Environmental DNA Metabarcoding as a Tool for Fast Fish Assessment in Post-Cleanup Activities: Example from Two Urban Lakes in Zagreb, Croatia

This study evaluated the effectiveness of eDNA metabarcoding in assessing fish communities in two urban lakes (First Lake and Second Lake) in Zagreb, Croatia, following IAS removal. Water samples were collected in April and June 2024 and analyzed using MiFish primers targeting the 12S rRNA gene. The results indicated that the cleanup efforts were largely successful, as several IAS previously recorded in these lakes were not detected (Ameiurus melas, Lepomis gibbosus, and Hypophthalmichthys spp.). However, some others persisted in low relative abundances, such as grass carp (Ctenopharyngodon idella), topmouth gudgeon (Pseudorasbora parva), and prussian/crucian carp (Carassius sp.). Species composition differed between lakes, with common carp (Cyprinus carpio) dominating Maksimir First Lake, while chub (Squalius cephalus) was prevalent in Maksimir Second Lake. Unexpected eDNA signals from salmonid and exotic species suggest potential input from upstream sources, human activity, or the nearby Zoo Garden. These findings underscore the utility of eDNA metabarcoding in biodiversity monitoring and highlight the need for continuous surveillance and adaptive management strategies to ensure long-term IAS control.