Fish communities in the coastal Baltic Sea arecurrently monitored using passive fishing gears, such asgillnets. In recent years, eDNA approaches have gainedpopularity for fish detection thanks to their non-invasivenature, faster processing, and more precise identification,although such methods have been underutilized incomprehensive fish community assessments in the Baltic Sea.This study reports fish diversity using eDNA metabarcodingwithin different depth profiles for the first time intemperature-stratified coastal bays in the Baltic Sea, whilealso offering some comparison with traditional net-basedapproaches. Comparing samples above and below thethermocline revealed exclusivity in fish species at bothdepths, emphasizing the importance of vertical sampling incapturing a comprehensive understanding of fish distributionpatterns in such systems. Results indicated that eDNAcaptured more fish taxa per sample compared to gillnetsampling, with similar or higher fish diversity, althoughvariations occurred between bays. This study highlights theimportance of incorporating eDNA metabarcoding, alongsidetraditional survey methods, to aid assessment of fishcommunities in aquatic environments.

Decades of overfishing and unsustainable management, together with habitat degradation and eutrophication, depleted the cod (Gadus morhua) stocks in the Baltic Sea. Accompanying severe oxygen deficiency and decreased salinity in their spawning grounds restricted successful spawning today to the Bornholm basin, resulting in decreased recruitment. In order for the species to recover, several different measures are required and proposed, among others restocking. We therefore investigated the possibility of producing laboratory-reared cod larvae acclimatized to the current environment in the Baltic Sea. For this, cod were reared from newly fertilized eggs to non-feeding yolk-sac larvae, testing the effect of different salinity reduction treatments during early development on mortality, hatching success, and neutral buoyancy. The results show that a sudden ambient salinity decrease after hatching has no strong effect on survival or hatching (around 60% and 95%, respectively), while it decreased neutral buoyancy of larvae from 18 to minimum 12.5 psu. Lowest buoyancy was reached in treatments with a salinity change in the early egg stage. Gradual salinity decrease starting early in the egg stage yielded to significantly increased mortality and reduced hatching success, but also lowest buoyancy of 12 psu. We showed that a decrease of ambient salinity enables the production of yolk-sac cod larvae with reduced buoyancy, which are potentially better acclimatized to survive in current environmental conditions in the Baltic Sea.