## 4 Discussion  
### Compositional Changes  
In 1900, Lake Kastoria's fish fauna comprised only 6 native species; through 2020, an additional 10 species have been introduced, bringing the number of fish species in the lake up to 16. Currently, the species richness is lower after the extinction of the native _Anguilla anguilla_, while for the introduced species _Ctenopharygondon idella_, _Hypophthalmichthys molitrix_, _and Hypophthalmichthys nobilis_ there are no records after 1995. In general, during the examined period (1900-2020) two steep increases in the fish assemblage dissimilarity among decades were observed: one from 1920 to 1940 with the introduction of species such as _Eosox lucius_, _Perca fluviatilis_, and _Tinca tinca_ to enhance fisheries; and another from 1960 to 1990 when most of the introductions occurred. From 2010 to 2020, the [PERSON]'s index  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **2010** & **2022** \\ \hline
**OMNI\({}_{\text{b}}\)** & 60.69 & 46.60 \\
**Introduced\({}_{\text{a}}\)** & 51.22 & 49.04 \\
**EQR-OMNI\({}_{\text{b}}\)** & 0.32 & 0.47 \\
**EQR-Introduced\({}_{\text{a}}\)** & 0.44 & 0.45 \\
**GLFI** & 0.38 & 0.46 \\ \hline \hline \end{tabular} \(\text{OMNI}_{\text{b}}\) represents the relative biomass of omnivorous species in the benthic gillnet catches. Introduced\({}_{\text{a}}\) represents the relative abundance of introduced species in the benthic gillnet catches.  
\end{table}
Table 2: Values of the two metrics, \(\text{OMNI}_{\text{b}}\) and Introduced\({}_{\text{a}}\), estimated for the years 2010 and 2022, along with their respective Ecological Quality Ratios (EQRs) and the total value of the GLFI, indicating the ecological quality class for each year.  
Figure 6: Benthic gillnet CPUEs for total catches and per depth zone (i.e., 1: 0–2.9 m, 2: 3–5.9 m, and 3: 6–11.9 m) based on the number of specimens (NPUE, specimens/gillnet) and biomass (BPUE, g/gillnet) in Lake Kastoria. The median (horizontal straight line) and mean (\(\times\)) values are given. The numbers in parentheses show the number of gillnets that were set in each depth zone.  
values remained at the same level, indicating that the introduced species already present in the lake were adapted and had established permanent populations in the ecosystem. Considering the Jaccard dissimilarity between reference (1900) conditions and present (2020) times, is becoming obvious that the lake's fish fauna has undergone significant compositional changes that are irreversible, with the establishment of several opportunistic species including _Lepomis gibbosus, Pseudorasbora parva_, and _Carassius gibelio_[46; 47; 48; 49].  
Such introductions, along with the extinction of native species, emerge as the predominant driver behind the phenomenon of \"biotic homogenization\" in freshwater fish assemblages on a global scale [50]. This term characterizes the escalating similarity in species compositions among freshwater ecosystems over time [51; 52], leading to diminished geographical turnover and historical distinctiveness within these assemblages [53]. This trend is already evident in water bodies across the European continent [54], while taxonomic homogenization has been confirmed for numerous other taxa worldwide [55; 52].