Assessment of the Lake Kartal (Danube Delta, Ukraine) Fish Assemblage Using Invasive and Non-invasive Methods

The aim of this study was to describe the fish assemblage of Lake Kartal, a Natural Protected Area in the Ukrainian Danube delta, using both invasive (multimesh gillnets) and non-invasive (eDNA metabarcoding) methods. This is the first ichthyological investigation in Ukraine to use non-invasive eDNA analysis. Gillnet sampling took place at four localities between March and August in 2019 to 2021, while water was sampled for eDNA at 15 localities in October2022 and May 2023, the samples being preserved and sent to a laboratory for further analysis. Twenty-nine fish species were registered using gillnet sampling, and twenty-eight fish taxa and two species complexes using eDNA. While eDNA confirmed four taxa absent in gillnet samples, 13 species recorded using gillnets were not detected using eDNA. Seven taxa (21% of total fish fauna) were non-indigenous, which could prove critical for the Natural Protected Area. Although species composition determined by the two methods was relatively similar, use of eDNA made it possible to estimate species composition and relative frequency with higher accuracy. Parallel studies using eDNA and standard fishing tools could help identify a greater number of species; however, such studies would be complicated, especially in lakes overgrown with macrophytes or in Natural Protected Areas.