Tired of your goldfish? Releasing them into nature may sound like a solution, but with a view to biodiversity, it turns out not to be very convenient.

When you hear the word goldfish, you probably first think of a fishbowl, aquarium or pond. But released or escaped specimens can also survive in the wild. Unfortunately, the species that naturally occur in such an area can suffer quite a bit. Goldfish are also called ranked among the most damaging invasive fish species in the world†

But what makes these popular pets such a risk? have that James Dickey from the Leibnitz Institute for Freshwater Ecology and Inland Fisheries now in their lab examined† And from those experiments they conclude: goldfish eat quite a lot. In addition, they are not afraid of novelties, which would allow them to spread faster.

Problematic Newcomers

For their research, Dickey and colleagues purchased a stash of goldfish from an Irish pet store. They also bought specimens of another popular aquarium fish: the Chinese danio† In addition, they caught some roadsidesfish that resemble goldfish in eating and group behavior, and minnowswhich have many similarities with Chinese danios.

In their lab, the goldfish turned out to be by far the largest food bag of the four. And not only do the goldfish have an almost insatiable appetite, Dickey and colleagues write in the magazine NeoBiotaThey are also not very picky: they easily adapt their diet to other circumstances. This increases the chance that as newcomers they will pose a problem for the species that can already be found in an area.

Major risk to biodiversity

One trait that can help a species spread across a new habitat is ‘braveness’. You can get an impression of this by hanging a new object in the aquarium and counting how often fish visit that object. At that point, the goldfish beat the Chinese danios.

All in all, the researchers conclude, the brave and voracious goldfish could pose a major risk to the biodiversity of the area they land in.

cloudy water

Joep de Leeuw, senior researcher in fish ecology and inland fisheries at Wageningen University & Research, thinks Dickey and colleague’s work is “a useful addition to the knowledge of the extent to which goldfish pose a risk”. What the study cannot do, he continues, is “predict exactly what will happen for any type of spawning, in any type of water, and for different communities of fish and other animals. But that’s a utopia anyway.”

De Leeuw also says that goldfish can also cause problems in other ways. “Invasive species released into the environment through human activity can also carry pathogens. For the goldfish, for example, it is Third Cyprinid Herpes Virus, which can be deadly to other carp-like fish species. Goldfish can also be unfavorable for amphibians by eating eggs and larvae and turning the bottom up, making the water too murky.”

In short: those apparently harmless fish can do quite a bit if you throw them out of their bowls into a ditch.