R. Eschbaum

Invasion of round goby to the temperate salmonid streams in the Baltic Sea

Round goby Neogobius melanostomus, initially originating from the Ponto-Caspian region, has become one of the most successful invasive species in the Northern Hemisphere. During the last three decades, this invasive fish has established numerous populations both in Eurasia and North America (reviewed in Kornis et al. 2012). In Europe, this species has invaded lakes, large rivers and also various marine environments. In Northern America, round goby has started to spread from lakes to several estuaries, marshes and smaller tributaries (Poos et al. 2010; Kornis et al. 2012). High tolerance to various environmental factors, short generation time, opportunistic diet preferences and aggressive behaviour allows this species to colonize different biotopes (Corkum et al. 2004; Johnson et al. 2005; Brown and Stepien 2008; Kornis et al. 2014). Rapid invasion of round goby has proved to have impact on native fish populations in several invaded regions. Declines in abundance of some benthic fishes as stone loach Barbatula barbatula, bullhead Cottus gobio and white-finned gudgeon Romanogobio albipinnatus have been suggested in European freshwaters (Jurajda et al. 2005). In the Baltic Sea, the expansion of round goby has been suggested to affect flounder Platichthys flesus, eelpout Zoarces viviparus and native gobies (Balážová-L’avrincı́ková and Kovác 2007; Karlson et al. 2007). In North America, several percid and cottid species compete with round goby for food, territory and spawning grounds (Jude et al. 1995; Janssen and Jude 2001). Round goby invasion to the Baltic Sea was confirmed in 1990, when several fish were caught from Puck Bay in Poland (Skóra and Stolarski 1993). In the Estonian coastal sea, round goby was found in 2002, from Pärnu Bay in the Gulf of Riga (Shpilev and Ojaveer 2003). However, first rapidly growing population occurred during the mid-2000s in Muuga Bay near Tallinn, in the southern coast of the Gulf of Finland (Ojaveer 2006). During the 2010s, round goby dispersed widely along the southern coast of Gulf of Finland and became particularly abundant in several small bays east of Tallinn. Since 2011, the round gobies were found from six salmonid spawning streams along the North Estonian coast (Fig. 1). Fishes were caught during the national annual salmonid parr density survey, using backpack electrofishing gear. Study sites (two or three per stream) were located in rapid sections situated on the lower reaches of the streams which are accessible to anadromous salmonids. All sites were fished twice and the interval between sampling runs was at least 30 minutes. The abundance of salmonids and all other fish species were recorded, all caught fish were measured to the nearest mm (total length), kept in water buckets and released afterwards (except round gobies). Based on the maximum summer water temperatures, streams were classified as cool (13.1–17.0 C; three locations) or temperate (17.1–21.0 C; three locations) (Järvekülg 2001). The length of the streams varied between 25 and 85 km, catchment areas between 84–479 km and mean discharges in lower reaches between 0.5–4.0 m/s (Loopmann 1979) [Electronic Supplementary Material (ESM) Table S1]. All the studied sites were located in freshwater, and distances along a stream from the sea varied from 0.5 to 2.6 km. In three streams, where round Electronic supplementary material The online version of this article (doi:10.1007/s10228-016-0537-4) contains supplementary material, which is available to authorized users.