Small hunters bring new hope for the Baltic cod

After being moved from the Bornholm Deep, the cod have now spent four months in laboratory tanks. Here they have had time to recover, feed, and adapt to the laboratory environment.

“They have really grown and become stronger - growth is around 20–30 percent,” says Vincent Néron, project manager for ReCod.

And now it has happened. The cod have begun to spawn.

The beginning of a new generation

Evidence of spawning can be seen in the egg collector, a device that gathers the floating eggs. So far, the quantities are small, but the significance is much greater. The fact that cod spawn in a laboratory environment shows that the right conditions are in place.

The right water temperature, sufficient salinity, and oxygen are crucial for successful spawning. This applies not only to the females’ eggs but also to the males’ sperm, which depend on the right water conditions to become activated. The fertilised eggs are also sensitive and require stable conditions to develop. In the laboratory’s recirculating aquaculture system (RAS), these factors can be controlled with high precision, which is essential for successful hatching.

The fertilised eggs are carefully collected and placed in special incubation units where their development is closely monitored. In these tanks, they undergo their first sensitive phase – from fertilised egg to yolk-sac larva and onward to a feeding juvenile.

From yolk sac to hunting

After 14 days at 7°C, the eggs hatch into small, almost transparent larvae. At this early stage, the larvae carry their own nutrition in a yolk sac and do not yet need to hunt to survive.

In previous years, the ReCod project released the cod already at the larval stage. But this year, the level of ambition has been raised.

“The goal is to rear the larvae further – all the way until they have developed into juveniles of around two centimeters,” Vincent explains.

This is where the real challenge begins.

“When the larva’s yolk sac is depleted, which happens just a few days after hatching, everything changes. The larvae must then start hunting and feeding on their own to survive. This transition is another critical phase in a cod’s life,” Vincent continues.

Cod larvae also undergo several rapid developmental stages during their first days of life. They develop jaws, sensory organs, and swimming ability – everything needed to detect and capture prey. At the same time, their nutritional needs change, meaning that the feed must be adapted in size, type, and timing.

“No one has fed Baltic cod larvae before, so there is no easy recipe to follow, which makes this work very unique. The goal is to find the key to successfully breeding resilient and healthy cod fry that will cope with life out in the Baltic Sea,” says Vincent.

Right now, copepods are on the menu – small zooplankton that the larvae seem to appreciate.

Small hunters under the microscope

Early signs of success are already visible. In the incubators, larvae are moving and actively hunting their first prey.

“You can see them grow day by day. It’s quite amazing,” says Vincent as he looks into one of the incubators.

Under the microscope, the development becomes even clearer – how tiny jaws form and stomachs gradually fill with food.

This is a crucial confirmation that the larvae are successfully transitioning to active feeding.

During the summer, Vincent and his team will conduct extensive studies to determine which type of feed provides the best growth, when and how often the larvae should be fed, and how the feed is best adapted to their rapid development.

The goal is to create optimal conditions for more larvae to survive and develop into strong juveniles – in a cost-effective way.

A shift in behaviour

When the cod are around 70 days old, another important change occurs: they develop demersal behaviour. This means they move from living freely in the water column to seeking the seabed – a crucial part of their natural life cycle.

It is at this stage that Vincent plans to release them into the sea.

“By rearing cod for longer than before, we hope to increase survival after release, as larger and more developed juveniles have better chances of surviving in the wild,” he explains.

If everything goes according to plan, the hope is to release cod juveniles in early August. But there’s still a long way to go. Much remains to be seen as to how the cod larvae grow and develop in the laboratory.

An intensive but hopeful summer

While many go on holiday, ReCod is entering its most intensive period. Work continues around the clock, collecting eggs, monitoring hatching and feeding larvae.

Every step is carefully monitored, every change documented.

The aim is to release the first juveniles later this summer – a concrete step towards strengthening the Baltic Sea’s cod stock.

But perhaps even more important is the knowledge gained along the way. In the laboratory tanks, it is not only cod that are being reared – methods and insights are being developed that could be crucial for future efforts to save one of the Baltic Sea’s most threatened species.