Scientists are creating global maps of fish migration routes, or swimways. The first global map has been completed; it includes 665 migratory fish species’ routes of passage. Illustration: Jeroen Helmer and World Fish Migration Foundation www.fishmigration.org.
“Dear people, it's over.” So begins a letter to the citizens of Utrecht, in the Netherlands—and to the residents of other cities and towns around the world. What was over was the spring season and the need for a fish doorbell developed by Dutch ecologists.
Thanks to the human inhabitants of Utrecht, and to anyone around the globe with access to the Internet, migratory fish may swim via the Oudegracht, or “old canal,” that runs through the center of the city to the Kromme Rijn, a river in the central Netherlands. There, they reproduce. To help the fish survive the journey to their natal streams, more than 440,000 people have pressed an online doorbell that effectively opens passage from the Oudegracht to the Kromme Rijn.
From March through May each year, fish such as bream (Abramis brama) navigate a route straight through Utrecht. Sometimes they must wait to swim through a canal lock where the gates do not regularly open. Now, conservationists in the Municipality of Utrecht, working with ecologist Mark van Heukelum of Utrecht's OAK Consultants, have placed an underwater camera at the lock. Anyone with an Internet connection may watch fish swimming up to the lock's gates and alert the gatekeeper by pressing a digital doorbell. The keeper receives the signal, then looks to see how many fish are milling about and decides whether the lock needs to be opened.
In spring 2021, fish watchers around the globe participated in the effort, including digital doorbell ringers from Canada, Germany, Spain, and Taiwan. “Every fish [at the lock gates] has been pressed on the bell,” concludes the letter from the municipality. “Utrecht has embraced its [migratory] fish, … and the fish doorbell works. We will be back in the spring of 2022.”
Migratory fish swim via the Oudegracht, the “old canal,” that runs through the center of the city of Utrecht in the Netherlands, to the Kromme Rijn, a river in the central Netherlands. Photograph: Japiot. (CC BY 3.0).
Protecting migratory fishes’ pathways
Piscine pathways, such as the run through Utrecht, are called swimways. Like birds use flyways, fish use swimways in their upstream and downstream migrations. Now ichthyologists are starting to look skyward, asking what they can learn from the flyways that led to bird conservation efforts stretching across regional, national, and international borders. Parallel programs should be developed for migratory fish, scientists say, by identifying and protecting swimways.
“Since the 1930s, people have used the concept of flyways for the conservation of birds,” says William Darwall, head of the Freshwater Biodiversity Unit in the International Union for Conservation of Nature's (IUCN)Global Species Programme. “That led to international policies for the protection of avian ecological hotspots. We can similarly use the developing concept of swimways for migratory fish to inform global policy.”
Swimways may span distances of more than 1000 kilometers, traversing oceans, lakes, and rivers. In more than 1100 fish species, migration is required for survival, according to The Living Planet Index for Migratory Freshwater Fish, a 2020 report by scientists affiliated with the World Fish Migration Foundation and the Zoological Society of London. For many fish, however, migration is akin to running a gauntlet. “Artificial barriers, such as dams, culverts, road crossings, and weirs impede the movement of migratory fish and reduce their ability to complete their lifecycle,” states The Living Planet Index.
Navigating swimways
Take the European eel. Its swimway winds along 6000 kilometers from the Sargasso Sea near Bermuda, across the Atlantic Ocean, and on to Europe's estuaries and rivers. It is the longest such path of any migratory fish. The European eel is a catadromous species that migrates down rivers to the sea to spawn. In contrast, anadromous species, such as coho and pink salmon, migrate upstream to reproduce.
Dams and other obstructions have blocked the eels’ migration from the Sargasso to freshwater habitats and have made their journey difficult to impossible, says Darwall. Overfishing has compounded the situation, with the result that European eels have declined by 98% since 1980, state Christian Sonne of Aarhus University and coauthors in a 2021 letter in Science on the eels’ risk of collapse. The species is now critically endangered. “To protect the European eel from further decline,” write Sonne and colleagues, “the EU must implement either a complete ban or restrictive quotas on harvesting.”
To reduce human impacts on migratory fish, Darwall and other scientists are developing maps of global swimways. Identifying these underwater paths in detail will lead to a worldwide overview of migratory fish and their swimways; provide decision makers with up-to-date, relevant information; and stimulate international information exchange, collaboration, and awareness of fish on the move, according to researchers.
At the top of the list is “evaluating the potential ecological impact of the proliferation of dams on migratory fish,” says Darwall, who is leading the effort. “We're working to inform decision-makers about how to avoid or minimize the environmental costs of dam construction when they're making plans to meet demands for water and hydropower.”
As part of the global swimways initiative, researchers at the IUCN, the UN Environment Programme World Conservation Monitoring Centre, the University of Cambridge, and the World Fish Migration Foundation have created the first global map of fish migration routes. The map includes 665 migratory fish species. Next steps focus on developing individual species swimways maps for many of the world's migratory fish species, according to Herman Wanningen, founder of the World Fish Migration Foundation.
100 pressing scientific questions on global fish migration
“Failure to understand how, why, when, and where different fishes migrate limits our understanding of migratory fish and their roles in aquatic and terrestrial ecosystems,” state Robert Lennox of the NORCE Norwegian Research Center AS in Bergen, Norway, and coauthors in a 2019 paper in Frontiers in Ecology and Evolution.
For the article—“One Hundred Pressing Questions on the Future of Global Fish Migration Science, Conservation, and Policy”—Lennox assembled a group of fundamental and applied scientists who study fish migration to identify pressing unanswered questions in physiology, navigation, locomotion, external drivers, and threats, as well as conservation management.
“The management of fish migration was developed primarily to restore the free movement of fishes in systems fragmented by dams and other in-stream infrastructure,” Lennox and colleagues report. Early efforts to provide passage at dams included royal decrees in 1215 in the Magna Carta to remove weirs from salmon rivers and the installation of fish ladders in Europe in the 1800s. A main question the authors pose: “Is there a time window in which habitat connectivity needs to be restored before population rebounds can occur in such migratory species?”
The European eel's swimway winds along 6000 kilometers from the Sargasso Sea across the Atlantic Ocean and on to Europe's estuaries and rivers. Photograph: Dmitriy Konstantinov (CC BY-SA 3.0).
Migratory fish: Bottoming out
“Who hears the fishes when they cry?” asks John Waldman in his 2013 book Running Silver: Restoring Atlantic Rivers and Their Great Fish Migrations, quoting Henry David Thoreau. Thoreau was dismayed by what he envisioned as fishes’ prospects during his 1839 trek along industrializing rivers in New England. He recounted his experience in A Week on the Concord and Merrimack Rivers, published in 1849.
Today, Waldman, a fish biologist at the City University of New York's Queens College, foresees the same bleak future, unless we change direction. Running silver, according to Waldman, refers to “times when there were so many metallic-scaled bodies churning their way up a river [that] it seemed the fish had become the water.”
Global migratory fish populations have plummeted by 76% on average since 1970, according to The Living Planet Index. “That is higher than the rate observed in terrestrial and marine species but in line with the overall decline observed for freshwater vertebrate populations as a whole: 83%,” states the document.
In Europe, migratory fish have nearly bottomed out, their numbers dropping by a staggering 93%. Those in Latin America and the Caribbean have not fared much better, declining by 84%. North America's numbers showed the least falloff at 28%. Overall, migratory fish populations affected by threats anywhere along their swimways showed an average decline of 94%. The findings were calculated using the Living Planet Database, with abundance information extracted for 1492 populations of 251 fish species listed on the Global Register of Migratory Species.
These fish, such as Pacific salmon, Atlantic salmon, shad, and anguillid eels, are vital to meeting global food security needs and to supporting the livelihoods of millions of people. They also play a critical role in keeping rivers, lakes, and wetlands healthy by supporting complex food webs. But their populations are threatened by hydropower development, climate change, overfishing, and pollution—and sometimes a combination of factors.
Dams + habitat loss + drought = fish in trouble
A river, say Waldman and other aquatic biologists, needs to run free. In fact, scientists such as Darwall and Wanningen maintain that the number one threat to migratory fish is dams. “The only way forward to rivers full of fish,” says Wanningen, “is to remove dams, starting with old and obsolete dams.” Living Planet Index data show that habitat degradation, change, and loss, largely due to river obstructions, account for some 40% of the problem depending on the region, and in Europe, more than 60%. According to the report, pollution and climate change together represent 10% of migratory fishes’ challenges, with invasive species and disease another 10% and exploitation or overfishing some 30%, again depending on the locale.
What happens, then, when these factors converge? In dammed river flows, salmon fry may miss cues to the best timing for heading to the ocean and becoming adults. Dams on rivers such as the Stanislaus, a tributary of the San Joaquin River in north-central California, are in effect compressing the migration timing of native Chinook salmon. The curtailment of high winter-into-spring river flows by dams, coupled with the loss of wetland habitat in the Sacramento–San Joaquin Delta, have left salmon in trouble. Less than 3% of the original wetlands in the delta remain.
Forested wetlands supply organic material, fallen trees, and shade to improve fish habitat in nearby open waters, according to the US National Oceanic and Atmospheric Administration. Floodplain forested wetlands are especially important as winter and early spring feeding and nursery habitat for salmon and other species.
In the Stanislaus, salmon fry that begin their migration in mid-spring, riding higher water flows to the sea, survive best and dominate adult returns to the river. Later-migrating fish suffer from rising summer temperatures that reduce their survival even though they migrate at larger sizes.
They may also face low water levels. After years of dry conditions, California is now in a moderate-to-exceptional drought, according to the National Integrated Drought Information System. In summer 2021, the Sacramento–San Joaquin Delta reached the highest category: exceptional drought.
Steps that restore natural fluctuations in river flows would benefit salmon, says Anna Sturrock of the University of California, Davis, lead author of a paper on the Stanislaus River salmon published in 2019 in Global Change Biology. Fry migrate in spring in such great numbers, Sturrock says, “that even small improvements in their survival rates could yield many more fish to boost adult returns.”
A herring gull flies off with a herring it caught just below the Watertown Dam, on the Charles River in Massachusetts. Photograph: Gouldingken (CC BY-SA 3.0).
It takes a (free-flowing) river
Only 37% of rivers longer than 1000 kilometers remain free-flowing over their entire lengths, and just 23% flow uninterrupted to the sea, according to a global assessment of 12 million kilometers of rivers by Gunther Grill, of McGill University, and colleagues, published in Nature in 2019. Very long free-flowing rivers are mostly found in remote regions of the Arctic and the Amazon and Congo basins. Fragmentation of rivers by dams and reservoirs is the leading reason for the loss of river connectivity, Grill and coauthors report.
More than 1 million barriers splinter Europe's rivers alone, according to a 2020 paper published in Nature. Barbara Belletti, of the Politecnico di Milano in Italy, and coauthors report that there are “at least 1.2 million instream barriers in 36 European countries, 68 percent of which are structures less than two meters in height that are often overlooked.” The findings, these scientists state, “could inform the implementation of the EU Biodiversity Strategy, which aims to reconnect 25,000 kilometers of Europe's rivers by 2030.”
If migratory fish need connected rivers, humans need them too. According to the 2021 report The World's Forgotten Fishes, freshwater fish provide a main food source for 200 million people across Asia, Africa, and South America and livelihoods for 60 million people. Healthy freshwater fish stocks sustain major global industries: recreational fishing, which generates more than US$100 billion annually, and aquarium fish collecting, which drives yearly trade worth up to US$30 billion. Aquarium fish are the world's most popular “pets.”
To reverse the steep decline of freshwater fish, the report recommends six pillars: let rivers flow more naturally, improve water quality in freshwater ecosystems, protect and restore critical habitats, end overfishing and unsustainable sand mining in rivers and lakes, prevent and control invasions by nonnative species, and protect free-flowing rivers and remove obsolete dams.
Razing dams should be at the top of the list, according to scientists such as Pieterjan Verhelst, of Ghent University in Belgium. Verhelst and colleagues published a “roadmap” to migratory fish conservation in the September 2021 issue of Frontiers in Ecology and the Environment. The scientists cite what they call “the Big Five considerations”: number one is removing barriers to migration, followed by installation of fish passages, habitat restoration, restocking depleted aquatic ecosystems, and improving fisheries management.
From a population perspective, the biologists write, “it may make more sense to focus efforts on improving a realistic number of river stretches in an integrated manner and address all major obstacles to successful population restoration, as opposed to investing in isolated management measures in as many rivers as possible.” The authors suggest identifying stretches of rivers that could serve as “diadromous species reserves.”
A Hurricane Sandy resilience project led to removal of the Centreville Dam in Maryland, providing 2 miles of unimpeded passage for river herring and American eels. Photograph: Jim Thompson/Maryland Department of Natural Resources.
If you build it, they won't come
One such location, perhaps surprisingly, may be New Jersey's Raritan River. “Lucky the folk who dwell beside a river,” wrote Richard Walsh in 1928 in Save the Raritan: Queen of Rivers. Across the United States, if not around the world, the Raritan's reputation as a fouled waterway has often preceded it. But efforts to revitalize the river by the Lower Raritan Watershed Partnership (LRWP), Rutgers University's Sustainable Raritan River Initiative, and other organizations are paying off.
The Raritan was once dammed in several places, keeping migratory fish from reaching their spawning grounds. “Dams are devastating to our stream systems and watersheds,” says Heather Fenyk, director of the LRWP. “They fragment our riverine habitat with cascading impacts, including blocking fish migration.” That said, “in many parts of the world, dams are a source of clean, renewable, and affordable energy,” adds Fenyk. “These shorter-term human-oriented uses are often prioritized over the significant longer-term habitat restoration benefits of removal.”
In the past decade, the Raritan's dams have begun to come down: First went the Calco Dam in 2011, then the Robert Street Dam in 2012, then the Nevius Street Dam in 2013. “Dam removal takes a big picture view of river restoration, advancing an approach that prioritizes restoring a river's ability to create and reconnect habitats,” says Fenyk. The removal of the Calco Dam ushered in what John Jengo, a hydrogeologist at Stantec Consulting Services who worked on the dismantling, calls the Raritan's “remarkable and rapid recovery” in the former dam area.
Based on observations at the upstream Island Farm Weir (IFW) and its fish ladder in the first spring migration season following the Calco Dam's removal (March through May 2012), the number of American shad migrating upstream increased by 500%, and the total number of fish passing through the IFW fish ladder went up by 200%. “It was an essentially instantaneous result,” says Jengo.
In the past decade, dams on New Jersey's Raritan River have begun to come down. First went the Calco Dam, pictured here in before-and-after views in 2011. Photographs: John W. Jengo, PG.
But the fish still faced hurdles. Biologists Olaf Jensen and Anthony Vastano of Rutgers (Jensen recently relocated to the University of Wisconsin–Madison) and their colleagues conducted weekly sampling each spring from 2012 through 2017 for American shad and river herring (the latter a catch-all term for blueback herring and alewife) at the IFW. Jensen and Vastano reported their results at the 2018 American Fisheries Society meeting. Every captured American shad and river herring was tagged with a PIT (passive integrated transponder) tag. The scientists deployed four antennas at the IFW fish ladder as well as a camera system to count and identify fish passing through.
In total, 50 American shad and 116 river herring were tagged. Some 38% of tagged American shad and 1.72% of tagged river herring successfully navigated the IFW fish ladder. The data are preliminary, says Vastano; he and colleagues are working on a paper to be submitted for publication. The main hurdles to passage, according to the researchers, appear to be locating the entrance of the fish ladder and progressing beyond its first chamber. Fish may have found the ladder's hydraulic height difficult. To make the transit easier, changes in the ladder's design to what Jengo calls a “nature-like fishway” may be in the offing.
The Veazie Dam was a hydroelectric dam on Maine's Penobscot River, located between Veazie and Eddington. The dam was removed in 2013, reopening the river to migratory fish passage. Photograph: Meagan Racey, US Fish and Wildlife Service Northeast Region.
“Although we want to celebrate successes, there is still a lot to learn about what makes for effective fish passage and how dependent it is on species,” says Dorothy Leonard, former director of the Maryland Department of Natural Resources Fisheries Service. “Many of the initial ladders were an obstacle for less energetic species and, as an alternative, pathways have been developed around some dams.”
Now Vastano and ecologist Julie Lockwood of Rutgers are using eDNA (environmental DNA) to monitor the comeback of river herring and American shad in the Raritan. “The goal,” says Lockwood, “is to assess the recovery of these migratory fish in habitat newly opened via dam removal in the Raritan watershed.”
Tigerfish patrol the Sabie River in South Africa's Kruger National Park (KNP). To date, at least 25 KNP dams have been breached or removed, but not all rivers in the park are free-flowing. Tigerfish photograph: Herman Wanningen, World Fish Migration Foundation. Sabie River photograph: Ossewa (CC SA 4.0).
In 2017, the Weston Mill Dam on the Millstone River, a Raritan River tributary, was demolished. With the dam's disappearance, American shad once again spawned in the Millstone. Juvenile shad were found there in 2018—the first shad sighted in the Millstone since 1845.
Raritan successes circle the globe
The news from Maine's Penobscot River is also uplifting, according to Joshua Royte of The Nature Conservancy, writing in From Sea to Source 2.0: Protection and Restoration of Fish Migration in Rivers Worldwide, a 2018 publication of the World Fish Migration Foundation. After the demolition of the Penobscot's Great Works Dam in 2012 and Veazie Dam in 2013 and the bypassing of the Howland Dam with a nature-like channel in 2016, river herring numbers increased from a few hundred to almost 2 million and American shad from 11 to 8000 by the spring of 2017.
Halfway around the world in Kruger National Park (KNP), South Africa, where some 53 concrete and earthen dams and weirs have been constructed along watercourses since 1931, the first dam removal, of the Black Heron Dam, took place in 2001. To date at least 25 KNP dams have been breached or removed, according to Robin Petersen of South African National Parks.
Not all rivers in KNP are free flowing, however. In one of the park's waterways, the iconic Sabie River, the aptly named African tigerfish swims. Marauding mobs of migratory tigerfish patrol the Sabie for hapless smaller fish. Remarkably, tigerfish also prey on barn swallows, the fish leaping out of the water and lunging for the birds as they skim the surface, according to Gordon O'Brien of the University of KwaZulu-Natal. In the 15 days of O'Brien's fish-bird study, published in 2014 in the Journal of Fish Biology, tigerfish “netted” 300 swallows. But life for tigerfish does not always bring an easy meal. The fish are restricted from migration to fertile lowland waters by the Corumana Dam downstream of the park.
Whether in South Africa or Maine or New Jersey, Thoreau's words in A Week on the Concord and Merrimack Rivers ring true today. He spoke of migratory fishes “still wandering the sea in thy scaly armor to inquire humbly at the mouths of rivers if man has perchance left them free for thee to enter.”
Message from the Municipality of Utrecht to migratory fish: Swim up to the lock gates. Thanks to a corps of bell ringers, ahead indeed is safe passage.
Author Biographical
Ecologist and science journalist Cheryl Lyn Dybas ([email protected]), a fellow of the International League of Conservation Writers, often writes about conservation biology for BioScience.
