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, a starker word than the "decline" or "pressure" that usually fronts these papers. Langangen, Kaartvedt, Hylland and Durant had spent years assembling the long-run numbers for coastal cod in the Oslofjord in one place, and their conclusion was blunt: the population is down 80 to 90 percent from historical levels. Their biomass proxy peaked in 2004 and had fallen below 20 percent of that peak by 2018. Registered catch went from around 150 tonnes in 2000 to around five tonnes in 2024.

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 gather them.

What the fjord used to be

The Norwegian word for cod is torsk. It turns up in place names, on coats of arms, and on drying racks along the whole coast. In the Oslofjord, torsk was not a distant offshore species you needed a trawler to reach. People caught cod from the quayside, from piers, and from rowboats in the inner harbour. Norway's first marine research station was built at Drøbak in 1894, and Johan Hjort ran it from 1897 to 1900. The site was chosen because the inner fjord was rich enough that scientists travelled there 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, biologists at Havforskningsinstituttet tracked a steady drop in juvenile recruitment. Fewer young cod survived each year, fewer adults reached breeding age, and the older generation died off without replacement.

In 2019 the Norwegian government imposed a total ban on cod fishing in the Oslofjord and along the Skagerrak coast, with no quotas and no seasonal exemptions. If you hook one by accident, you are legally required to release it immediately. The ban still stands.

It was not enough. An environmental DNA study published in Scientific Reports in 2024 detected 63 fish species in water sampled near Drøbak between March 2022 and January 2024, which sounds healthy until you separate diversity from abundance. The species are still on the list, but the numbers behind them have gone.

Then came the escalations. From 1 October 2025, recreational fishing in the fjord was restricted to hand-held gear: rod, handline or similar, nothing set and left. On 1 January 2026 a new Oslofjord fishing regulation took effect and runs to 31 December 2035. It keeps cod fishing banned across the whole fjord and adds three zero-fishing zones, in the inner fjord, around Færder and around Hvaler, where all fishing and harvesting is prohibited.

Five problems at once

Overfishing was one of five things killing the cod simultaneously, which is why the fishing ban alone cannot fix it. Each problem makes the others worse.

Overfishing

Decades of commercial bottom trawling tore up habitat along the fjord floor while removing breeding-age fish faster than they could reproduce, and recreational fishing added pressure on top. Lobster shows what that does over a long enough run: Havforskningsinstituttet's catch-per-effort series from Flødevigen recorded a 92 percent decline between 1928 and 2019. That series covers the Skagerrak coast rather than the Oslofjord specifically, and the fjord sits inside it. When the latest lobster restrictions were announced, fishers rushed out to pull as many pots as possible before the deadline.

Sewage and runoff

1.6 million people live in the 26 coastal municipalities around the Oslofjord. Their sewage, along with agricultural runoff and stormwater, feeds into the water continuously. During heavy rain the combined sewers overflow. Oslo alone has around 200 such overflow points, where diluted, untreated wastewater goes straight into rivers and on into the fjord. The Norwegian term is overløp, and it happens more often than most Oslo residents realize. The excess nitrogen and phosphorus feed algae blooms that block light on the way down and consume oxygen when they decompose on the bottom.

Warming water

Atlantic cod is a cold-water fish, and the Oslofjord sits near the southern edge of its range, which is the part of a distribution that gives way first. Havforskningsinstituttet's climate status for the North Sea and Skagerrak recorded above-normal surface temperatures in every month of 2025, with July the warmest relative to normal. Warm surface water pushes cod deeper, and the deep basins of the inner fjord are frequently too low in oxygen to live in. Marine biologists call the result a habitat squeeze. The livable zone keeps shrinking from both ends.

Habitat loss

Kelp forests and eelgrass meadows are the nurseries of the fjord. Juvenile cod, juvenile lobster and dozens of other species depend on them for shelter and food through their first months. NIVA's 2025 status report puts kelp forest cover down 21 percent and finds 67 percent of the eelgrass meadows showing signs of weakening. Much of the lost sugar kelp has been replaced by a brown filamentous algae Norwegians call lurv, which translates roughly as "shaggy mess". Lose the nurseries and recruitment stops, and recruitment is the part of the problem a fishing ban cannot reach.

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 basins behind it, Vestfjorden at 164 meters and Bunnefjorden at 154, hold stagnant, oxygen-poor water for months at a time. Braarud and Ruud documented hydrogen sulphide in the Bunnefjord deep water as early as 1937, and those dead zones never went away. Open water would have currents to flush them. Here the sill works like a dam, and what sinks behind it mostly stays.

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 shore crabs have exploded in the shallows, and both work through the blue mussels. A 2020 study in Frontiers in Marine Science put stones carrying newly settled mussel spat into twelve outdoor tanks: with shore crabs and goldsinny wrasse both present, the mussels were gone within five hours.

Blue mussels are filter feeders that clean the water column. Fewer mussels means murkier water, less light reaching the kelp, and fewer hiding places for juvenile cod, with no step in that sequence pushing back the other way.

Then there are the newcomers. The Pacific oyster turned up in Norway in 1980 off Kragerø, was recorded at several sites in the Oslofjord by 2007, and is now classed as a high ecological risk. The brush-clawed shore crab (penselkrabbe, Hemigrapsus takanoi) was first found in Norway at Tjøme in 2020 and had established itself across the Oslofjord by 2022. At some stations it now accounts for roughly 70 percent of the crabs caught, against the native shore crab.

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.

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. Snorkel in the shallows near one of the outer islands and you will see wrasse, gobies and crabs, all of them small. What you will not see is cod. Twenty years ago you probably would have. We pass over deep water where the sounder reads 80, 90, 100 meters, and I know that somewhere below us the oxygen is thinning 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 treats 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 have 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 taken out the kelp nurseries the cod depend on. If the water quality and temperature problems go unsolved, the cod will not come back regardless of how long the ban lasts.

The plan, and its problems

In 2021 the Norwegian government published Helhetlig tiltaksplan for en ren og rik Oslofjord, the Comprehensive Action Plan for a Clean and Rich Oslofjord, running to 63 technical measures, plus 19 aimed at better knowledge of the fjord. Oslo's biggest piece of plumbing had already been rebuilt by then: the Bekkelaget treatment plant expansion went into service in 2021 on an approved budget of 2.74 billion NOK, handles wastewater from more than 500,000 people, and now removes over 70 percent of the nitrogen and over 90 percent of the phosphorus passing through it. On paper the response looks serious.

The problem is pace. A progress report from November 2023 found that none of the responsible parties were on track against their own goals: not the municipalities, not the county authorities, not the state agencies. Of roughly 50 treatment plants in the catchment that need nitrogen removal, about 10 had begun planning for it. Some of that has moved since. Hokksund got nitrogen treatment in 2025 and seven municipalities are building new plants with it. But treatment plants take years to build, cutting agricultural runoff means changing practice across an entire watershed, and the warming does not pause while that happens.

The precedent

Recovery is possible, and the evidence comes from the fjord's own history. In the 1970s the inner fjord was an open sewer. Untreated wastewater flowed straight into the water, oxygen levels at the bottom dropped so low that nothing survived below a certain depth, swimming was discouraged, and the fjord smelled.

Then Norway invested in wastewater treatment. Modern plants were built, nutrient concentrations dropped, oxygen levels improved. Fish returned, and the harbour porpoises followed them back in. The fjord proved it can recover when the pressure comes off.

The difference now is scale. Building treatment plants solved the 1970s. Nothing on today's list is that self-contained: warming water, shifting species ranges, invasive arrivals, microplastics, and nutrient loads from a population that has grown since the last cleanup.

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 lies 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. Mackerel pour through every summer in schools you can see from the surface, having taken over the top of the food chain the cod left empty. The fjord is diminished and heading in the wrong direction, and it is still here.

If you visit, respect the fishing regulations. The current rules are set out in the Oslofjord fishing regulation. 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. It swallowed the fish whole and dove again, and 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. When Havforskningsinstituttet pulled the beach seine through the inner fjord in 2022, they got zero adult cod and zero juveniles across the seventeen stations inside Drøbak. You cannot rebuild a population from that with a rule about hooks.

Fishing rules and stock figures change. Lovdata carries the regulation in force and Havforskningsinstituttet publishes the survey numbers. Last reviewed: July 2026.