Eels In The Susquehanna: A Surprising Success Story

Annapolis MD

20 October, 2021

12:56 PM

Description

By Karl Blankenship, the Chesapeake Bay Journal Oct 18, 2021 A decade ago, Steve Minkkinen and a team of biologists pulled into a boat ramp along a tributary to the West Branch of the Susquehanna River in Pennsylvania. Their pickup was hauling a blue tank filled with hundreds of squirming eels, ranging in size from large earthworms to small snakes. To some, they had about as much appeal. One woman watched as the creatures poured from the tank into the creek. "Well," she told Minkkinen, who heads the Maryland fisheries office of the U.S. Fish and Wildlife Service, "I'm never swimming in this river again." She was only getting a glimpse of what was to come. Efforts to bring the slithery fish back to the East Coast's largest river have accelerated. This year alone, more than 620,000 eels were returned to habitats they had dominated until the last century. Although eels were once an abundant food for American Indians and early settlers, the river was devoid of them just two decades ago — the result of massive dams built in the early 1900s, which blocked their migrations. Eels fell off the radar of the public and resource managers alike. Restoration attention — and funding — turned to getting the higher profile American shad back upstream. Those efforts absorbed tens of millions of dollars but have largely failed to date: Since 2008, shad restoration efforts have succeeded in moving fewer than 2,400 fish beyond the four dams on the lower Susquehanna. During that same period, biologists trucked 800 times as many eels upstream — more than 2 million — at a fraction of the cost. Started on a whim with a shoestring budget, the American eel restoration effort is transforming the ecology of the river. Surveys show they are not only surviving, but growing fast and spreading throughout the Susquehanna basin, where they once accounted for a quarter of all fish biomass. Dave Harp Biologists hope the returning eels will prey on, and slow the spread of, rusty crayfish, a troublesome nonnative invader. They also predict that the eels will boost populations of water-filtering mussels, which may eventually help improve water quality in the river. That's not all. Biologists now hope the surprising Susquehanna success will have consequences that reach far beyond the river or the Chesapeake Bay. Eel populations have plummeted throughout their North American range in recent decades and are considered "depleted" today. But the Chesapeake region retains the greatest abundance of eels along the East Coast, and 40% of the habitat there is found in the Susquehanna basin. Biologists hope that returning that vast area to productive eel habitat could help bolster eel numbers from South America to the Arctic Circle. A life full of mystery Eels have been surprising people for nearly as long as humans have existed. They live in a greater variety of habitats than any other fish in North America, from deep ocean waters to tiny headwater streams. They even crawl into ponds with no connection to any creek. Especially perplexing for centuries was the question of where eels come from, as most have no sex organs, and no one knew where they spawned. Aristotle thought they were spontaneously produced in mud. Some thought they came from earthworms. Scientists now know — at least they're pretty confident — that American eels come from the Sargasso Sea, a large expanse of the Atlantic Ocean off Bermuda bordered by strong ocean currents that is known for massive beds of seagrass. No one has actually seen a spawning eel, nor even a dead post-spawn eel. But plenty of eel larvae are found in that area. The larvae float with ocean currents for about a year until transforming into small, transparent "glass" eels that are capable of swimming, allowing them to break free of currents and head toward the coast. They gain green-brown pigmentation, becoming "elvers" as they move into brackish coastal habitats, like the Chesapeake Bay, and upstream into rivers. At around 4 inches, they transform into larger yellow eels, an appearance they will retain for years or decades — not that most people see them, as they also become nocturnal and live under rocks and roots or in the mud. They remain sexless until they are nearly ready to transform into their final stage: the silver eels, which are 2– to 3-feet long. These mature eels then make an enormous migration back to the Sargasso Sea to spawn. They have thicker skins and larger eyes to help survive the journey. This is the opposite of anadromous species, like shads, salmons, river herring, striped bass and sturgeon. Those fish briefly visit freshwater rivers to spawn but live most of their lives in the ocean. "Eels do everything backward," Minkkinen said. Eels are the only "catadromous" fish in North America, breeding in the ocean but living most of its life in brackish or fresh water. Eels face a dam problem That unusual lifecycle may have contributed to the demise of eels — and is the reason the Susquehanna might be a key to their comeback. An eel's sex is not determined until later in life, and research suggests that those in dense populations tend to be mostly males. Those that reach sparsely populated headwaters are almost exclusively females. By congregating eels downstream, dams may be restricting the production of females needed to help the coastwide stock reproduce. Some crawl over, or around, smaller structures, but each can reduce the number of eels that get by. Large dams — like the 94-foot-high Conowingo — can totally shut down their passage. If the river were to be fully reopened, Minkkinen estimates that the Susquehanna alone could eventually support million mostly female eels. Some think that number is low. That's important because, unlike anadromous fish that return to their native rivers to spawn, the entire eel population breeds as a group in the Sargasso Sea. Their offspring are flung across the coast by ocean currents, rather than returning to a specific river. Therefore, a rejuvenated Susquehanna population, biologists hope, could help rebuild eel numbers all along the coast, which is near its all-time low. But it's hard to say for sure because of the eel's unique life cycle, and they are poorly studied compared with anadromous species. It may be that eels from some places never get back to their spawning ground in the Sargasso Sea. In terms of reproduction, eels from some areas may be significantly more important than others. Are fewer big females with lots of eggs from one location more important than lots of smaller females with fewer eggs from someplace else? No one knows. "It's really hard to fit eels' life history into a quantitative model," said Kristen Anstead, a stock assessment scientist with the Atlantic States Marine Fisheries Commission. "Eels just become — highly scientific term — weird all the time." If Susquehanna eels do boost spawning, measuring their impact could be difficult because an increase in small eels would be spread from South America almost to the Arctic Circle. But, "it is pretty standard to think that the habitat loss with dams is a really big issue with eels," Anstead said. "Conowingo does seem to be a pretty big success story right now," she added. "I hope that does mean something for the population." Once prized, then forgotten The steady success on the Susquehanna is a surprise because it was so improbable. Eels were once an important part of the river. They were a major food for American Indians because they packed far more calories than other fish. Early colonists reported Onondagas roasting eels along the Susquehanna's headwaters. The river is still filled with stone weirs constructed hundreds, if not thousands, of years ago, to capture eels. European settlers developed a taste for them, too. A report from the Pennsylvania Department of Fisheries more than a century ago stated that the number of eels migrating up the state's rivers each spring was "simply enormous." But their days on the Susquehanna ended in the early 1900s when a series of hydroelectric dams closed the river to migrating fish. The last straw was Conowingo Dam, completed in 1928, leaving just 10 of the river's 440 miles unblocked. Pennsylvania periodically supported efforts to truck eels around the dams until around 1980, but the numbers found in the river declined, and fisheries vanished. In recent decades, they essentially stopped showing up. Despite their historic significance, eels became a low restoration priority. When resource agencies and utilities that owned the hydroelectric dams negotiated operating licenses decades ago, they called for huge investments to get migrating shad upstream. Eels were ignored. Tens of millions of dollars were spent building fish elevators to carry shad over dams. Those fish tend to migrate during the day and follow strong midriver flows. The elevators were never suited to move small, juvenile eels that migrate at night along slower currents at the rivers' edge. About two decades ago, Minkkinen became intrigued with eels after seeing efforts to improve eel passage on the Shenandoah River. If you could give eels a hand there, he wondered, what about the Susquehanna? Minkkinen kept hearing stories from people working with shad near the Conowingo Dam. "There are eels crawling up the rocks," they told him. "That," Minkkinen said, "is when I had the idea that it could be a really good place to catch eels." He had no budget to work with, but he got permission from Exelon Corp, which owns the dam, to collect eels that pile up below the structure. Working with low-tech materials from hardware stores, Minkkinen's team constructed a small tray that angled down the steep rip-rap lining the river's edge. They ran a small trickle of water down the tray hoping it would attract migrating eels. The first year, 2005, they caught 42. The next had 19. They tinkered with equipment and location, and in 2008 collected more than 42,000 eels and began trucking them upstream. Their best year — shortly before Exelon took over the task and upgraded the eel-catching devices — was in 2013, when they collected 293,141. "For a device that we probably spent less than $2,000 on, we had something that worked," Minkkinen said. A surprising link to mussels Around the time Minkkinen and his team were tinkering with eel collection below the dam, biologists at the U.S. Geological Survey's Northern Appalachian Research Laboratory, located hundreds of miles upstream near Wellsboro, PA, were focused on seemingly unrelated work with freshwater mussels. Mussels also have a complex life cycle: Successful reproduction requires that larvae attach to a fish, where they live as a parasite for a time before dropping off and growing on their own. While some mussels use many fish species, others are picky about which serve as hosts. The USGS biologists wanted to understand the decline of rare mussels. They wondered whether too few fish hosts might be contributing to the problem. Their big surprise, though, came when they looked at the river's most common mussel, the eastern elliptio. "They're so common you would think they use everything," said Bill Lellis, the biologist who led the work. Instead, he found that the eastern elliptios relied almost exclusively on eels. Karl Blankenship Follow-up surveys showed that the Susquehanna had fewer elliptios than the neighboring Delaware River. Further, the Susquehanna mussels were old. It appeared that the eel-less river was not producing young mussels, unlike the Delaware. Like oysters, mussels filter the water. In the Delaware, Lellis estimated that approximately 280 million elliptio mussels had the potential to filter 2 billion to 6 billion gallons of water and remove 78 tons of sediment from the water each day. With far fewer mussels in the Susquehanna, their filtering capability was dramatically lower. A recent report from the Bay Program's Scientific and Technical Advisory Committee reached a similar conclusion, estimating that a robust mussel population in the Susquehanna might be able to remove 8% of the annual nitrogen load — a key Chesapeake Bay pollutant. Today's depleted mussel population eliminates only a fraction of that. That's spurred more interest in hatchery-raised mussels that could boost the population. But mussel abundance could still be limited if they lack the right host fish to help them reproduce. Minkkinen's fledgling effort to get eels upriver quickly adjusted to test the notion that more eels could help produce more eastern elliptio mussels. From 2010 to 2013, his team stocked 118,742 eels in Buffalo Creek and another 122,049 in Pine Creek, both on the river's West Branch. The reintroduced eels thrived and produced an uptick in young mussels. In Pine Creek, juvenile mussels increased from zero to 21%. The increase in Buffalo creek was smaller, only a couple of percent. The watersheds are greatly different, which suggests other issues may be at play. "I think that's due to water quality issues," Minkkinen said. Pine Creek is less developed and overwhelmingly forested. Buffalo Creek has large amounts of agriculture — in places where the biologists did their surveys, the dominant "aquatic" animal seemed to be cows, rather than eels. A gauntlet of dams The effort to return eels to the Susquehanna still has something to prove. It has to show they can leave the river. While trucking small elvers upstream has been relatively easy, it's another matter for mature eels, now several years old and several feet long, to migrate on their own downstream, past the dams. "Everything isn't a success yet," Minkkinen said. "Not until you show that they actually are going to get out [of the river] and contribute to the population." To help understand whether that happens, Minkkinen and three other biologists were wading through Buffalo Creek one day in late August, carrying battery-powered electroshocking equipment and nets. They were sending streams of electricity into the water, stunning fish just enough so that they could rise to the surface and be counted before swimming away again. "There's a big one in there," Minkkinen said, pointing to a river bank with branches covering the water. It took several minutes of work, shocking and reshocking the water, pressing a longhandled net into the vegetation and muck until, at last, Sheila Eyler of the Fish and Wildlife Service pulled out their target. It was an eel — a female — more than two feet long. Though still predominantly yellow and green, it was starting to darken, beginning the process of "silvering." Having endured a remarkable life that started in the Sargasso Sea, a year or more in the ocean, a migration up the Bay and then a truck ride to Buffalo Creek, the eel was now sedated and given a small incision so biologists could insert a 1.5-inch transmitter. The eel, designated number 60053, can now be tracked as she begins migrating out of the river in the next year or so. During that trip, she will further transform, absorbing her digestive track to provide added fuel for the 1,500 mile swim ahead. It will be a one-way journey. Biologists want to learn if her trip is cut short. At the dams, eels can spill over the top, where they may smash on rocks below, or — more commonly — go through turbines, which can cut them to pieces. Receivers have been established along the river in Harrisburg, above the dams, and in Havre de Grace at its mouth. They will pick up signals from the transmitters and allow biologists to determine the fate of 60053 and hundreds of other eels tagged in years ahead. Right now, Eyler said, the goal is to have an 85% survival rate at each dam. Ultimately, that means that only about half of the eels that migrate past Harrisburg would make it to the Bay. The good news, she said, is that most of the dams are big, with larger, slower turbines. A small test at Conowingo a few years ago produced a survival rate of 90% for eels passing through turbines. But York Haven Dam, the first dam they encounter when moving downstream, has small, fast turbines. "We've had reports of several hundred dead eels downstream of that project in both 2019 and 2020," Eyler said. Gaining respect The fact that anyone cares about getting eels upstream or downstream is a remarkable reversal from 2005, when Minkkinen's team made their first attempt to catch eels at the base of Conowingo. Now, a new operating agreement for Exelon Corp., which took over the eel trucking at Conowingo in 2016, ensures that efforts to move eels upstream will continue for decades to come. Most upstream dam operators also have eel-related obligations. "They're on the radar now, and before they weren't," Minkkinen said. "That's a big change right there. So I'm really happy about that." He and others are hoping that people will come to embrace rather than fear the remarkable and still mysterious creatures as they return. Tracking by the Susquehanna River Basin Commission shows that eels are now turning up in most of the river's drainage. They have appeared near Clearfield on its western border and near Cooperstown, NY, near its northern edge. Both are far upstream from stocking locations. Aaron Henning, a commission biologist, said that eels account for 40% of the fish biomass in one creek he's monitoring. "They look weird and may be a nuisance when you catch them, but I think everyone is starting to understand their ecological value," he said. "There's not a person living in the basin who would disagree with the statement, 'we need more mussels.' People are ready for a feel good story like that." Cover photo: A mature eel swims along the rocky bottom of the West Branch of the Susquehanna River in Pennsylvania. (Dave Harp) The Chesapeake Bay Journal is a nonprofit news organization covering environmental issues in the Bay region. Sign up for a free subscription at BayJournal.com.