First record of round goby , Neogobius melanostomus ( Pallas , 1814 ) in the Sava River , Croatia

In June 2011, 34 specimens of round goby (Neogobius melanostomus) were caught at three different locations in the Sava River in Croatia, which is a first record of this invasive species in Danube tributaries in Croatia. Size, structure and condition of populations indicate that the species is well adjusted to its new habitat and that further range expansions are highly likely.

Round goby is a species with a broad and flexible diet (Diggins et al. 2002;Corkum et al. 2004;Copp et al. 2008;Polačik et al. 2009), aggressive behaviour, tolerance of a wide range of environmental factors, early sexual maturation (L'avrinčíkova and Kováč 2007), an ability to spawn several times a year (Kulikova and Fandeeva 1975;Moiseyeva and Rudenko 1978;Moiseyeva et al. 1983;Stammler and Corkum 2005) and male parental care that facilitates successful recruitment (Sapota 2004).Native to Azov, Black and Caspian Sea basins (Kottelat and Freyhof 2007), during the last several decades it has been rapidly spreading throughout most of the Europe (Jurajda et al. 2005;Ojaveer 2006; Czugala and Woźniczka 2010), and, probably via ballast water, even reached North America (Great Lakes) (Charlebois et al. 2001).
Compared to several other Ponto-Caspian gobies, the round goby invasion of the upper parts of Danube basin was reported relatively late (Harka and Biro 2007).In 1997, its presence was observed in Serbia around Prahovo (downstream from the Sava River) (Simonović and Nikolić 1996;Simonović et al. 1998).Several years later, its presence was reported around Vienna, Austria (Wiesner et al. 2000), while in 2001, it was found near Budapest (Guti et al. 2003).Though both latter reports are upstream from the Sava, in the light of possible discontinuous range expansion of Ponto-Caspian gobies (Jurajda et al. 2005;Harka and Biro 2007;Borcherding et al. 2011), it is unclear whether it was present in the relatively short (137 km) stretch of the Danube in the far-eastern part of Croatia, that acts as a border with Serbia, before the reported findings (Mustafić 2005;Polačik et al. 2008;Francová et al. 2011) (Appendix 1).As yet, no findings were reported in any other locality in Croatia.
Its identification was performed based on the following morphological characteristics: a large black spot on the posterior part of first dorsal fin; nape scale coverage; pelvic-disc fraenum with small rounded lobes and length less than 1/6 of width at base; first branched ray of second dorsal about as long as the penultimate ray (Kottelat and Freyhof 2007;Froese and Pauly 2011).A typical sample is shown in Figure 2.

Results and discussion
Total length (L t ) of the sampled specimens was in the range from 4.5 to 9.7 cm, while weight (W) was from 0.89 to 12.5 g.Condition factor (CF), as WL t -3 100, was 1.17.The length-weight relationship is expressed as W=aL t b (Ricker 1975), where relationship parameter values were: a=0.0058 and b=3.38 (95% confidence limits 3.283-3.477).Correlation coefficient was r 2 =0.993 (p<0.01).In comparison, b value of L-W relationship of round goby specimens from its native range in Turkey (where L t was from 6.8 to 18.4 cm) varied from 2.87 to 3.27 (Tarkan et al. 2006), indicating a high positive allometry of Croatian specimens and, indirectly, their high successfulness in adapting to new habitat.
The presence of round goby in locations up to 125 km away from the confluence of Sava and Danube, the number of specimens caught, the presence of several different size-classes and L-W relationship all suggest that round goby has established a stable and well-adapted populations in this part of the Sava River.As yet, it is uncertain how far it has spread further upstream and whether it has entered other reaches of the Sava.As the morphologically, biologically and genetically closely related species monkey goby (Neilson and Stepien 2009) has already spread as far as the middle reaches of the Kupa River (Piria et al. 2011), and given the high invasiveness of round goby (Corkum et al. 2004;Jurajda et al. 2005)

Figure 1 .
Figure 1.Location of the sampling area.

Figure 2 .
Figure 2. A typical Neogobius melanostomus specimen, with clearly visible black spot on dorsal fin, caught in the Sava River (30th June 2011, photo by T. Tomljanović).