Tvären Bay: From stardust to the next generation of cod

At first glance, Tvären looks like many other bays along the Sörmland coast. Between the islands and skerries, a tranquil archipelago landscape stretches out, with no clear signs of the dramatic event that once shaped the place.

Yet Tvären is far from an ordinary bay – the circular inlet was, in fact, created by a meteorite impact. At the same time, it is more than just a geological phenomenon. Today, Tvären has become an important site for research and, not least, for BalticWaters’ work to strengthen the Baltic Sea’s threatened cod stock.

When the meteorite fell over the Trosa archipelago

Tvären is one of several Swedish impact craters that probably originated from the same event in the asteroid belt between Mars and Jupiter. Around 470 million years ago, a large asteroid collided with another celestial body there and shattered into millions of fragments, causing a sharp increase in the number of impacts on Earth over the following million years.

One of these fragments eventually reached what is now the Trosa archipelago. Around 460 million years ago, a meteorite struck the shallow sea that then covered the area. The meteorite is thought to have been around 100 metres in diameter and was travelling through space at a speed of nearly 20 kilometres per second.

The force of the impact was enormous. In just a few moments, the seabed was reshaped as the bedrock was hurled away from the impact site whilst the surrounding ground was compressed. The newly formed crater changed rapidly as material from the sides collapsed and filled in parts of the depression. When the process was over, nothing remained of the meteorite itself, which had completely vaporised. All that remained was a 2-kilometre-wide crater.

For a long time, it was a well-founded hypothesis that Tvären was formed by a meteorite impact. However, through drilling and geological surveys of the area, researchers have been able to confirm that the bay’s distinctive shape is indeed the result of an ancient collision between the Earth and a meteorite.

From crater to coastal landscape

Since the meteorite impact, ice ages have come and gone. The landscape has changed and the land has gradually risen out of the sea, forming the inlet that is now known as Tvären.

Even today, the meteorite impact event shapes the bay’s topography. Whilst many inlets along the Sörmland coast are 30–40 metres deep, Tvären reaches depths of 75–80 metres in its central parts. At the same time, the deep basin is separated from the surrounding sea by shallower threshold areas that limit water exchange.

This is precisely what makes Tvären special. The deep basin and the limited water exchange create conditions that in many ways resemble those of the Baltic Sea as a whole. Differences in temperature, salinity and oxygen levels build up between different water layers, and in the deepest parts, oxygen depletion occurs periodically. At the same time, Tvären’s varied conditions create a mosaic of habitats. Shallow areas near the shore, deep basins and transition zones between different seabed types provide a home for many different species and ecological processes.

That is precisely why Tvären is sometimes described as a ‘miniature Baltic Sea’. Within a relatively small area, many of the environmental conditions that characterise the sea as a whole can be found, making the bay a valuable site for research.

A new chapter for Baltic cod

Right next to Tvären, in Studsvik, lies the BalticWaters Fish Laboratory. Over the past year, the ReCod project has been developing methods for rearing cod here. The aim is to increase the number of juvenile cod in the sea through restocking, as a step towards strengthening the unique eastern cod stock.

The project has previously succeeded in rearing cod larvae, which were then released into the sea. However, an important milestone was reached in 2026. For the first time, ReCod succeeded in rearing the larvae to fry measuring around two centimetres in length. This is a significant step forward, as larger fry generally have a better chance of surviving the transition from a laboratory environment to life in the sea.

Tvären was not chosen as a release site by chance. For one thing, the bay is a known cod habitat where large individuals have been observed in the past. It is also located close to the laboratory where the juvenile cod are reared, reducing transport time and minimising stress on the fish prior to release.

But it is also Tvären’s unique characteristics that make the site particularly interesting. ReCod’s project manager, Vincent Néron, explains:

“The ancient meteorite crater forms a clearly defined and sheltered inlet with great depths and limited water exchange with the surrounding sea. This means that the juvenile cod are not expected to disperse into the open Baltic as quickly as they would if released in a more exposed coastal area,” he says.

For the researchers, this means better opportunities to monitor the fish’s development and, later on, to link recaptured individuals back to the project.

However, Tvären's depth makes the bay interesting for other reasons too. From the warmer surface waters to the colder depths, there is a greater range of temperatures here than in many other parts of the Baltic Sea.

“We expect the juvenile cod to seek out cooler waters and move through the water column in search of suitable conditions,” says Vincent.

The bay also provides access to the food resources the juveniles need during their first months of life.

“We have observed strong primary production in the bay, including phytoplankton and zooplankton, similar to the food we provide for the juveniles in the laboratory. This should help the juveniles and support their survival after release,” he says.

From impact to recovery

Over hundreds of millions of years, Tvären has been shaped by geological processes, the forces of the sea, and the gradual return of life following the meteorite impact. Today, the bay is both a natural heritage site and a place where new knowledge about the Baltic Sea is being developed.

It is therefore no coincidence that ReCod has chosen Tvären as a release site. The geological conditions created by the impact have made the bay a valuable environment for research into the future of Baltic cod.

From a rock hurtling through space to the release of juvenile cod into the Baltic Sea, Tvären tells a story of change, adaptation and recovery. And right now, the next chapter of that story is being written.