If that lockdown goldfish is starting to lose its lustre, think twice before throwing it in the river or canal – the creatures may look innocent but their voracious appetite, tolerance for cold and have-a-go habits compared with native species can be catastrophic for local wildlife.
New research shows that goldfish consume much more than comparable fish in UK waters, eat more than other invasive fish and are also much more willing to aggressively take on other competing species.
That means goldfish pose a triple threat, according to Dr James Dickey of Queen’s University Belfast, the lead author of the study.
“Not only are they readily available, but they combine insatiable appetites with bold behaviour,” he said. “While northern European climates are often a barrier to non-native species surviving in the wild, goldfish are known to be tolerant to such conditions and could pose a real threat to native biodiversity in rivers and lakes, eating up the resources that other species depend on.”
The study was not able to gauge whether more goldfish were being released into the wild by pet owners who bought new fish during lockdown, but anecdotal reports have suggested this could be the case.
“Whilst our research didn’t focus on whether this problem has got worse since lockdown, there is reason to believe that this is, or at least will be, the case,” Dickey said.
“There have been recent news stories about released Amazonian catfish being found in Glasgow, which may be linked. It may also be that there is a time lag, and it may only be this summer, when normality kind of resumes and, for example, people want to go travelling [and leave their pets behind], that we start to see the effects.”
The study, published in the journal NeoBiota on Wednesday, examined the two most commonly traded fish species in Northern Ireland: goldfish, which is an invasive species around the world, and the white cloud mountain minnow, which has yet to establish much of an invasive foothold. Both species are members of the carp family and are native to east Asia.
The researchers established a new method for assessing and comparing the impacts of both species by looking at availability, feeding rates and behaviour. By these standards, goldfish far outcompeted the white cloud minnow, and were shown to be capable of wreaking havoc on native wildlife populations in UK ponds, rivers and streams.
Goldfish prey on tadpoles and other small fish when released into UK waterways, disrupting natural ecosystems.
In the US, goldfish have been found to grow to more than 30cm (1ft) in length in some waterways, owing to their ability to adapt.
Another way to limit the damage, according to the study, would be for pet shops to stock more of alternative species that do not pose such an invasive risk.
“Goldfish are high risk,” Dickey said. “Limiting the availability of potentially impactful [species, such as goldfish] alongside better education of pet owners is a solution to preventing damaging invaders establishing in the future.”
Flushing an unwanted fish down the toilet is also a no-no, according to Dickey. But he said some pet shops would take fish back, though not usually with refunds, and there are websites such as Preloved.co.uk where they can be given away or exchanged.