Underwater Oslofjord: the cod is gone, and the food web inverted

In 2022 Havforskningsinstituttet pulled the beach seine through the inner Oslofjord for the hundredth consecutive year. It is the longest continuous marine survey in Norway — started in 1919, one hundred stations, the same nets hauled the same way at the same spots. For the first time in the survey’s history, it caught zero adult cod. Zero.

Juvenile cod inside the Drøbak threshold: also zero. Outer fjord did slightly better — 82 juveniles across five stations — still below the long-term average. The cod that defined Norwegian cuisine, the fish that built an industry, is effectively gone from the fjord.

The food web did not collapse. It inverted. Different animals now run the underwater Oslofjord. Here is what they are and where you find them.

The cod collapse, briefly

80–90 percent decline since the 1970s. A combination of overfishing, warming water, loss of spawning habitat (eelgrass meadows choked by algae), and reduced oxygen in the deep basins. Commercial cod fishing was banned from the Swedish border to Telemark (Kragerø) in 2019. Further restrictions 2024–2025. The ban has not reversed the decline yet; the survey suggests the population may be below the point where recovery is automatic.

A 2025 genetics paper from the University of Oslo found something worse: the Oslofjord cod was not just any cod. It was a distinct genetic population unique to this fjord. When samples from 2009 and 2011 were analysed, researchers identified a third cod type, separate from the known coastal and Atlantic stocks. Now near total collapse. The Oslofjord cod might already be, in genetic terms, extinct.

Who took over

Mackerel as top predator

The new apex in the fjord is the mackerel (makrell, Scomber scombrus). Warm-water fast-moving pelagic that came north with climate change and filled the predatory niche cod left. A single mackerel school eats through surface herring and sprat in a way cod — which hunts cold, deep, ambush-style — never did.

This matters for everything else. Mackerel keeps small pelagic fish numbers down, which means less food for seabirds, which is part of why the seabird colonies are struggling. Mackerel also competes with adult cod for prey — making cod recovery even harder.

Shore crab explosion

The shore crab (strandkrabbe, Carcinus maenas) has spread north all the way to Finnmark with warming water. A single female spawns 150,000 to 200,000 eggs per season, up to twice a year. Cod used to eat them. No cod, no predation, and the crabs proliferate.

If you put your hand on any rock below the tide line between Hovedøya and Malmøya this summer, odds are you will encounter a shore crab within thirty seconds. They are in every tide pool, every harbour stone wall.

A newer invader: penselkrabbe (Hemigrapsus takanoi), recently established in the outer fjord. Havforskningsinstituttet now recommends not fishing shore crab in the outer Oslofjord, so the natives can compete with the invader.

Sculpins and mesopredators

Ordinary sculpin (vanlig ulke) and dwarf sculpin populations have exploded in the absence of cod. They are mesopredators — mid-sized fish that eat cod larvae. A 2023 paper in Ecology and Evolution described a cod-recovery trap: with cod gone, sculpins explode; sculpins then eat every cod larva that tries to establish. The more absent cod becomes, the harder it is for cod to come back.

Jellyfish years

Warm winters favour jellyfish. 2024 saw blue jellyfish (blåmanet) all the way from Oslofjord north to Harstad. Warmer bottom water in winter means more polyp offspring the following summer, which means more jellyfish swarms in Oslo beaches in August.

The kelp forests

Two species matter here. Stortare (Laminaria hyperborea) — the tall kelp with a thick stipe that forms the classic underwater forest. Sukkertare (Saccharina latissima) — sugar kelp, smaller, more delicate, forms lower meadows.

Stortare still functions as forest in the outer fjord. Dense tareskog between Heia and Torbjørnskjær in the Hvaler archipelago holds the classic assemblage: cod (what little remains), crabs, mussels, shrimp, anemones, wrasses, nudibranchs. If you want to see what the fjord’s underwater used to be like everywhere, that is where to dive.

Sukkertare tells a different story. NIVA’s 2025 report on the state of the Oslofjord shows kelp forest coverage reduced by 21 percent. Sørlig sukkertareskog (southern sugar-kelp forest) is now red-listed. It has been replaced in many stations by lurv — filamentous fast-growing opportunistic algae that smothers perennial habitat-formers. Driver: nitrogen and phosphorus pollution from agriculture and sewage, combined with warmer water.

“Lurv” has become a technical term. Norwegian marine scientists use it as a category. That is the scale of the shift.

Eelgrass meadows

Eelgrass (ålegress, Zostera marina) forms underwater meadows in sheltered shallow bays. They are cod nurseries — juvenile cod shelter among the blades before graduating to deeper water. They are also blue-carbon sinks, sequestering more carbon per hectare than most terrestrial forests.

NIVA’s 2025 report: 67 percent of Oslofjord eelgrass meadows show signs of weakening. Same drivers — algal overgrowth, nutrient pollution, physical damage from anchors and trawling. The most intact meadows are in Hvaler and around Færder. If you have ever anchored in Sandspollen on Hurum you have sailed over an eelgrass bed.

The lobster question

European lobster (hummer, Homarus gammarus) is still in the fjord but under heavy fishing pressure. From 1 October 2025, Norwegian rules tightened:

• Season opens 1 October (same traditional date).
• Minimum size 25 cm, maximum 32 cm. Undersize and oversize must be released.
• Maximum 5 pots per person per vessel (down from 10).
• Pots must have escape holes and openings.
• Expanded hummerfredningsområder in the inner fjord — Asker’s zone now extends to Bærum.
• Total ban on all lobster fishing and pot-use inside the protected zones.

Read the full fishing-rules guide for detail. The short version: if you are not sure what you caught or what zone you are in, release it and ask.

What about kongekrabbe?

It is not here. The red king crab (Paralithodes camtschaticus) was introduced to the Murmansk coast by Soviet scientists in the 1960s and has spread west into Finnmark and Troms. Norwegian policy tries to confine it east of 26 degrees east longitude — keep it north, keep it controlled. It has likely established local populations in Troms but remains a very long way from the Oslofjord.

If someone tells you they caught a kongekrabbe off Bygdøy, they are either confused or lying.

Marine protected areas

Two national parks with marine components: Ytre Hvaler (2009) — Norway’s first — covering Hvaler islands, Torbjørnskjær, Heia, the Hvalerrenna trench (460 metres deep). And Færder (2013), 340 km² of land and sea including the Bolærne islands and 309 red-listed species.

In the inner fjord: roughly 110 protected areas, mostly small reserves on islets. Fishing regulations are layered on top of these — the Miljødirektoratet and Fiskeridirektoratet have been consulting on expanded nullfiskeområder (zero-fishing zones) through 2024–2025, focused on the inner fjord and around the NPs.

Diving and snorkeling

Year-round possible; April–October warmest; September–March best visibility because algal blooms have cleared. Accessible entries: Sørenga in central Oslo (shore dive, surprisingly lively in autumn), Nakholmen (boat or shore), Malmøya, Steilene. The better sites are all outer-fjord: Hvaler kelp, the wreck sites around Drøbak.

Reading the water from the boat

Even without diving, the surface tells you things. Surface feeding mackerel — brief splashes, quick circular disturbances — means a pelagic pulse through that part of the fjord. Seabirds diving in a concentrated patch: a herring shoal. Big flat calm patches that look oily: often nutrient-rich upwelling. Dense green surface film: algal bloom, usually late summer.

If you want to see the new Oslofjord, its underwater side, go out in September. The water clears. Mackerel shoals have thinned. Seabirds concentrate on what food remains. From the deck of a private cruise you can read more of the fjord’s biology than any other time of year — the surface is calm enough, the light is angled enough, and the captains have six seasons of muscle memory telling them where to look.

For the fishing side — what you can legally catch, where, and what the rules protect — our fishing guide has the full breakdown. For the cod story specifically, we have a longer piece on the collapse. The Oslofjord is changing. Watching it change is part of understanding it.

Twenty years ago you couldn’t put a hook in the water at Drøbak without catching a cod. Now you can fish a full day and not see one. The 2025 genetics paper showed why it matters: the Oslofjord cod was its own genetic population, separate from the Atlantic stock.