A first for the Baltic Sea: reared cod juvenile released
After months of care at BalticWaters Fish Laboratory, they are ready. Around 500 juvenile cod are now leaving the safety of the laboratory for their new home in the Baltic Sea. Join us as the project ReCod carries out a historic release and takes another step towards developing methods that could give the threatened Baltic Sea cod a better chance of recovery.
From the laboratory to the depths of the sea
With careful, methodical movements, Johnnie Stjernberg, animal caretaker within ReCod, transfers the juvenile cod from the laboratory tanks to the transport containers that will carry them to the boat waiting in the harbour. Today, around 500 juvenile cod will move to their new home: the Baltic Sea.
“The earliest stages of a cod’s life are highly sensitive, making it difficult for the species to recover in the wild. Survival rates are low, and the journey to adulthood is long. But these juveniles have had a really good start,” says Johnnie as he inspects the small cod.
Johnnie (in the background) and Daniel (in front) prepare the juveniles for transport. Photo: Frida Tornberg
The boat Arne. Photo: Frida Tornberg
Once on board, there is a sense of anticipation. The project ReCod has previously released newly hatched yolk-sac larvae, but releasing juvenile cod measuring around two centimetres is entirely new. Vincent Neron, ReCod project manager, has been waiting a long time for this year’s releases.
“We have succeeded in rearing Baltic Sea cod from larvae to juveniles, which is a major advance in itself. But the goal is not simply to release fish. It is also to build knowledge and develop methods that can support future restoration efforts,” says Vincent.
“This is the first of three releases this year, so today feels especially special,” he continues, looking out across the bay.
The boat approaches the northern reaches of Tvären Bay, where the juvenile cod will soon encounter the sea for the first time. The release site was selected with great care. The sheltered bay offers favourable conditions, with suitable salinity, oxygen levels and temperature. It also has an abundance of plankton, an important food source for the young cod during their first period at sea.
Once the boat is anchored, preparations for the release begin. Rather than releasing the juveniles directly at the surface, where threats such as predators are greater, they are transferred to a specially designed release unit. This container is lowered to the required depth and opened underwater.
The release unit. Photo: Frida Tornberg
The juveniles in transport containers. Photo: Frida Tornberg
“Today, we will release the juveniles at a depth of around 20 metres, where both the temperature and salinity closely match what the juveniles need,” says Vincent.
A crane slowly lifts the release unit containing the juvenile cod over the side of the boat and lowers it into the depths of the sea.
After a short while, small bubbles suddenly rise towards the surface. The crew erupts in cheers. At a depth of 20 metres, a hatch has just opened, and hundreds of juvenile cod are now swimming out into the Baltic Sea.
The release unit is lowered into the depths of the sea. Photo: Frida Tornberg
Vincent and Johnnie, delighted by the successful release. Photo: Frida Tornberg
The juveniles’ future
It will be around a year before the first research fishing surveys can provide answers about how the juveniles have fared at sea. Because they have been marked with strontium, a substance that creates a marker in the fish’s otolith, Vincent hopes to monitor the release effectively.
“If we recapture cod next autumn, we will be able to analyse the fish’s otolith and determine whether it is one of the project’s cod,” says Vincent.
Until then, work at the laboratory continues. The adult cod, the parents of the juveniles, will remain in the laboratory tanks for a while longer before being released back into the sea. In just seven months, they have grown considerably and will soon be moved to larger tanks.
“It will be great to see them swimming in their larger tanks, continuing to grow and, hopefully, producing even more fertilised eggs for next season. For now, our focus is on continuing to create the best possible environment for the parent fish,” says Johnnie.
Today’s release is an important step forward in efforts to strengthen the threatened Baltic Sea cod stock. Hundreds of small cod have now been given the chance to begin their lives in the Baltic Sea, while the project team takes another step towards developing methods that could contribute to the recovery of Baltic Sea cod.
Would you like to help bring the cod back?
ReCod’s work is made possible by people who care about the sea and Baltic Sea cod. By donating to BalticWaters, you can support research and rearing efforts and help more juvenile cod be released into the Baltic Sea. Read more about how you can support the sea here, or make a donation via Swish to 9007790. Thank you!
