Reproductive Parameters of the Western Tubenose Goby (Proterorhinus semilunaris) from Žitný Island, Slovakia, in Connection with Its Invasive Potential

Three populations of the western tubenose goby were analysed during the reproductive season of 2024 in the area of Žitný Island, Slovakia. Specimens were processed in the laboratory, where meristic and reproductive parameters were detected. Populations were female-dominated and all in good condition. Males showed a decrease in condition, probably due to different roles during reproduction. Females were characterised by increased values of the absolute (114–3997; mean 1299 oocytes) and relative number of oocytes (114–3206; mean 1225 oocytes), gonadosomatic index (3.02–93.27%; mean 19.49), and oocyte size (0.10–1.93 mm) compared to other native and/or invasive areas of occurrence. Comparing populations from 2024 with 2015, we found that females from the present study have smaller body sizes, higher values of absolute and relative number of oocytes, but lower values of gonadosomatic index and larger sizes of oocytes. This would suggest a shift to a more generalistic strategy based on a hypothesis derived from the theory of alternative ontogenies and invasive potential. Although the western tubenose goby is regarded as a native species in Slovakia, its reproductive parameters suggest a shift to more opportunistic behaviour. This pattern is consistent with its invasiveness in other non-native regions, such as parts of Europe and the Laurentian Great Lakes.