Humans have massively changed many river systems over the last decades. In order to improve ship passages, rivers have been straightened, deepened, and river banks reinforced. Often the natural river bed has been altered. On top of that water temperatures are rising as a result of climate change. All of those changes actually provide perfect conditions for the round goby (Neogobius melanostomus), a fish traditionally found in the lower stretches of the Danube and along the coasts of the Black Sea. Today, however, Neogobius melanostomus can be found in the headwaters of the Danube as well as in the Rhine, the Baltic Sea and even the Great Lakes in North America.
Scientists at the Technische Universität München have teamed up with the Bavarian State Collection of Zoology to investigate the strategies round gobies use to conquer new stretches of river and the impact on the local ecosystem and existing food webs. The researchers focused on the Danube River especially its headwaters where the species arrived only a few years ago probably as a stowaway in the ballast water of ships.
Founding individuals are usually particularly strong and powerful fish that are able to consume a broad range of food and thereby outperform other species. Gradually this process eliminates native fish species such as the barbel (Barbus barbus) and European chub (Squalius cephalus). Established round goby population already account for over 70 percent of the entire fish stock in some areas. The round goby is also reducing the diversity and abundance of invertebrates. In particular stoneflies, caddisflies and mayflies quickly become the preferred prey of the fish in their new habitat.
The round goby is not the only fish species advancing upstream from the Black Sea. There are currently five different goby species from the lower Danube making their way up the river. And the fish are not the first new settlers. They have already been preceded by many exotic snails, mussels and freshwater amphipods, some even hailing from other continents, e.g. Asia. Like the gobies, a number of these animals dominate the native fauna now. The goby invasion also has led to a 'novel ecosystem' in the headwater of the Danube comprising previously unknown combinations of species. This is not unique to the Danube, however. There are similar developments in the Rhine and in the Great Lakes of North America.
Jürgen Geist is one of the authors of this publication and he states: "What we are observing is a very flexible and robust network of different species that adapts itself perfectly to new environments...". "Biodiversity is declining and once the original ecosystem is lost, we can never go back."