Insights into past and present status of a Balkan’s lentic fish fauna hotspot

Lake Lesser Prespa, (Southeastern Adriatic), is known for its unique fish fauna which includes seven endemic species. However, over recent decades, numerous fish species have been introduced into the lake, establishing populations and leading to natives’ decline. To understand changes over time in the taxonomic and functional composition of the lake’s fish fauna, a comprehensive assessment was conducted. Specifically, the Jaccard’s similarity index was used to estimate alterations in the fish fauna composition over seven decades (since 1950). Subsequently, compositional, and functional dissimilarities between decades were calculated along with the trait dissimilarity between species. Quantitative data from field samplings implemented in 2011 and 2022 were subsequently used to provide insights into the current structure of the fish fauna. The ecological quality of the lake was also assessed, using a two-metric fish index. Notably, introduced species such as Pseudorasbora parva, Lepomis gibbosus and Economidichthys pygmaeus have successfully established populations and become abundant. The analysis revealed functional similarities among various introduced and native fish species, suggesting potential niche overlaps posing threats to the viability of the latter. Higher compositional and functional dissimilarities were estimated between the decades 1950 and 1990. No significant changes were observed in species contribution to benthic gillnet catches between 2011 and 2022. However, a clear dominance of the newly introduced Economidichthys pygmaeus in the littoral zone was evident. These results provide a comprehensive knowledge of Lake’s Lesser Prespa fish fauna that is essential for understanding the dynamics of this ecosystem and for implementing effective conservation strategies.