. In Pennsylvania, they have been spotted in numerous locations, including far up the Juniata River, where they were never stocked.

“These individuals have traveled hundreds of miles in just a few years and are growing rapidly,” said Aaron Henning, a biologist with the Susquehanna River Basin Commission who has begun tracking reports of eels in the watershed.

Their return to long-lost habitats may change some of those streams in unknown ways. In headwater streams, for instance, they can account for a quarter of all fish biomass. Henning has begun collecting data in nine small Susquehanna basins to document how the predators may affect populations of other species over time.

“These were such a major component of the fish community 100 years ago, but have since disappeared,” Henning said. “Now we’re bringing them back. That’s a significant change to the system.”

Biologists are hopeful they might take a bite out of the growing populations of invasive rusty crayfish, an aggressive species from the Midwest that has invaded this region in recent years.

“Every time we look at (eel) stomachs, they’re full of crayfish,” said Steve Minkkinen, who heads the USFWS Maryland Fishery Resources Office.

But biologists are also hoping to see other changes in the rivers, including a boost in the populations of eastern elliptio mussels.

As he waded in Buffalo Creek, Minkkinen picked up several large eastern elliptio mussels, about three inches across. “These would be about 50– to 60-years-old,” he said.

But young mussels in the Susquehanna are scarce. Recent research has shown that eels appear to be critical for reproduction of elliptios, historically the most common mussels in the river.

Mussel larvae temporarily attach to the eels before they drop off and begin growing on the bottom. While other fish may also serve as “hosts” for the larvae, none is nearly as effective as an eel. As a result, there’s been little mussel reproduction in the river for most of the last century — those that remain are decades old. That’s also removed a powerful water-filtering capacity from the Susquehanna.

In the neighboring Delaware River, where mussel populations are more robust, biologists have estimated the 250 million eastern elliptios have the potential to filter 2 billion to 6 billion gallons of water and remove 78 tons of sediment from the water each day.

One of the hopes of the eel reintroduction effort is to boost mussel reproduction in the Susquehanna, and in turn, improve water quality.

Mussels grow slowly so finding young ones in the wild is difficult, but Minkkinen said biologists have seen evidence of reproduction, though primarily in Pine Creek — possibly because Buffalo Creek has problems like cows standing in the stream.

“We think there are some issues with Buffalo Creek with all of that agriculture there,” he said. Much of Pine Creek is forested, but Buffalo Creek is more heavily farmed, making it more “flashy,” he said, with changes in water flow and temperature happening more abruptly — something that could affect the brief window of opportunity in which mussel larvae have to connect with their host. But a growing number of eels may increase the odds of the larvae and eels being in the right place at the right time to make the connection, he said.

And more are on the way. Another 22,597 were trucked above the dams this year. Under an agreement with Exelon, the utility will take over the job of catching and transporting eels upstream through 2030, at which time eelways are to be completed at all four lower Susquehanna dams, allowing eels to migrate upstream on their own.

Not only should the river and the mussels benefit, but Minkkinen thinks the overall eel population will, too. Eel numbers along the East Coast are near record lows, with climate change, loss of habitat because of dams, pollution and other factors among possible contributing causes.

But one concern is that the number of large females in the spawning population has decreased — and the restocking effort may be helping there as well. The sex of eels is not determined until they are several years old. Those that swim farthest upstream into smaller, sparsely populated headwater streams almost always become females. By returning eels up the Susquehanna — historically their largest spawning river on the East Coast — Minkkinen believes more will reach headwater habitats that produce large, egg-filled females.

“The eels that travel farthest away from the ocean grow at low densities and they become the largest egg-bearing females,” he said.

Already, some of the largest eels on Buffalo Creek are showing signs of “silvering up,” — transforming into mature, dark silver eels with larger eyes — ready to make the swim back to the Sargasso in a few years.

(As originally posted, this story misidentified the New York city in the Susquehanna River watershed where eels have been found. It is Hornell. The Bay Journal regrets the error.)