Canada Fenced the Carp Out of a Dying City Marsh — What Came Back Is Unreal

In the spring of 2025, aquatic plants filled Coutts Paradise Marsh from end to end, for the first time since the 1930s. Not in patches, not in scattered clumps near the shore, from end to end. 600 hectares of wetland on the western edge of Lake Ontario, wedged between a steel city and a Staff at the Royal Botanical Gardens had limestone escarpment, covered in green. been monitoring the marsh for over three decades. They had watched it die. They had watched it sit empty. And now they were watching submerged vegetation spread across the entire basin in a single growing season, as if the marsh had remembered what it was supposed to look like. Tis Thaismeyer, the senior director of ecosystem stewardship at the gardens, wrote that Coots Paradise once again looks and functions like a marsh. That sentence sounds simple. It is not. It compresses nearly a century of loss and 30 years of painstaking recovery into nine words. This is the story of how Canada fenced the carp out of a dying city marsh, and what came flooding back once they were gone. To understand why that sentence carries so much weight, you need to know what Coote's Paradise was before it broke. The marsh sits at the far western end of Lake Ontario, just three kilometers from Hamilton City Hall. It is the largest remaining coastal wetland on the western shore of the lake. It drains 17 streams across 270 square kilometers of watershed. It is part of the Niagara Escarpment World Biosphere Reserve. It holds more than 750 native plant species, roughly 24% of Canada's entire flora, and 38% of Ontario's. It is designated a National Historic Site, a nationally important bird area, and an important amphibian and reptile area. On paper, this marsh is one of the most ecologically significant patches of land in eastern North America. For thousands of years, it functioned like a living engine. Indigenous peoples used it as a gathering and portage site after the glaciers retreated. The marsh sat at the northern edge of the Carolinian life zone, where southern tree species like sassafras and tulip tree pushed up against the boreal north. Dense stands of cattails and bulrushes filtered the water flowing in from the escarpment. Wild celery and wild rice grew thick across the shallows. Sago pondweed carpeted the bottom in beds so dense that diving ducks could feed without surfacing for minutes at a time. The marsh produced enormous quantities of fish, that migrated into Lake Ontario and fed the broader food web. In the early 1800s, a settler named Charles Durand described vast flocks of waterfowl and abundant beaver, mink, muskrat, and otter. In 1826, a British engineer named John McTaggart praised the extraordinary variety of wildfowl. By the 1920s, hunters and naturalists described marshlands, where ducks gathered in the thousands every autumn until freeze-up. The water was clear enough to see the bottom in most of the shallows. The marsh was not just beautiful, it was productive. It filtered water, sequestered nutrients, buffered floods, nursed fish, and fed birds on a continental migratory flyway. A 1928 biological survey documented three salamander species, six snake species, six frog species, one toad species, and four turtle species. The system was complete, and it did all of this for free. Then humans began to dismantle it, piece by piece. The first petition for protection was filed in 1866 by John W. Kerr, Canada's first fisheries overseer. It was ignored. Between 1826 and 1837, workers had already dug the Desjardins Canal straight through the heart of the wetland, connecting Hamilton Harbour to the town of Dundas for shipping. In 1851, a railway straightened the canal further, and a railway berm sealed off the marsh's natural outflow to the harbour. The marsh was no longer a flowing system. It was a basin with a plug. The water that once flowed freely between the marsh and the harbour now sat stagnant, controlled by an infrastructure that was designed for commerce, not ecology. Pollution followed. Hamilton was a steel town. Its industries dumped waste into the harbour. Sewage from surrounding communities flowed in through the tributary streams. Sediment from farms and parking lots and subdivisions washed downstream and settled on the marsh bottom. But the single most destructive force was not a factory or a pipe. It was a fish. Common carp were introduced to Lake Ontario in the 1870s. They were brought from Asia as a food fish, and they spread rapidly through the Great Lakes. By the 1930s, carp dominated Cootes Paradise. To understand what that means, you need to understand how a common carp feeds. It is a benthivore. It roots along the bottom like an underwater pig, sucking up mouthfuls of sediment to extract invertebrates and plant material. A single adult carp can disturb several square meters of lakebed every day, multiply that by tens of thousands of fish, and the result is a marsh that never settles. The water stays permanently turbid. Sunlight cannot reach the bottom. Submerged plants cannot photosynthesize. They die. Without rooted vegetation to anchor the sediment, waves and wind churn the mud even further. The turbidity feeds itself. The carp do not just damage the ecosystem, they lock it into a new state. John Hall of Fisheries and Oceans Canada once explained that carp have locked themselves into a biological niche that sustains their own conditions at the expense of every other species. A 1935 survey found roughly 85% of the marsh covered by emergent plants like cattails and bulrushes. By 1984, that number had collapsed to about 15%, and most of what remained was It was non-native invasive vegetation. By 1990, nearly all submerged and emergent plants had been lost. The marsh held an estimated 70,000 adult carp. Their combined biomass was around 800 kilograms per hectare, roughly 240 metric tons of carp stirring the bottom of a single wetland. Only four of 21 submerged plant species that had once grown in the marsh could still be found. Only 28 of 190 hectares of emergent vegetation survived. Bald eagles, once common along the Lake Ontario shoreline, had been pushed to near extinction in the region by DDT. By the early 1980s, the entire Great Lakes basin had just four active bald eagle nests. Cootes Paradise was not a marsh anymore. It was a shallow, muddy lake. The water was opaque. You could stand at the edge and not see your boots beneath six inches of brown water. The plants were gone. The fish were almost all carp. The frogs had fallen silent. The turtles, painted and snapping and northern map, were declining year after year because there was nothing left to sustain their young. The birds that depended on vegetation for nesting and feeding had moved on or died out. Bittons, marsh wrens, black terns, the species that had once defined this place, were absent. A place that had once been one of the most productive wetlands in eastern Canada had become an ecological dead zone, surrounded by a city of half a million people, and almost nobody outside of Hamilton was paying attention. The turning point came in 1993. The Royal Botanical Gardens, which had managed the Coutts Paradise Nature Sanctuary since 1941, launched an initiative called Project Paradise. It was part of the Hamilton Harbour Remedial Action Plan, a binational effort under the Great Lakes Water Quality Agreement, to clean up the most degraded sites in the basin. Hamilton Harbour had been designated one of 43 areas of concern across the Great Lakes, and Coutts Paradise was its most visible wound. Peter Rice and Len Simser led the project for the gardens. They brought in researchers from McMaster University and the Canada Centre for Inland Waters. They consulted Ontario government ministries and enlisted the Hamilton Naturalists Club, which had donated $32,000 toward fish and wildlife restoration. The ambition was extraordinary. Rice acknowledged openly that no textbook existed for what they were attempting. He said there was no off-the-shelf technology to restore a wetland ecosystem of this scale. They were improvising. But they had one central insight that changed everything. The marsh could not recover while carp controlled the bottom. You could plant all the cattails you wanted. You could filter every stream. You could dredge every inlet. None of it mattered as long as 70,000 carp were ripping the roots out of the sediment every day. The plants would never establish. The water would never clear. The fish community would never diversify. Every other restoration goal depended on solving the carp problem first. The solution was deceptively simple in concept and enormously complex in execution. They would build a fence, not a wall across the marsh. A selective barrier at its only connection to Hamilton Harbour. The Desjardins Canal, the same canal that humans had carved through the wetland in the 1820s, was the single point where fish moved between the marsh and the lake. If you could control that choke point, you could control what got in and what stayed out. Beak Consultants designed the structure. It was a novel concept. Nothing like it had been attempted on the Great Lakes before. Most fish barriers are one-directional. They keep things out. This one had to work in two directions, blocking invasive carp while still allowing native fish to migrate freely between the harbour and the marsh for spawning. Construction ran through 1996 and into 1997. When it was finished, the Coutts Paradise Fishway became the first carp barrier and two-way fishway on the entire Great Lakes. It was not a dam, it was a sorting machine. A steel grate system spanned the canal from shore to shore. The bars were spaced five centimeters apart. Any fish smaller than that could swim through freely. Larger fish, the ones too wide to pass through the grate, were caught in steel baskets submerged in the water. Every morning during the spring spawning season, starting around mid-April, staff would arrive at the fishway to begin the sort. Cranes lifted the heavy baskets out of the canal and dumped their contents into large holding tanks. Biologists waded in and began identifying each fish by hand. Native species, pike, bass, suckers, bowfin, perch, were measured, weighed, and released into the marsh to spawn. Common carp and other invasive species were removed. Some were tagged with microchips to track their movements. Others were destroyed and in some cases composted for use as soil nutrients. It was physical, tedious, meticulous work. Two chutes directed fish into separate channels, letting the team block non-native species, while native fish passed through to their spawning grounds. The fishway operated through the spring and into early summer, then partially opened in fall when water temperatures dropped and carp activity slowed. Staff recorded sex, length and weight for every migrating fish that passed through. They built a dataset that now spans nearly 30 years. The early results were modest and uncertain. Carp did not vanish overnight. The water did not clear in the first season. Meanwhile, a community volunteer planting program had been running since 1993, organized by a local group called Ecowise. Volunteers waded into the shallows with bundles of nursery-grown cattails, bulrushes and arrowheads, and pushed them into the muck by hand. Of 44 exclosures planted with eight emergent species, only three established reliably. Researchers tested protective enclosures around transplanted vegetation, rectangular plastic barriers designed to keep carp from uprooting young sprouts. When the panels were removed in late summer, muskrats destroyed many of the plants within weeks. Staff learned to wait until early spring or late winter before opening the enclosures, giving the roots time to anchor. Even with the fishway operating, the water remained turbid from decades of accumulated nutrient pollution. Sago pondweed was the only submerged species that grew consistently in the windswept, murky water. Storms tore up fragile seedlings. Algae blooms smothered new growth. In the first few years, it would have been easy to look at the marsh and conclude that nothing much was changing. But the biologists at the fishway kept sorting. Every spring morning, they lifted the baskets. Every carp that went into the removal tank was one less fish churning the bottom. It was incremental. It was unglamorous. It was the kind of work that does not make headlines because the payoff is measured in centimeters of water clarity per year. But something was changing. Slowly, and then not slowly at all. By the time researchers compiled the data over the following decades, the numbers told a story that nobody had dared to predict. In 1997, the fishway's first year of operation, 17,861 fish passed through the barrier. Roughly 82% of them were carp and other non-native species. By 2022, the catch composition had flipped almost completely. Out of 15,847 fish handled that year, about 91% were native. Carp and their relatives had dropped to approximately 9% of the total. The population collapse was even more dramatic. Ecologists had estimated roughly 70,000 adult carp in the marsh before the fishway was installed. By the mid-2020s, that number had fallen to fewer than 1,000. Carp biomass dropped from approximately 800 kilograms per hectare in 1995 to roughly 10 kilograms per hectare by 2024. That is a 98.7% reduction. Let me put that in context. The marsh went from holding 240 metric tons of bottom-feeding invasive fish to holding a fraction of that, and the ecosystem responded as if a switch had been thrown. The vegetated area of the marsh expanded from roughly 27 hectares in 1995 to 166 hectares by 2024. That is more than a six-fold increase. Submerged plants went from scattered patches to covering approximately 40% of the marsh. Emergent plants like cattails and bulrushes reclaimed another 19%. White water lilies, once gone from the western bay, began spreading again. American lotus flowered for the first time in 2025. Southern wild rice, a culturally significant plant that had nearly vanished, was reintroduced and started spreading to new sites. Hundreds of plants appeared in the meadow marsh ponds. The water itself was transforming. Here is a detail that rarely makes it into the conversation about wetland restoration. The suspended sediment concentrations dropped from 65 mg per litre in 1995 to 16 mg per litre by 2024, a 75% decrease. Total phosphorus fell from 270 micrograms per litre to 85. E. Coley counts plummeted from over 10,000 per 100 milliliters to 173. Water clarity, measured as the depth at which you could still see a white disk lowered into the water, improved from under 30 centimeters to 52 centimeters. The marsh was still far from pristine. But for the first time in living memory, you could see into the water. Scientists expected modest improvements. What they found was a system accelerating beyond every projection. And here is where the cascade begins. Clearer water meant more sunlight reaching the bottom. More sunlight meant submerged plants could grow. More plants meant more root systems anchoring the sediment. Anchored sediment meant less resuspension. Less resuspension meant even clearer water. The cycle that carp had locked in reverse was now spinning forward. Each improvement enabled the next. With vegetation came habitat. With habitat came species. In 2013, something happened at Cootes Paradise that stunned even the most optimistic researchers. A pair of bald eagles built a nest on the north shore of the marsh, and successfully hatched two eaglets. Tees Theismeyer spotted the first chick in mid-February and checked the nest weekly from a nearby trail. On March 22nd, he saw a small head poke up from the nest. The next day, a second eaglet appeared. These were the first bald eaglets born on the north shore of Lake Ontario in decades. Some sources say it was the first successful nest in the region since populations crashed in the 1980s. Heismeyer called the moment a testament to the restoration efforts of Project Paradise. The eagles were not alone. Trumpeter swans returned to the marsh, their deep calls echoing across water that had been silent for decades. Osprey appeared, circling above the shallows and diving for fish that were finally visible from the air. Leased bitterns and prothonotary warblers were spotted in the recovering vegetation. Gray tree frogs recolonize the habitat, their trilling chorus audible from the boardwalk trails on warm spring nights. Hundreds of migratory bird species now use Cootes Paradise as a seasonal staging area, including white pelicans, Caspian terns, and black-crowned night herons. The marsh functions as a fish nursery that produces an estimated 5 to 20 million young fish for Lake Ontario every year, depending on water levels and conditions. In 2025, white sucker became the most abundant species recorded at the fishway for the first time in its history. White sucker is a native fish, an important forage species in the deeper waters of Lake Ontario. Its dominance at the fishway was a signal that the native fish community was reclaiming the marsh. By 2026, the fishway recorded the fewest large common carp since operations began. The marsh had achieved five consecutive years of positive habitat recovery. Aquatic ecologist Jennifer Bowman continued to identify American eels at the fishway. Migrants that had traveled all the way from the Sargasso Sea to spawn in a marsh that a generation earlier would have offered them nothing. The system was rebuilding itself. Not the way engineers rebuild things, the way nature rebuilds things. Slowly, then suddenly. Here is the detail that rarely makes it into the conversation. The cost of the fishway, and its decades of operation, is a rounding error, compared to what Hamilton would have spent trying to achieve the same ecological outcomes through engineering. A constructed wetland treatment system to filter the same volume of water would cost tens of millions of dollars to build, and millions more annually to operate. The fishway used a steel grate, some baskets, a crane, and a small team of biologists. The marsh did the rest. It filtered the water. It sequestered the nutrients. It rebuilt the habitat. It nursed the fish. It fed the birds. It did all of this because someone had the insight to remove the one thing preventing it from functioning. This is not romantic nature worship. This is hard-nosed restoration economics. Remove the barrier to recovery, and the ecosystem becomes its own restoration engine. But this is also where the story gets honest. Because Coots Paradise is not fixed. It is recovering. And those are two very different things. The marsh's water clarity has improved from under 30 centimetres to 52. But the goal set by the remedial action plan is over 100 centimetres. The marsh is still less than halfway there. Total phosphorus has dropped dramatically, but at 85 micrograms per litre, it remains nearly three times higher than the target of 30. The reason is straightforward and deeply frustrating. The fishway can keep carp out, but it cannot stop what flows in through the 17 tributary streams, and what flows in is sewage. Coutts Paradise still receives both treated and untreated wastewater from the city of Hamilton. Combined sewer overflows send raw sewage into Chidoke Creek and Westdale Creek during heavy rainstorms. In 2025, the city completed two overflow capture projects to reduce the worst spills. But the system is old, the city is growing, and the storms are getting heavier. Upstream development in Ancaster-Dundas, Waterdown and Burlington continues to add impervious surface to the watershed, increasing runoff volumes and nutrient loads. The streams that feed the marsh carry the signature of every parking lot and subdivision and farm field they cross. The consequences are visible in the data. Coots Paradise still records near zero dissolved oxygen levels in summer, creating conditions that favor pollution-tolerant species. In 2025, young-of-year fish surveys found that goldfish, another invasive species likely descended from released pets, made up 65% of the juvenile fish in Coots Paradise. Native sunfish dominated in nearby Grindstone Marsh, where the last known major sewage discharge ended in 2011. The contrast is stark. Grindstone Marsh shows what is possible when you address both the carp and the water quality. Coots Paradise shows what happens when you solve one problem, but not the other. There is also the question of forces that no fishway can control. Lake Ontario is a regulated lake, and its water levels are managed for shipping, hydropower, and flood control downstream. In 2017 and 2019, record high water on the lake pushed waves and carp back into the marsh, temporarily undoing years of progress. The fishway cannot stop a lake. When water rises above the barrier, carp swim over it. Staff responded by removing the fish gates during low water events in 2007 and 2012, herding carp out of the marsh before conditions changed. But these are reactive measures, not permanent solutions. Hamilton Harbour itself remains a designated area of concern under the Great Lakes Water Quality Agreement, one of just a handful of Canadian sites that have not yet been delisted. The marsh's recovery is a prerequisite for delisting. And delisting is a prerequisite for proving that the decades-long remedial action plan has worked. The stakes extend far beyond one wetland. Cootes Paradise is a test case for whether degraded Great Lakes coastal ecosystems can be meaningfully restored without relocating every source of pollution in their watersheds. And the honest answer is that the marsh has Rush has exceeded expectations, but the watershed has not kept pace. Keith Rogers, who coordinated the Hamilton Harbor Remedial Action Plan during the early years of Project Paradise, said the marsh would never return to pristine conditions. The goal was a reasonably balanced system. Len Simser, one of the original project managers, stressed that the real indicators of success were not the return of individual species, but their population levels and distribution. He said that 15 to 20 black terns spread out evenly across the marsh in discrete territories would be a better sign than a single bird spotted once. That distinction matters. A bald eagle nest is a triumph. But a self-sustaining population of bottom-dwelling native fish in water clean enough to support them is the real benchmark. The marsh is not there yet, and there is a tension that extends beyond the ecology. Hamilton is a working city. Its steel mills and factories employed generations of families. The harbor was the economic engine that made Hamilton what it is. The people who built the mills and the railways and the subdivisions were not villains. They were workers building livelihoods in a system that did not account for the cost of what it destroyed. Many of them lived in neighborhoods just blocks from the water. They swam in the harbor as children. Their parents swam in it before them. And then one generation watched the water turn brown and the signs go up warning them to stay out. And they accepted it because the mills paid the bills. Now the remedial action plan asks the city to invest hundreds of millions of dollars in infrastructure upgrades, sewer separations, and stormwater management to fix what the old economy broke. Those costs fall on taxpayers. Property taxes in Hamilton have risen steadily. Some residents, particularly those in the working class neighborhoods closest to the harbor, wonder why a marsh matters more than roads or housing or transit. That question deserves a real answer, not a dismissal. The answer is that Coutts Paradise is not just a nature reserve. It is municipal infrastructure that works for free. It filters It absorbs floodwaters. It sequesters carbon. It processes nutrients that would otherwise accelerate algal blooms in Hamilton Harbour and Lake Ontario. It produces millions of fish that support recreational fishing economies across the Western Lake. It provides habitat for migratory birds that draw birders and tourists and researchers who spend money in Hamilton. A 2022 review estimated that the marsh and surrounding lands within the Coutts to Escarpment EcoPark system protect nearly 2,200 hectares of ecological services that would cost vastly more to replicate with concrete and steel. The marsh is not an indulgence. It is an investment that compounds. Now come back to the image we started with. A 600 hectare marsh, once given up for dead, covered in green from shore to shore, submerged plants visible through water that a generation ago was opaque mud. Bald eaglets in a nest that no one believed would be built. Native fish outnumbering invasive species at a barrier that was designed as a temporary measure and has now operated for nearly 30 years. White sucker, a fish most people have never thought about, becoming the most common species at the fishway because the habitat finally supports it. Nobody built that recovery. Nobody engineered the cascade of water clarity, leading to plant growth, leading to sediment stabilization, Leading to more clarity, leading to more plants, leading to habitat, leading to fish, leading to eagles. No computer model predicted that American lotus would flower in a marsh that had been bare mud within living memory. No engineer designed the moment when a biologist wading into the shallows could see the bottom for the first time in her career. All anyone did was install a steel grate across a canal and spend three decades patiently sorting fish, one basket at a time. The marsh did the rest. That is not a coincidence. That is balance. Reasserting itself through a system that humans had thrown out of alignment for over a century. What Cootes Paradise teaches is something that most environmental policy still has not absorbed. You do not have to rebuild an ecosystem from scratch. You do not have to engineer every outcome. You do not have to replant every species. In most cases you just have to identify the one thing that is preventing the system from healing itself, and remove it. At Coots Paradise, that one thing was a fish. Not pollution. Not development. Not climate change. A fish. Remove the carp, and the water clears. Clear the water, and the plants return. Bring back the plants, and everything else follows. The models were not wrong about the direction. They were wrong about the scale. Nobody predicted that a single grate across a canal would trigger a recovery visible from space. And here is the part that rarely gets mentioned. This is happening in other places too. At Delta Marsh in Manitoba, one of the largest freshwater wetlands on the continent, Ducks Unlimited Canada, installed carp exclusion screens in 2013. Within six years, submerged vegetation in the eastern marsh increased sevenfold. Diving duck densities rose 339%. Canvasback daily use jumped from roughly 200 birds to 3,300. The pattern is the same. Remove the carp. Watch the system rebuild. The screens cost a fraction of what a single water treatment upgrade would run. The recoveries are enormous. Dr. Lauren Bortolotti and research biologist Paige Kowal at Ducks Unlimited Canada documented these results over years of careful monitoring. The data was clear. Carp exclusion works. It works fast. It works at scale. And most people have never heard of any of it. So here is what I want to know from you. Did you know that a steel grate and a team of biologists with baskets could trigger the largest coastal wetland recovery on Lake Ontario? Did you know that a marsh three kilometers from a steel city could produce 20 million fish in a good year? Did you know that bald eagles came back to a place where they had not nested in decades, not because anyone bred them or relocated them, but because the water cleared, and the fish returned, and the habitat rebuilt itself from the bottom up? Tell me in the comments which part of this story surprised you most. If you are not subscribed yet, now is the time. We cover stories about restoration, and the people who are fixing what is broken. Hit subscribe. See you in the next one.