Ecosystem restoration through restocking hatchling cod in the Baltic Sea: Overcoming critical challenges

Top predators play a crucial role in maintaining healthy and resilient ecosystems, as they regulate prey populations, stabilize food webs, and support biodiversity. In the Eastern Baltic Sea, cod (Gadus morhua) once played this role, but the stock has declined to critically low levels and shows no sign of recovery despite a ban on targeted commercial fishing. This thesis investigates whether a hatch and release approach could support future stock recovery and how this can be achieved. First, gillnets and environmental DNA were compared as tools for assessing fish communities and for identifying predators and competitors at potential release sites. Both methods detected key species such as sprat and herring, but revealed different fish communities, highlighting the importance of combining approaches for site selection. Second, reproduction of captive cod was investigated to understand variation in egg and larval characteristics throughout the spawning season. Substantial differences in traits were observed between broodstock groups of different fish sizes and across the season. Third, methods to acclimate eggs and larvae to Baltic Sea salinity conditions were tested, showing that the lowering of incubation salinity around hatching brought neutral buoyancy closer to Baltic conditions without impeding survival. This step is essential for restocking, as larvae must be able to avoid sinking into oxygen-poor deep layers. Finally, different otolith-marking techniques were evaluated to enable reliable distinction of released larvae from wild individuals, ultimately making it possible to evaluate restocking success. Both immersion in strontium enriched water, as well as low-dose alizarine complexone produced clear marks suitable for identifying released larvae, with strontium showing strong potential for large-scale use. Together, these results provide key methodological steps for an optimized cod restocking strategy, while emphasizing that improved environmental conditions remain essential for long-term recovery.