Cod juveniles are tagged and prepared for life in the Baltic Sea
Just a few months ago, they were almost invisible. Tiny, transparent cod larvae drifting through the water, sustained only by the energy stored in their yolk sacs. Today, they have grown into juveniles around 2.5 centimetres long and developed several vital senses and functions needed to survive independently. As part of BalticWaters project ReCod – release of small cod in the Baltic Sea, they are now being prepared for a future life at sea. But before that, they will receive a unique chemical fingerprint, a mark that will help the project team follow the fish at the next stage of their journey.
A great deal can happen in just a couple of months. Since the cod larvae hatched earlier this summer, they have developed rapidly. Their almost transparent bodies have grown, and step by step the larvae have undergone the transformation needed to face life outside the laboratory.
They have developed jaws, fins and a functioning digestive system. The characteristic pigmentations are beginning to appear on their bodies, small flecks that will eventually make them easy to recognise as cod.
When it comes to food, the plankton cultures in the live-feed laboratory have laid out a buffet of living organisms.
“The larvae have grown well and adapted to several different types of plankton, which gives us important insights for future seasons,” says Vincent Néron, project manager for ReCod.
Exactly which diet produces the best growth remains one of the major questions. Studies of optimal feeding strategies will therefore continue next season.
A 50-day-old cod juvenile. Photo: Madeleine Kullenbo
The art of swimming
Raising cod involves far more than simply keeping them fed.
Over the summer, Vincent and the project team have carried out several experiments to better understand the development of fish larvae. One of the experiments investigates when cod larvae develop the ability to control their own buoyancy.
Early in life, fish larvae are largely carried by currents. As they develop, so do their swim bladder and sense of balance, enabling the larvae to actively control their position in the water. This transition is one of the processes the project team has examined more closely.
“By testing different salinities and different ages we examined when larvae begin to control their buoyancy, “ says Vincent.
This knowledge helps the project team determine when the juveniles are ready to leave the laboratory and face life in the Baltic Sea.
“We raised most juveniles at 12 psu, the salinity of neutral osmoregulation. At this salinity, they don’t need to drink or expend energy regulating salinity. However, they show negative buoyancy and must swim actively to maintain their position. Around day 50 and onwards, we observed that the fish started showing active buoyancy control at the lowest salinity which correspond to Tvären salinity” says Vincent.
“We want to understand as much as possible about the fish’s early development before they leave the laboratory. The more we know about their behaviour and needs at these critical stages the better the conditions we can provide,” Vincent continues.
Cod juvenile, 50 days after hatching. Photo: Madeleine Kullenbo
Developed cod juvenile ready for a life in the Baltic Sea. Photo: Madeleine Kullenbo
A chemical fingerprint
How will Vincent know whether a cod that is recaptured comes from the ReCod project? To evaluate the project and the releases of cod juveniles, Vincent is testing a new marking method this year using the element strontium.
“Before release, the juvenile fish are exposed to strontium for approximately 24 hours. Because strontium behaves much like calcium, it becomes incorporated into the fish’s growing ear stones, known as otoliths.” Vincent explains.
If a cod is recaptured later, its otoliths can be analysed to determine whether it carries traces of the mark. This makes it possible to distinguish cod from the ReCod project from wild individuals. Every marked fish that leaves the laboratory therefore carries the potential to provide new knowledge about one of the Baltic Sea most iconic species.
Vincent tags cod juveniles with strontium prior to release into the Baltic Sea. Photo: Frida Tornberg
Strontium is added to a water tank that gradually pumps the mixture over to the cod. Photo: Frida Tornberg
A tank containing some of ReCod’s juveniles. Photo: Frida Tornberg
Young cod swimming in a tank at BalticWaters Fish Laboratory. Photo: Frida Tornberg
Conditions in the Tvären Bay
Before the juveniles are ready for release, they must reach another important milestone. They need to begin moving down towards the seabed, where adult cods spend most of their lives. While Vincent waits for this next stage in their development, preparations for the release are in full swing.
Over the summer, water samples have been collected from Tvären Bay outside Nyköping, where the release is planned. By monitoring temperature, salinity and other environmental factors, Vincent aims to ensure that conditions are as favourable as possible when the young cod are eventually released. At present, conditions in the bay appear promising, with suitable temperature and salinity levels at a depth of around 20 metres.
The project team has also investigated what food will be available to the juveniles in Tvären. Once released, the cod juveniles are expected to feed on small zooplankton and benthic organisms that occur naturally in the bay. Unlike in the laboratory, however, they will have to find and catch every meal for themselves.
Multiparameter probe used to monitor water conditions. Photo: Vincent Néron
Zooplankton from Tvären, similar to the live feed provided at the laboratory. Photo: BalticWaters
Soon, it will finally be time for the first release of cod juveniles this year, and we look forward to continuing to follow their journey!