Population characteristics of the non-indigenous round goby, Neogobius melanostomus (Actinopterygii: Perciformes: Gobiidae), in the eastern Gulf of Finland

The round goby, Neogobius melanostomus (Pallas, 1814), is a fish of Ponto-Caspian origin that has been invading the Baltic Sea since the 1990s. Currently, it is abundant and commercially important in some areas of the sea. This species was first reported in the eastern Gulf of Finland (GoF) in 2012. Its occurrence increased thereafter, however it has remained largely unstudied in this region. The aim of this study was to investigate the population characteristics of the round goby in the eastern GoF to better understand its expansion trend and whether it will become abundant enough to be exploited by the local fishery. Fish were caught using multi-mesh gillnets (12–60 mm mesh) and a beach seine (0.5–10 mm mesh). Occurrence, density, catch per unit effort, biomass per unit effort, relative number and biomass in catches, as well as age, size and sex ratio were studied. The species regularly occurred in samplings along the southern coastline of the GoF, as well as some central areas and along the northern coast. Within 2012–2019, its occurrence in catches increased, with the highest frequency in 2015–2019 in shallow waters (<1.5 m) of Koporye Bay (70%) and in the deeper waters of Narva Bay (74%). Similarly, the highest density in the shallow waters was also observed in Koporye Bay (10.0 ind. ∙ 100 m−2), and offshore in Narva Bay. Relative abundance and biomass usually did not exceed 23%, although it reached 93% in Narva Bay. The oldest specimen was five years old. Young-of-the-year (YOY) juveniles predominated in the shallow waters (85%), while three-year-olds prevailed in deeper waters (75%). Among the fish older than two years, females were more predominant (mean ratio 3 to 1), and males were larger than females. Specimens in all life stages were found in the eastern GoF, and their abundance increased annually, suggesting that the round goby has successfully colonized this region of the Baltic Sea. However, compared to other areas of the Baltic Sea inhabited by longer-established populations, its population size is still relatively low.