Steve Rose investigates a catastrophic pollution incident at Furnace Brook, a fishing lake in East Sussex, where thousands of fish suddenly died and the local ecosystem collapsed. Neighbours suspect pollution from nearby Morgans Farm, and Rose examines whether their concerns are justified. In doing so, he uncovers broader national problems with water pollution, illegal waste dumping, and the limited power and resources councils and regulators have to tackle them.

He headed about half a mile up the valley to a tributary joining the stream from a neighbouring property named Morgans Farm. “You could clearly see the water upstream [of the tributary] was totally normal, a sandy colour with clay silt in it, and what was below it had gone a blue-black colour. You could see dead brook trout, wild eels, and there was a dead crow or raven, which may well have eaten something poisonous from the river.” In some places, he says, there was a thick layer of blue-grey scum on the water, which he later learned is sometimes called “bacterial cheese”. “There’s so much nutrient [sic] in the water that it allows for a bacterial colonisation explosion.” The bacteria take up all the oxygen, which causes most of the fish, especially the older, larger ones, to die.

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For more than four months, Brian, a commercial fisherman in his late 40s, had been waiting for the weather conditions to be right for us to go fishing outside Bagenkop, a small harbor town on the southern tip of the Danish island of Langeland. 

“One more problem has come up,”  Brian told me over the phone in early May. “I have lost my fishing license." He was no longer allowed to sell fish for a living. Instead of preparing his boat to go out to sea and to show me how fishing in Denmark had changed over his lifetime, Brian was now removing the registration number from its hull. 

Across Denmark, coastal fishing communities are shrinking as catches decline. At the same time, warming seas, overfishing and pollution are putting increasing pressure on marine ecosystems. 

“Plenty of fish in the sea”? Not anymore, say UN experts, with 35% of the world’s marine fish stocks being fished faster than they can be biologically replaced. Today, more than half of the fish we consume worldwide comes from fish farms. 

This summer, sea temperatures along Denmark’s beaches reached 28 degrees Celsius, a record high. On September 2, the UN Environment Program warned that the world is likely to cross 1.5 C of warming above pre-industrial levels within the next few years. As the climate warms, the pressure on marine ecosystems grows.

In August, Iceland voted not to restart accession talks with the European Union, in part because of concern that it would lose sovereign control of its fishing industry. While fishing is not as major a part of Denmark’s GDP as it is Iceland’s, the EU’s impact has meant independent Danish fishermen have struggled to survive in the face of corporate competition.

For Brian, though, it’s about more than regulations. It is about watching the ocean he grew up with slowly disappear.

Nets covered in black slime

His story reflects a larger shift happening along the Danish coastline. Coastal fishermen, once a central part of the country’s history, are a vanishing breed.

For Brian, it was never about the money. Even when he was a teenager and he told his father he wanted to be a fisherman, his father said it was a bad idea. But the pull of the sea was irresistible. His life as a fisherman began as a hobby, accumulating experience and knowledge just by spending as much time as possible on water. He learned how to read the currents, the weather, and fish movements through experience rather than education. In 2019, despite the uncertainty surrounding the industry, he bought a commercial fishing boat. “When I started as a teenager, there were fish everywhere,” he said. But over the years, the numbers have dwindled starkly. Every few years, another Danish harbor became economically unsustainable. First Bogense, then Kerteminde, later Nyborg and Spodsbjerg. Each one of them followed the same pattern: fewer fish, declining landings, and growing questions about whether it was possible for small fisheries to continue their operations. 

While working on this story, I spoke to ten small-scale Danish fisheries across the country. Half of them said they do not expect to still be fishing five years from now. In 2019, they said, ten nets could bring in between 150-200 kilos of plaice on a single trip. Two years later, the same nets would return with only two kilos. As the catches became smaller, so did the possibility of keeping a commercial license. Brian no longer caught and sold enough fish to keep his permit.

One day still stands out to him as perhaps the single worst he has experienced at sea. He had three nets in the water. The first one he hauled in looked normal. The second held fewer fish than expected. Then he started pulling in the third — the net was completely empty and covered in thick, black slime. 

Marine biologist Stiig Markager told me that what Brian saw was a tell-tale sign of oxygen depletion. In 2025, around 40% of Danish marine areas were affected by low oxygen levels. Oxygen depletion happens when too many nutrients, mainly nitrogen and phosphorus from agriculture and wastewater, enter the sea. These nutrients fuel large algae blooms. 

When the algae die and sink to the seabed, bacteria begin decomposing them, consuming enormous amounts of oxygen in the process. Climate change exacerbates the problem. “Warmer water contains less oxygen. At the same time, biological processes speed up when temperatures rise.” In practice that means algae grow faster, oxygen is consumed more quickly, and deep waters struggle to recover. Together, these processes reinforce each other. “When oxygen disappears, it damages the seabed ecosystem,” Markager said.

A new global study argues that aquatic deoxygenation is approaching an “unsafe space” and should be considered a potential tenth planetary boundary. The global ocean has lost around 2% of its oxygen over the past 50 years. The consequences for marine life are catastrophic, forcing either flight or suffocation.

Marine biologists have studied oxygen depletion in the Danish seas for decades. But, Brian says, “fishers are often the first people to see the effects of that depletion, directly in their nets.” It is a first-hand experience, fishers say, that is often discounted or ignored. Many independent coastal fishers say they feel increasingly excluded from political conversations about the ocean, despite spending their working lives at sea. 

“It feels like we’re not really welcome anymore,” Brian said. He believes small-scale fisheries like his are being pushed out while decisions about the seas are increasingly made by those on land. “I think they don’t want so many people out there seeing what’s actually happening.”

For years, debates about oxygen depletion in Denmark have become deeply political. Marine scientist Stiig Markager says that the results of environmental research have repeatedly clashed with agricultural and political interests. “There is complete opposition from agriculture and from Venstre (a major center-right political party) to making research publicly available,” he said. “There is a clear political strategy from Venstre that there must be no interference with agriculture.” 

According to Markager, tensions between environmental researchers and agricultural interests intensified during the early 2000s, as scientists linked nitrogen runoff from farming to oxygen depletion in Danish waters. Despite decades of plans and political promises, Markager says, the recovery many researchers hoped for never fully arrived.

For this story, I also contacted both the Minister of Environment and the Minister for Food, Agriculture and Fisheries to ask for an interview about the state of Danish waters and the future of small-scale fishing. Neither had time to answer questions.

What remains is behind glass

One harbor that has felt the impact of the changing nature of Danish waters is Bønnerup Strand, which has transformed from a fishing town into a tourist destination. The harbor now features an ice cream shop, a hot dog kiosk, a seafood restaurant, and  a mini golf course. The iconic blue Danish fishing boats are conspicuous only by their absence. Instead, boats built for leisure, not commercial fishing, are docked at the harbor. 

Along the quay, an outdoor exhibition has been installed behind glass panels. It started last year, created by a local group of volunteers trying to preserve what Bønnerup Strand used to be. The pictures show fishing nets piled high, men in blue overalls, boats unloading their catch, and fish laid out in crates.

Movement. Noise. Purpose. It was a harbor bustling with life. Hans, in his 60s now, was once a fisherman in Bønnerup Strand. He walks past the diorama and tells me that twenty years ago “there were 60 or 70 fishing boats here, now there are two.” Some of the independent fishers chose to leave, their lives no longer making economic sense. Others lost their licenses. The rest have disappeared, not all at once, but gradually. Some left before it was too late, others had their boats seized by the bank when their revenue became too low. “Quite a lot of my colleagues had their boats taken by them," says Hans, referring to banks who repossessed boats because independent fishermen no longer made enough money to pay for them.

You could say that the gradual suffocation of the Danish seas began in 1909 with the invention of the artificial nitrogen fertilizer that transformed global agriculture. Artificial fertilizer rapidly became central to modern-day farming. By 1959, nearly 75% of farms were using fertilizer to increase crop production. At first, the growing amounts of nitrogen entering the sea did not appear harmful. During the 1960s and 70s, the extra nutrients actually increased algae and plankton growth, creating more food for some fish species. “There were actually more fish,” says Markager, the marine scientist, “but by the mid-1980s, the consequences had become impossible to ignore.”

On October 8, 1986, on Danish television fishermen in Gilleleje were seen pulling dead lobsters from the sea. The images became the subject of alarmed national conversation. For the first time, the public saw what oxygen depletion meant for life beneath the surface of the sea. 

Hans, the fisherman from Bønnerup Strand, told me he was part of the television coverage himself. “Fishers had talked about pollution long before the dead lobsters,” he said. “But suddenly people started paying attention.” The public outrage pushed Denmark to act. In 1987, the country introduced its first Water Environment Plan, aiming to reduce phosphorus emissions from wastewater by 80% and nitrogen emissions by 50% within five years. For many fishers and marine scientists, it became the first real sign that the environmental crisis in Danish waters was finally being taken seriously. During the 1990s, nitrogen and phosphorus pollution declined significantly. Water in several fjords and coastal areas became clearer, and conditions slowly began improving. For a moment, it appeared the damage could still be reversed.

But by the early 2000s, many coastal fisheries were once again experiencing declining catches and, with it, a growing uncertainty about the future. Hans was one of those affected. Coming from a family that had fished for five generations, the harbor was never a tourist destination for Hans. It was a way of life. But on a sunny Friday in April back 2005, he sailed out to Kattegat for plaice. He knew it was over. “I said to my employee: “this is the last trip. I start working on land from Monday.”” He had made the decision months earlier, as the numbers of fish dwindled and regulations tightened. 

It wasn’t an easy choice, but he had four children to think of. Climate change had taken away his livelihood. “You can’t do anything about it,” he said. “If you think too much about it, you go crazy.” Researchers describe this kind of environmental grief as solastalgia, the distress caused by watching a familiar environment change beyond recognition. Unlike homesickness, solastalgia happens while still remaining in the same place. For coastal fishers, the sea remains physically present, but the ecosystem they once knew and depended on, is disappearing.

Adapting has always been part of the job, says Hans. “There has always been a species that disappeared and then another one came.” But eventually, nothing was replacing what was lost anymore. 

In 2002, marine scientists Stiig Markager and Peter Bondo Christensen from DMU, the Danish Environmental Research Institute, published research showing that around 75% of Denmark’s nitrogen emissions originated from agriculture through waterways and atmospheric pollution. The findings intensified tensions between marine scientists, politicians, and agricultural organizations about who was responsible for the declining marine environment. 

In response to the worsening marine crisis, Denmark established DMU in 1989 to monitor oxygen depletion, nutrient pollution, and ecological changes beneath the surface. But their findings were inconvenient. By 2007, after nearly two decades as an independent institution, DMU was folded into Aarhus University as part of a wider restructuring of Denmark’s research sector. The decision sparked criticism from several environmental researchers and organizations, who feared the changes could weaken the independence of Danish environmental monitoring. 

“Agriculture succeeded in closing DMU,” former researcher Bo Normander told the media. “Nature has lost.” In 2015, the Danish government introduced the controversial Landbrugspakke, easing environmental regulations on fertilizer use in an effort to increase agricultural production. 

Critics warned that the policy could worsen nitrogen pollution in Danish waters at a time when oxygen depletion remained a recurring problem. In the years that followed, researchers from Aarhus University reported rising nitrogen emissions instead of the reductions politicians had promised. Markager argues that despite decades of environmental warnings, nitrogen pollution reductions largely stalled. While nitrogen pollution in Danish waters declined significantly after the first Danish Water Environment Plans in the 1980s and 1990s, researchers say the reductions have largely stagnated over the last 20-25 years. 

Now, Denmark is trying to reverse that trend. In the first week of September, the country passed a historic new law changing how agricultural nitrogen is regulated. The law will take effect in 2027 and represents a significant tightening of the rules introduced in 2015. 

Under the system introduced in 2015, farmers were allowed to fertilize closer to what was considered the agronomic optimum for their crops, while still operating within nationally defined nitrogen quotas and other rules. The new system moves in the opposite direction. Instead of relying primarily on broad national rules, nitrogen regulation will increasingly take the environmental sensitivity of individual areas into account. 

Where a farm is located will thus matter much more. The requirements will vary depending on factors such as whether fields drain towards vulnerable rivers or coastal waters, and whether they lie above areas important for groundwater and drinking-water protection. For farmers, this means that the amount of nitrogen they can use will become more restricted in areas where the surrounding environment is particularly vulnerable. 

For fishermen like Brian and Hans, the law represents a change they have long been asking for: an attempt to redress the damage done to Danish seas. 

But understanding how the water became polluted is more complicated than it first appears. Much of the pollution data commonly referred to in public debates focuses on point-source pollution, which emanates from identifiable sources like wastewater treatment plants, industrial discharge, and fish farms, and is generally easier to measure. But most nitrogen pollution in Danish waters actually comes from diffuse sources, mainly agricultural runoff carried from fields through rainwater, rivers, groundwater, and the atmosphere. Unlike point sources, diffuse pollution is far harder to measure directly and comes with greater uncertainty in the data. As a result, wastewater pollution can often appear to be the biggest issue, even though agriculture remains the dominant source of nitrogen input into Danish waters.

For Hans, the consequences appear almost irreversible. These days, the fish often do not return. “There are still plaice here, but they’re too thin. They don’t gain weight.” Scientists point to several overlapping causes, and the suggested solutions are just as diverse. Activists call for stricter rules, fishers say there are already too many. Farmers reject blame, and governments insist wastewater is already being purified. Plenty of fingers are being pointed, but the problem remains.

Hans doesn’t expect it to change. “I don’t think we’ll have fish out here ever again.” These days, he’s on the other side of the industry, buying catch from fisheries and selling it to customers. “People have to get used to the fact that we don’t always have everything available on demand. With a changing climate, fish shops may no longer offer cod, salmon, or plaice like we’ve been used to.” Instead, he says, squid is becoming the new normal. 

Still, while independent fisheries disappear, the demand for fish continues to grow. Instead of restoring the ocean, parts of the fishing industry are increasingly moving onto land. 

In northern Denmark, companies like Skagen Salmon are now raising fish entirely inside closed artificial systems, where the water, oxygen, and temperature are carefully controlled. Outside, Skagen Salmon looks like a factory. No windows, just a large grey box near the coastline. 

If the building had windows, the fish inside could look out at the sea. But these fish will never see, taste, or feel those waters. Instead, they are raised in artificial tanks until they reach market size. They are then slaughtered and sold to supermarkets and fish shops across the country.

https://youtu.be/XKDIe7PXMCw

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