Where Did All the Cod Go? The Collapse of Oslofjord Fish Stocks

In January 2026, four researchers at the University of Oslo published a paper in Frontiers in Marine Science with the word “collapse” in the title. Not “decline.” Not “pressure.” Collapse. Langangen, Kaartvedt, Hylland, and Durant had spent years building the first comprehensive quantitative analysis of coastal cod in the Oslofjord, and their conclusion was blunt: the population has dropped 80 to 90 percent from historical levels. The cod are functionally gone from waters where they once defined the ecosystem.

I read the paper the week it came out. None of the numbers surprised me. What surprised me was that it took this long for anyone to put them all in one place.

What the fjord used to be

The Norwegian word for cod is torsk. It appears in place names, in coat of arms, dried on wooden racks from Lofoten to Stavanger. In the Oslofjord, torsk was not some distant offshore species you needed a trawler to reach. People caught cod from the quayside. From piers. From rowboats in the inner harbor. When Johan Hjort established Norway’s first marine research station at Drøbak in 1897, he chose the location because the inner fjord was so biologically rich that scientists came just to study it.

Through the 1970s and 1980s, research surveys still found cod across all size classes: juveniles in the kelp forests, breeding adults in the deeper channels. Lobster traps came up full. Blue mussels covered every rock in the shallows. Dense stands of sugar kelp (Saccharina latissima) formed underwater forests that sheltered young fish and crustaceans in the thousands. The system had been working for as long as anyone had bothered to measure it.

A slow collapse, then a fast one

The decline crept in over decades, masked by the fact that the fjord still looked blue and the beaches still filled every summer. Through the 1990s and 2000s, marine biologists at the Institute of Marine Research tracked a steady drop in juvenile recruitment. Fewer young cod surviving each year. Fewer adults reaching breeding age. The older generation dying off without replacement.

By 2019, the Norwegian government imposed a total ban on cod fishing in the Oslofjord and along the Skagerrak coast. A complete prohibition on taking torsk from these waters, with no quotas or seasonal carve-outs. If you hook one by accident, you are legally required to release it immediately. The ban remains in effect today.

It did not work. Or rather, it was not enough. By 2024, even with the fishing ban in place, the population had continued to shrink. An environmental DNA study detected 63 fish species in the Oslofjord, which sounds healthy until you understand the distinction: species diversity is holding, but abundance has collapsed. The fish are technically still here. Just not in numbers that sustain anything.

Then came the 2025 and 2026 escalations. In October 2025, recreational fishing gear was restricted to hand-held equipment only: no nets, no pots, no bottom-set lines. In January 2026, three large zero-fishing zones were established in the inner fjord. A ten-year ban on all commercial fishing took effect. The government had moved from regulating the problem to trying to seal it off entirely.

Five problems at once

The reason the fishing ban alone cannot fix this is that overfishing was only one of five things killing the cod simultaneously. Each problem makes the others worse.

Overfishing

Decades of commercial bottom trawling destroyed habitat along the fjord floor while removing breeding-age fish faster than they could reproduce. Recreational fishing added pressure on top. When the lobster restrictions were announced, fishers rushed to pull as many lobsters as possible before the deadline. Norwegian media called it a fishing frenzy. The result: 92 percent of the Oslofjord’s lobster population gone.

Sewage and runoff

Approximately 1.6 million people live around the inner Oslofjord. Their sewage, along with agricultural runoff and stormwater, feeds into the fjord continuously. During heavy rain, combined sewer overflows dump raw wastewater directly into the water. Single overflow events have released up to 100 million liters of untreated sewage. The Norwegian term is overløp, and it happens more often than most Oslo residents realize. The excess nitrogen and phosphorus fuel algae blooms that block light and consume oxygen as they decompose on the bottom.

Warming water

Atlantic cod are a cold-water species. They become stressed above 16–17°C. In the summer of 2023, surface temperatures in the Oslofjord hit 22–23°C, roughly five degrees above the long-term normal. Sugar kelp begins dying at sustained temperatures above 20°C. So the cod get squeezed: the surface is too warm, and the deep water (where they retreat) is often too low in oxygen. Marine biologists call this habitat squeeze. The livable zone keeps shrinking.

Habitat loss

Kelp forests and eelgrass meadows are the nurseries of the fjord. Juvenile cod, juvenile lobsters, dozens of fish species depend on them for shelter and food during their first months of life. Since the late 1990s, the sugar kelp has been dying back, smothered by a brown filamentous algae that Norwegians call lurv, which translates roughly to “shaggy mess.” No nurseries means no recruitment. No recruitment means no recovery. The loop closes.

The Drøbak Sill

The narrow underwater ridge at Drøbak, where the fjord is only 19.5 meters deep, restricts deep-water exchange between the inner and outer fjord. The deep basins of the inner fjord (Vestfjorden at 164 meters, Bunnefjorden at 160 meters) trap stagnant, oxygen-depleted water for months at a time. Oslo harbor was first found anoxic in the 1940s. Those dead zones have not gone away. In an open body of water, currents would flush them. Here, the sill acts like a dam. Everything that sinks to the bottom stays at the bottom.

What happens when the top predator disappears

Remove cod from the food web and everything below it rearranges. Ecologists call it mesopredator release. Without cod controlling their numbers, wrasse and green crabs have exploded in the shallows. The wrasse, in turn, are devastating the blue mussel population. A single study found 450 small mussels in the stomachs of 187 wrasse. That number stuck with me.

Blue mussels are filter feeders. They clean the water column. Fewer mussels means murkier water. Murkier water means less light reaching the kelp. Less kelp means fewer hiding places for juvenile cod. The feedback loop runs in one direction, and every component makes the next one worse.

Then there are the newcomers. The Pacific oyster has established itself in the Oslofjord, thriving in the warmer water and competing with native shellfish. The Asian shore crab arrived between 2019 and 2020 and reproduces up to four times per year. These are not hypothetical arrivals from a risk assessment. They are here, breeding, expanding with every degree of warming.

What it looks like from the boat

From the surface, the Oslofjord looks fine. That is part of what makes this so difficult to talk about. When we head out from Tjuvholmen on a private cruise, the water is clear enough to swim in. The islands are green. Seabirds circle the skerries. On a calm evening, with the light coming low across Nesodden, you would never guess that beneath the hull, an ecosystem is coming apart.

But there are things you notice if you know what to look for. The rocks below the waterline have a brown, fuzzy coating. That is lurv. If someone snorkels in the shallows near one of the outer islands, they will see wrasse, gobies, crabs. Small things. What they will not see is cod. Twenty years ago, they probably would have. We pass over deep water where the depth sounder reads 80, 90, 100 meters, and I know that somewhere below us the oxygen is thinning out and the bottom is functionally lifeless. There is a particular shade of green the water turns in late summer when the algae are blooming, and every year I notice it earlier.

The fishing ban and what it misses

The fishing ban was overdue. I have no reservations about saying that. But it is treating the most visible symptom while the underlying disease runs unchecked. Commercial fishing firms have argued that pollution should be addressed first, and they have a point, even if their motives are obvious. Norwegian lawyers challenged whether the ban is legally sound. The political fight has been loud.

My honest assessment: you cannot fish your way out of a crisis driven by warming water and nutrient overload that has wiped out the kelp nurseries the cod depend on. If the water quality and temperature problems are not solved, the cod will not come back regardless of how long the ban lasts. A fish does not care about a regulatory boundary if the kelp forest it needs has been replaced by lurv.

The plan, and its problems

In 2021, the Norwegian government published the Helhetlig tiltaksplan for en ren og rik Oslofjord, which translates to “Comprehensive Action Plan for a Clean and Rich Oslofjord.” Sixty-three measures across eight focus areas. The Bekkelaget wastewater treatment plant in Oslo is getting a 1.5 billion NOK expansion. New fishing restrictions keep coming. On paper, the response looks serious.

The problem is execution. A progress report from late 2023 found that none of the responsible parties were on track to meet their own goals. Not the municipalities. Not the county governments. Not the state agencies. Everyone agreed on the diagnosis. Nobody was moving fast enough on treatment. Of approximately 50 wastewater treatment plants around the fjord, only about 10 had even begun planning for nitrogen removal. Sewage plants take years to build. Agricultural runoff reduction requires changing farming practices across an entire watershed. Climate change is not waiting.

The precedent

Recovery is possible. The evidence comes from the Oslofjord’s own history. In the 1970s, the inner fjord was an open sewer. Untreated wastewater flowed directly into the water. Oxygen levels at the bottom dropped so low that nothing survived below a certain depth. Swimming was discouraged. The fjord smelled.

Then Norway invested in wastewater treatment. Modern plants were built. Nutrient concentrations dropped. Oxygen levels improved. Fish returned. The harbour porpoises followed the fish back in. It worked. The fjord proved it can recover when the pressure is reduced.

The difference now is scale. The 1970s problem had one fix: build treatment plants. Today’s crisis involves warming water, shifting species ranges, invasive arrivals, microplastics, and nutrient loads from a population that has grown since the last cleanup. Same fjord. Harder equation.

Talking about it on the water

On a full-day tour heading south through the fjord, we pass over all of this. Guests ask about wildlife. They ask about the islands. Sometimes someone asks about the fishing ban because they noticed a sign at the marina or read something before the trip. And then we talk about it.

Being on the water changes the conversation. You are floating above the thing being discussed. The fjord is right there, looking calm and blue and normal. And underneath it, thirty years of decline. I did not plan this as part of the tour experience. It is just what is happening.

Sixty-three species

The Oslofjord is not dead. That eDNA study still picked up 63 fish species. Porpoises still hunt in the deeper channels. Seals haul out on the outer rocks. Mackerel pour through every summer in schools you can see from the surface. The fjord is diminished and heading in the wrong direction, but it is not gone.

If you visit, respect the fishing regulations. They exist for a reason and they are enforced. Do not discharge anything into the water. If you see the brown fuzz on the rocks below the waterline, you now know what it is and what it means.

Last autumn I was anchored off Nakholmen, engine off, waiting for guests to finish swimming. The water was flat. A cormorant surfaced about three meters from the hull with a fish in its beak. Too small to identify. Swallowed it whole and dove again. Efficient. The whole interaction took maybe four seconds. I sat there thinking that in a healthy fjord, that fish might have been a juvenile cod. Impossible to know. But the thought keeps coming back.

The ban came too late. The population was probably already past the point of self-recovery when it was put in place. The 2025 Havforskningsinstituttet survey found zero juvenile cod inside the sill. Zero — across the entire inner fjord, summer of the survey.