Greedy fish fills gap in the market

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BY Marion de Boo | PHOTOGRAPHY Maurits Giesen

November 2018

Why do one percent of nature’s newcomers grow in numbers until they are a plague? Biologists study the resilience of the ecosystem in the face of exotic species.

As a result of globalization, plants and animals are ending up outside their original habitats, sometimes with disastrous consequences. Like the box tree moth or the Louisiana crayfish that has driven out indigenous crayfish. The original Asian tiger mosquito can transmit as many as 10 diseases, including the Zika virus and Dengue fever. New diseases and pests are continually appearing on the scene in agriculture and horticulture, from avian flu to the tomato leafminer moth. Water hyacinths are taking over ditches and ponds, nature areas are overgrown with giant hogweed and Japanese knotweed.

Research on the invasion of the round goby. VIDEO Marijn Flipse and Helena Ligthert

The success of all these ‘exotics’ depends not only on their own competitive advantage, but also on the resilience of the host ecosystem. And that is what Wageningen biologists Leo Nagelkerke and Bart Pollux are studying in collaboration with colleagues from WUR and several other universities. They chose the round goby, a fish from the Caspian and Black Seas, as a model species for their research on biological invasions.

Worldwide advance

“The round goby is spreading all around the world, especially since the Main-Danube canal was opened in Germany in 1992, connecting the watersheds of the Rhine and the Danube,” says Nagelkerke.

“The fish reached the Netherlands in 2004, under its own steam. Thanks to its aggressive behaviour, it is supplanting indigenous species such as the gudgeon and the bullhead. Round gobies snatch away their food – crabs, mussels and other small riverbed animals – and take over their spawning grounds. They build nests and defend them forcefully, and they take good care of their young. Smaller specimens sometimes play the cuckoo and secretly inseminate the females in someone else’s nest.”

Biologist Leo Nagelkerke is researching a round goby.

From left to right: A round goby in a sink with a tweezers and callipers for taking morphological measurements; measuring mouth size to study maximum prey size; probing the gills.

Of all the plants and animals that are transported around the world, whether by accident or design, about 10 percent thrive. Just one percent become a pest. Nagelkerke: “What makes that one percent so successful? Up to now, people sought the explanation in the characteristics of the newcomer themselves. But we think it also has something to do with the characteristics of the host ecosystem. It could vary in the degree of immunity and resilience. We are researching the interaction between the exotic species and the ecosystem.”

‘Thanks to its aggressive behaviour, the round goby is supplanting indigenous species such as the gudgeon and the bullhead’

The round goby looks for food on rocky riverbeds and spawns there too. These kinds of rocks are not found naturally in the Netherlands but thanks to water management measures such as the insertion of basalt blocks and stone breakwaters, we have made the ecosystem more attractive for this invasive exotic. Nagelkerke: “If you get to know more about those kinds of connections, you can make an ecosystem more resistant to invasive exotics, for example by removing basalt from rivers wherever possible.”

The strange thing is that in the course of their global advance, the round gobies adapt their physical characteristics – and probably their physiology too – to new circumstances. Nagelkerke: “Examples would be salt levels, new types of food or different natural enemies. It might be that great adaptation capacity that makes it particularly successful. We want to study whether this might be a case of super-fast microevolution.”


Leo Nagelkerke PhD


Assistant professor in Aquaculture and Fisheries at Wageningen University & Research, fish ecologist and functional morphologist

Resilience research

Research on the resilience of the ecosystem when exotics arrive


Leo van Nagelkerke collaborates on this research with a team of scientists from Wageningen University & Research, the University of Amsterdam, Radboud University and the Netherlands Institute for Ecology, who are specialists in the fields of aquaculture, marine zoology and ecology

Specimens from all over Europe

Round gobies like a diet of small shrimps and mussels. They have teeth in their mouths and a second set of jaws in their throats. If the available food includes more mussels, for example, the fish needs more powerful jaws than it does when the menu is mainly made up of mosquito larvae. Through their extensive international network, the researchers are now trying to collect specimens from all over Europe so as to make a closer study of the relation between the type of jaw and the food available in the habitat in question.

‘It might be that great adaptation capacity that makes it particularly successful. We want to study whether this might be a case of super-fast microevolution’

A successful exotic probably fills a “gap in the market”. In a healthy, diverse fish community, groups of fish species all play their own roles. There are slug-eaters and plankton-eaters, mosquito larvae-eaters, plant-eaters and fish-eaters. If there is a gap somewhere in the food web, which the invasive species fits, then its food supply is secured.

Passed the peak

Nagelkerke: “Of the 60 to 70 fish species in the Netherlands, only about 40 are indigenous. The others are successful newcomers such as the zander and the asp. A newcomer doesn’t necessarily cause problems, and this too depends on the resilience of the ecosystem. It seems as though the round goby has passed its peak, anyway. More and more specimens with skin diseases are being seen.”

Leo Nagelkerke: “A newcomer doesn’t necessarily cause problems, and this too depends on the resilience of the ecosystem.”

The researchers want to make a kind of damage scale which indicates how resilient an ecosystem is. The better the risks exotic species bring with them can be estimated, the better policymakers can ensure that steps to keep them out are taken in the right places.

It is also possible to ban the sale of certain exotics at garden centres. The study should also produce guidelines for setting up ecosystems in ways that make them resilient, giving exotics less of a chance. The ministry of Infrastructure and Water Management is already making sure there is more dead wood in the river system so as to promote biodiversity, and it has had superfluous rocks removed.

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