The summer of lurv

The pollution from salmon farming peaks exactly when it is most harmful to underwater forests.

Lou Luddington & Aleksander Nordahl (text and visuals)

It’s February 9, 2025 and Aleksander Nordahl is preparing to freedive with a companion. They have travelled to a fjord in the far southwest of Norway to document the seabed underwater. The plan is to swim out from the shore to where salmon hatchery waste water flows from long pipes into the fjord. After visiting in summer, Nordahl resolved to return to the same spot in winter, to create a visual time series underwater through the seasons.

Ålfjord in Vestland County, February 2025.

The air temperature is below zero as they slip into wetsuits and gather their dive gear. Sliding into the water they feel its icy sting as it reaches their exposed cheeks. Kicking with long fins, they surge through the frozen surface, cracking a seapath to deeper water and the pipes, 100 metres offshore. A few metres below them, the seabed is barren and black as a lava landscape with stark patches of bacterial mats.

Among these are humps of decomposing lurv, hair-like algae that in recent years have come to dominate Norway’s underwater habitats. They bloom in summer, then die off in winter to become food for bacteria that deplete the seabed of oxygen. The scene is littered with their decaying, lifeless tangles.

Swimming towards the rocks Nordahl notices splashes of crimson, orange and rose poking through the shrinking layer of lurv. Dahlia sea anemones dotted here and there, bloom like flowers in the sunshine, and look out of place in these gloomy surroundings. But they belong here, and are profiting from the free buffet of organic waste spilling from the pipes. Usually found below the kelp forest canopy, only a few disheveled kelp plants remain. The divers leave the water feeling a sense of loss at having witnessed an ecosystem in decline.

Ålfjord in Vestland County, February 2025.

At the beginning of August 2025, Nordahl returns. The water is cloudy with waste from the hatchery pipes and lurv grows in thick carpets, shrouding the seabed. The forlorn kelps and opportunistic sea anemones are still there but swamped by the expanding lurv. Along with the rocky reef, the kelp and stones on the seabed, Nordahl feels as though he is lost in a sea of summer lurv.

Ålfjord in Vestland County, Summer 2025.

In a new study, Sunstone Institute scientists found that nutrient pollution from salmon farms peaks in summer. This favours lurv and disfavours kelp- whereas lurv parties on summer’s profusion of fish farm nutrients, warmth and sunlight Norway’s underwater forests struggle and shrink. Based upon a perilous mismatch in biology between salmon, lurv and kelp this produces a slow but relentless battle that is being won by lurv.

Hidden in the seasons

Regulators judge a salmon farm's environmental footprint using annual averages, treating fish waste as a steady, year-round drip. But because salmon are cold-blooded, their feed intake and waste excretion rise and fall with seasonal water temperatures. Salmon growth is optimal at 10–14 °C and coincides with summer sea temperatures in Norwegian waters. This is when growth is fastest and they are fed more than double the winter amount. In 2025, feed consumption rose from 140,617 tonnes in March to a peak of 244,739 tonnes in September (Norwegian Directorate of Fisheries, 2025). This extra feeding and raised metabolism creates a spike in waste nutrient levels in summer waters—exactly when lurv thrives.

Lurv outside Stamsund, Lofoten, June 2025.

According to Sunstone’s calculations, an 80% increase has been recorded for all nutrients in just five months. When converted to the equivalent nutrients in human sewage, nitrogen from Norway’s salmon farms ballooned from a low of 12.45 million in March to a staggering 22.40 million in September in 2025. For phosphorus, 14.44 million became 25.97 million and organic carbon (uneaten feed and fish faeces) 21.40 million rose to 38.49 million.

By relying on national annual averages (equivalent to approximately 17, 19 and 29 million people for nitrogen, phosphorus and carbon) regulators miss the messy reality of summer’s overload, especially in densely farmed areas. Instead of an annual average equivalent to three times the population of Norway, the summer level shoots up to more than four times. Hidden among the averages used by authorities to monitor salmon farm waste, the summer spikes get overlooked. This results in the statistical dilution of risk in a single year and an industry that continues to grow - in 2025 salmon farming in Norway boasted the highest annual salmon production since 1980.

The biological mismatch

The overlooked seasonal ebb and flow of salmon farm waste amplifies the lurv catastrophe unfolding along Norway’s coasts. To understand how, requires a dive into the natural biological cycles that have helped shape these underwater ecosystems over millions of years.

In a healthy ecosystem, kelp show distinct seasonal patterns of growth in tune with natural environmental fluctuations. In summer, when daylight is plentiful, the focus is on making and storing sugars through photosynthesis. At this time growth is slow. In winter and early spring growth peaks, as storms churn the fjords, bringing deep-water nutrients up to the surface. Tapping into this rough weather bounty, and using carbohydrates stored during the summer, native kelps have evolved to maximise growth in winter.

By the time summer arrives and the waters calm, natural nutrient levels in the sea plummet and the kelp switches to accumulating sugar reserves for winter again. This strategy allows them to avoid natural nutrient and light limitations through the seasons.

First visit to the strait of Midsund, April 23, 2025.

Two months later in the same place, June 24, 2025.

Two months later in the same place at 6m depth, June 24, 2025.

But the salmon farm industry upends this cycle. Just as coastal waters reach their natural, nutrient-scarce summer phase, waste from open-pen farms peaks. The sudden influx of fertiliser cannot be absorbed by the native kelps. Instead it feeds the lurv; fueled by a spike in nitrogen equivalent to 22.4 million people excreting directly into the sea, these fast-growing, opportunists thrive. With unlimited nourishment and profuse sunlight their shaggy carpets spread across the seabed, overgrowing and shading out the seaweed gardens and kelp forests below. In a study of sugar kelp in southern Norway, scientists found that shading by lurv significantly slows kelp growth and increases death rate. Overgrowth also hinders reproduction by blocking the production of spores from the kelp frond surface and by smothering the seabed where young kelp might otherwise settle and grow. For kelp, lurv is bad news.

The new normal

Summer provides ideal conditions for lurv to bloom and take over- plentiful sunlight for photosynthesis and warmer temperatures and high nutrient load in the water from human sources. Industry, agricultural run-off, municipal waste water and sewage, and run-off from built up areas all contribute, but in Norway, aquaculture is the single largest source of nutrients nitrogen, phosphorus and organic carbon to the sea.

When asked about the relationship between lurv and salmon farm waste, senior scientist and kelp expert Hartvig Christie says, “It is highly likely that lurv growth in summer comes from salmon farm waste,” although he admits that it is not scientifically documented. “It is very difficult to design studies and document changes because there are no control areas left.” With the whole coast of Norway now affected by lurv, there are few places to compare it to. Throughout the world’s ocean the loss of natural baselines has led to Shifting Baseline Syndrome (SBS) which describes the gradual shifting of the accepted condition of natural environments. Each new generation of humans accepts the depleted state they grew up with as normal. Sadly it is a widely accepted global phenomenon prevalent in modern day ecology and is a symptom of sustained environmental degradation.

For years people believed it was impossible for lurv to grow in Saltstraumen, today it is common. August 2025.

The effects of climate change are already apparent in Norwegian coastal and fjord areas, leading to increased water temperatures, more frequent marine heat waves, higher snow and rainfall, more runoff from the land, lower salinity and reduced oxygen levels in the water. These changes are leading to less robust ecosystems and reduced tolerance to environmental impacts from salmon farming. Predictions show these changes intensifying into the future.

Some studies suggest the ocean is simply getting too warm for native, cold-adapted kelp to survive. This may also be true, but it is amplified by the added stress of seasonal nutrient loading and lurv overgrowth compromising kelp growth, reproduction, recovery and adaptation to a changing ocean.

The situation is made worse by kelps sensitivity to warming sea, “Kelp are stressed by high temperatures, ” Christie tells me. As baseline ocean temperatures rise, Norway's kelp become more vulnerable to the effects of nutrient pollution. Kelp forests are being lost to the creep of lurv all along the coast yet they are vital for ocean health. As keystone species they provide food, shelter and nursery areas for a rich biodiversity of life, as well as a wealth of ecosystem services including coastal protection, carbon sequestration and improving water quality.

Beyond average

Using annual averages to evaluate nutrient pollution from salmon farms underestimates the ecological pressure on Norwegian coastal ecosystems. For kelp forests the timing of pollution is critical. A concentrated pulse of nutrients in summer amplifies pressure on these underwater forests just when they are gathering stores for winter growth, and are most vulnerable to smothering by lurv. Understanding the nuances of both kelp and salmon lifecycles, highlights the importance of assessing nutrient pollution from a seasonal perspective. For authorities charged with regulation, the fluctuations revealed by monthly data give a clearer picture of ecological risk than yearly averages.

According to Christie, in some areas Norway’s underwater forests have reached a tipping point. They have been lost to a summer of lurv, where the story playing out is one of tragedy, not romance, for these ancient Arctic jungles. But it’s not too late. As with so many challenges facing the ocean today, we already have the data, knowledge and technology to turn things around. The next chapter will be written by regulators and decision makers willing to reevaluate, and respond with the urgency required.

Additional credits:

Data analyst/story design: Carter Ronald Brown

Story Editor: Ingerid Salvesen
Managing editor/creative director: Aleksander Nordahl
Publisher: Jan Grønbech

Special thanks to external experts Tom N. Pedersen (Senior advisor on aquaculture) and Hartvig C. Christie (Marine biologist and senior scientist)

Aquaculture Data

The waste beneath the waves

The summer of lurv

Mapped: Fish farms, foul waters