Watch Out, Here Comes the Round Goby: How One Fish Changed the Danube River

Round Goby (Neogobius_melanostomus)

As big as a pickle and about as intimidating as one, too, the round goby pictured above doesn’t look like an aggressive invader. But this little fish is naturally invasive and uses more than its meager looks to work its way up waterways, conquering native species, settling new habitat, and creating a completely new ecosystem along the way.

Understanding the invasion process of this fish may help scientists better estimate the ecological impacts that invasive species have on their new environments. The round goby is actively invading the German stretch of the Danube River and the authors of this PLOS ONE study used this opportunity to monitor the characteristics of the invasion to better understand the the process. Specifically, they measured the round goby’s population composition, physical characteristics, and feeding and sexual behavior at ten sites as it invaded a 200 kilometer stretch of the upper Danube River from 2009 to 2011. The sites were divided into ten stages of invasion, ranging from already established populations in the lower portion of the river to the ‘invasion front,’ or where they anticipated the goby would invade next.

Scientists found that the invading goby populations differed from their established counterparts when they compared sex ratio, age, size, feeding, and sexual behavior. Contrary to the researcher’s predictions, invaders were more likely to be female, even though male gobies are more exploratory. The authors suggest that a female-dominated invasion front may allow the established, primarily male population to better handle their roles in parental care and territorial defense.

What’s more, the invaders were physically larger than the established populations and appeared to use their size to establish populations in the ‘invasion front,’ rather than depending on rapid reproduction, or a “frequency in numbers” approach. The ‘invasion front’ contained a wider range of food sources, and the invading gobies’ ability to shift their diet from insects and crustaceans to mollusks may have contributed to their increased size. Overall, these results suggest that the round goby’s ability to physically and behaviorally adapt to changing habitats may play a role in invasion success.

All in all, it took the round goby two years to invade and establish populations in the Danube River study area—a rapid change, ecologically speaking. Fierce territorial defenders, 73% of the fish population is now goby in the settled study sites, leaving behind only fish and invertebrate populations able to compete with the goby for food and territory.

The impacts of these changes remain relatively unknown, but studying them may help researchers estimate impacts on ecosystems during future invasions. The Danube River is not the only habitat facing invasion, as this native of both the Black and Caspian Seas has also hitched a ride to the Great Lakes.

Citation: Brandner J, Cerwenka AF, Schliewen UK, Geist J (2013) Bigger Is Better: Characteristics of Round Gobies Forming an Invasion Front in the Danube River. PLoS ONE 8(9): e73036. doi:10.1371/journal.pone.0073036

Photo: Round goby fish by Eric Engbretson

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