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The recent expansion of the round goby (Neogobius melanostomus) in tributaries of the lower Danube (Bulgaria) prompted assessing potential negative effects on local fish fauna such as competition for food resources. Diet overlap between the round goby and native fishes was analyzed using samples of fishes and macroinvertebrates collected monthly for a one-year period. Significant dietary overlap between N. melanostomus and Barbus petenyi, Perca fluviatilis, Gobio gobio, Vimba vimba, and Neogobius fluviatilis was observed. Dietary overlap in the Iskar River was found to be considerable mainly in the spring months. Conversely, overlap in the Vit River was observed during summer and late fall. The main reason for the overlap noted was the preference for Chironomidae and Hydropsychidae larvae and Gammaridae. The results suggest that there is potential for competition between the round goby and native fish species inhabiting the tributaries of the Danube. Potentially harmful effects on rare, protected fish species in the area are discussed. [ABSTRACT FROM AUTHOR]

Reproductive traits are among the most important biological characteristics that facilitate or impede the establishment of non-native fish into new environments. In several tributaries of the Lower River Danube (Bulgaria), the round goby (Neogobius melanostomus Pallas, 1814) is a recent invader. Specimens from the edge of the invasion front were collected monthly (March 2017 to May 2018) from each river. The pioneer individuals displayed relatively small body sizes. First maturation of females occurs at 49 mm total length. Based on the dynamics of oocyte size and GSI, it was established that spawning begins in early spring, at a relatively low water temperature (6-7 °C). The breeding season extended from March to June. Average absolute fecundity was 162 ± 62 oocytes, while average relative fecundity was 94 oocytes/g ± 24. Female fecundity is linearly dependent on the length and weight. Both of the known male alternative reproductive tactics were observed. Body condition factor was lowest during April for both sexes. The relationship between reproductive traits and invasive potential in N. melanostomus is discussed. [ABSTRACT FROM AUTHOR]

Reproductive traits are among the most important biological characteristics that facilitate or impede the establishment of non-native fish into new environments. In several tributaries of the Lower River Danube (Bulgaria), the round goby (Neogobius melanostomus Pallas, 1814) is a recent invader. Specimens from the edge of the invasion front were collected monthly (March 2017 to May 2018) from each river. The pioneer individuals displayed relatively small body sizes. First maturation of females occurs at 49 mm total length. Based on the dynamics of oocyte size and GSI, it was established that spawning begins in early spring, at a relatively low water temperature (6-7 °C). The breeding season extended from March to June. Average absolute fecundity was 162 ± 62 oocytes, while average relative fecundity was 94 oocytes/g ± 24. Female fecundity is linearly dependent on the length and weight. Both of the known male alternative reproductive tactics were observed. Body condition factor was lowest during April for both sexes. The relationship between reproductive traits and invasive potential in N. melanostomus is discussed. [ABSTRACT FROM AUTHOR]

Fish established outside their native range often express various changes in external morphology, which may result in significant variation between individuals inhabiting different regions. In the current study we aim to explore differences in morphology among several populations of the round goby (Neogobius melanostomus), an invasive fish in Europe and North America. A total of 753 round gobies were collected from several invasive and native populations, including marine, brackish, freshwater, lake and river ecosystems belonging to the Black Sea, Baltic Sea and Great Lakes watersheds. We analysed 35 metric and meristic characters using ANOVA, discriminant analysis and allometric coefficients. Our results indicate significant differences between most of the populations in characters such as inter orbital distance, eye diameter, head width, second dorsal fin length and depth. River dwelling round gobies had elongated ventral fins compared to those from standing water bodies—a possible adaptation for upstream dispersal in flowing habitats. Most of the morphologic characters expressed positive allometry with significant differences in allometric coefficients between populations. Probably due to their recent invasive history, some of the head characters and allometric models grouped together round gobies from the Great Lakes watershed with those from a newly established population in the Danube tributaries. Similarities between geographically distant round goby populations might reflect common adaptations in external morphology during the course of invasion. [ABSTRACT FROM AUTHOR]