In 2021, Neele Schmidt, a PhD student at Uppsala University, began her PhD project as part of the BalticWaters project ReCod – release of small cod in the Baltic Sea. Now, five years later, she has submitted her doctoral thesis. The thesis provides new insights into how future cod releases can be designed and which biological and methodological steps are crucial for success.
2020 startade projektet ReCod med målet att undersöka om stödutsättning av torsklarver kan öka antalet småtorskar i havet. I sin avhandling undersöker Neele om, och på vilket sätt, den metod som används inom projektet kan bidra till att stärka det utsatta östra torskbeståndet i Östersjön. Avhandlingen presenterar nya forskningsresultat inom fyra områden:
1. Mapping of predators and competition at release sites
By comparing traditional net sampling with eDNA, the study shows that the methods capture different parts of the fish fauna. Both methods identified key species such as sprat and herring, but provided complementary insights into the ecosystem. The results underscore the importance of combining methods when selecting release sites.
2. Variation in eggs and larvae during the spawning period
The study shows that egg and larval characteristics vary significantly between different broodstock and across the season. This variation needs to be taken into account to optimise which individuals are used in a release programme.
3. Adaptation to the Baltic Sea’s low salinity
One of the biggest obstacles to release is that cod larvae risk sinking into oxygen-depleted deep waters where they cannot survive. Neele’s study shows that a gradual reduction in salinity at hatching can shift the larvae’s neutral buoyancy closer to Baltic Sea conditions, without adversely affecting survival. This is an important step towards enabling future releases.
4. Tagging of released larvae
In order to track the larvae that are released, a reliable method is required to distinguish them from other larvae. The thesis shows that both strontium enrichment and low doses of alizarin complexone produce clear and stable marks in the fish’s otoliths. Strontium appears particularly promising for large-scale use.
An important contribution to future management
Together, the results provide a concrete methodology for designing a release programme for cod larvae – from the selection of site and parent fish to hatching, acclimatisation and marking. At the same time, the thesis emphasises that improved environmental conditions in the Baltic Sea are a prerequisite for restocking to contribute to long-term recovery.
Read Neele’s full thesis here.
About ReCod
ReCod – release of small cod in the Baltic Sea is one of BalticWater’s largest and longest-running projects. The work began in 2020 at the Ar research station on Gotland, where we conducted trials involving the release of 4-6 day-old cod larvae at a few selected sites along the east coast to investigate whether the larvae survive and manage to establish themselves.
In total, nearly 1,000 cod have been housed at the station, and together they have produced almost 1.5 million cod larvae between 2022 and 2024, which have been released into the Baltic Sea.
From 2025, ReCod will be run at BalticWater’s new fish research laboratory in Studsvik, outside Nyköping. Here, the cod larvae will be reared until they are 2–3 cm long fry. The aim is to identify the age and size of fish that are best equipped to survive in the wild. If the trials prove successful, there is potential to reintroduce cod to the Baltic Sea in more locations, thereby increasing the chances of preserving and protecting the unique eastern stock.
ReCod is being carried out and funded by BalticWaters.
