Why You Shouldn’t Dump Your Goldfish in a Pond

Releasing an invasive species can lead to catastrophic changes.

swimming goldfish

Roberto Machado Noa / Getty Images

The family goldfish is being ignored and someone has the bright idea to set it free in the neighborhood pond or flush it down the toilet. Researchers (and likely the goldfish) explain why that’s a rotten idea.

Invasive species destroy ecosystems around the world and are one of the main causes of biodiversity loss. The pet industry is to blame for about one-third of aquatic invasive species.

When people release a fish or a frog into the water, thinking it will have a better life than in a tank, the results can be catastrophic, says lead author James Dickey, who did the research during his Ph.D. work at Queen’s University, Belfast. Dickey is now at the Freie Universität Berlin and Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB).

To understand the impacts posed by pets, Dickey and his research team studied the two most commonly traded fish species in Northern Ireland: goldfish and the white cloud mountain minnow. Goldfish are one of the world's oldest domesticated fish and have since developed non-native populations globally. White cloud minnows have had little invasive impact.

Their findings, which were published in the journal NeoBiota, suggest a new method for assessing the ecological impacts of pet invaders, based on their availability, feeding rates, and behavior.

Dickey talked to Treehugger about the study, the Teenage Mutant Ninja Turtles effect, and why people release their pets in the first place.

Treehugger: What was the impetus for your study?

James Dickey: My Ph.D. was focused on developing methods of predicting and assessing the impacts of non-native species, but during this time I became fascinated by the role played by the pet trade in this. Earlier work saw me studying freshwater turtle species, and in doing so, I read a lot about the red eared slider, a North American species that has since been established on every continent except Antarctica and is now banned in Europe. One contributing reason for this was the fanfare around Teenage Mutant Ninja Turtles (in which the protagonists are red eared sliders) in the ‘90s. Of course, people buy them when they're small and cute, not realizing they live for ages and get huge, and then mistakenly think they're doing the right thing by releasing them into the wild. 

But this isn't just a turtle thing, and basically, any pet is a potential invasive species, and invasive species are major drivers globally of biodiversity loss. As part of that turtle study, I'd done a survey of pet stores in Northern Ireland to see what freshwater species were available, and capable of surviving in our climate. Goldfish and the other species featured in the study, the white cloud mountain minnow, were the two most common, and the idea for the study grew from there.

How common of a problem is it where you live that people release goldfish and maybe other pets into the wild? (I would imagine the problem is consistent in the U.S. and Canada, too?)

We've got a certain level of protection due to a northern European climate, but goldfish have been found in our waterways and I've seen non-native turtle species with my own eyes. The recent findings of Amazonian catfish in the River Kelvin in Glasgow probably aren't too concerning due to the environmental conditions being totally unsuitable for them, but they demonstrate that people are releasing pets. Crayfish plague, a type of water mold that causes 100% mortality in the native endangered white-clawed crayfish, has been found in rivers in Ireland, linked with people releasing North American crayfish into the wild. With the number of pet purchases soaring during the lockdown, it may be that this could be a bigger problem as post-Covid "normality" returns.

What are the main reasons pet owners do this?

There are many reasons for this. It could be size, with many species small at the time of purchase. Bigger animals need bigger tanks, and they can be pretty expensive. For many it's a matter of time and effort, it can be an incompatibility with other pets, it can be moving house, or moving abroad and not being able to easily take pets with you. 

Of course, these don't directly lead to releases but often finding someone to take the pets off your hands can be tricky. You can go back to the store you got them, but they're under no legal obligation to take them back. You can try and speak to aquarist groups, animal sanctuaries, or online rehoming marketplaces (back home there's a website called preloved.co.uk, and here in Germany we have eBay Kleinanzeigen) but these can lead to issues too. Ultimately people doing the research before getting a pet and knowing what they're signed up for is crucial.

How did you measure the impacts of these released species?

Our study proposes that species in the pet trade can be assessed on three main fronts: 1) availability (previous studies have shown that the more available a species is, the greater the chances of it being released), 2) resource consumption relative to native species, and 3) behavior. While goldfish are getting all the attention from our press release, we also had the white cloud mountain minnow as our second study species. The idea behind this was, we knew both species were prevalent within the pet trade and both are capable of surviving our climate, but goldfish have a non-native populations worldwide, whereas the white clouds have only established in three countries outside of their native range, with no real documented impacts.

We determined availability as the prevalence of stores in which goldfish and white cloud mountain minnows featured. We determined resource consumption using the "comparative functional response" method, which essentially looks at prey consumption over different prey densities, and has been used a lot in the past decade to highlight damaging invaders relative to similar native species.

Finally, we assessed behavior using a "novel object test" which essentially assesses the "boldness" of a species, which has been deemed to correlate to dispersal in a non-native range. So, this is a long way of saying we asked how available they are, what predatory impacts they have, and what's their risk of spreading in a non-native environment. Goldfish, with their global range and known impacts, scored highly in all three categories, whereas white clouds, while readily available, had low predatory impacts and lacked the "boldness" of the goldfish. We propose that this method could be used to highlight potentially risky pet trade species, and could inform legislation to limit the availability of such species.

People may not think of goldfish as a harmful invasive species, but what damage can they do to an ecosystem?

Excellent question. Our study showed a voracious appetite for live insect larvae, but they actually have a very broad diet and will eat other fishes, fish eggs, amphibians, and aquatic plants. Their very presence has been shown to affect the breeding behavior of newts too. The way they eat has major impacts too, with them turning up a lot of sediment that decreases the water clarity, which in turn affects sunlight getting to aquatic plants, can cause algal blooms, affects competition with other species, can hide them from predators (pretty handy when you're bright orange!) and even regulate the temperature of the water. They also seem happy in ponds, lakes, or rivers, can tolerate a broad range of temperatures, salinities, and low oxygen levels, and one study found they can travel up to 4 kilometers per day.

Why are these findings important? (And why do you find them fascinating?)

In terms of importance, the global pet trade is notoriously poorly regulated. Often species can't even be identified to their species name, or are misidentified. As well as the risk of the species being bought and sold, there is the risk of hitchhiking species and parasites being introduced to new ecosystems. As a high-profile example, there was the issue of zebra mussels, a notorious invader from the Ponto-Caspian region, being found on moss balls in the U.S. and Canada.

While historically most businesses was done in "bricks and mortar" pet stores, increasingly pets are becoming available through pet store websites and online marketplaces, the latter being especially difficult to regulate. We hope that the methods we put forward can be used to add a bit of scrutiny to the species sold, and hopefully limit the availability of risky species relative to less risky ones. More broadly, I hope the media coverage that this study is getting can increase awareness of the problem, can educate potential and current pet owners, and hopefully down the line lead to easier options for pet owners who can't look after a pet any longer.

Are there other pet species that may have similar impacts? (We have a 15-year-old African clawed frog and my husband jokes that we should release him in the neighborhood lake to give him a better life! We would never do that, but I wondered if there are other pets that people might release.)

Another excellent question, and actually, African clawed frogs have established non-native populations with impacts on biodiversity! The problems posed by species in the pet trade are incredibly dependent on the regions in which they're introduced. The likes of Florida is full of non-native reptiles and amphibians, but many of these species won't pose too big a risk to, say, Belfast. You then have interesting systems like the Gillbach in Germany, which is a thermally polluted stream, and it's full of African cichlids, South American guppies, Asian shrimp species... It's crazy, but so far the species there are pretty limited in terms of the damage they can do due to dependence on the warm water. However, adaptation is possible. 

The lionfish, a marine species, has been spread by the pet trade and has entered the Loxahatchee River in Florida, and there's evidence of it slowly adapting to much lower salinity levels than it would be expected to survive. Then you have marbled crayfish, a species that just seemed to "appear" in the German pet trade, and is capable of cloning itself, which is a pretty neat trick for an invader. I guess my point is, there's a lot we don't know about species in the trade, and while the likes of the European Union's List of Union Concern has proved relatively effective at banning the sale, import, and breeding of species (such as the red eared slider), invasion ecologists are always playing catch up. That is another reason why the method proposed by our study, one that is proactive rather than reactive, could help us get one step ahead of potential future invaders.

View Article Sources
  1. Dickey, James W. E., et al. "Threats At Home? Assessing the Potential Ecological Impacts and Risks of Commonly Traded Pet Fishes." Neobiota, vol. 73, 2022, pp. 109-136., doi:10.3897/neobiota.73.80542

  2. Dick, Jaimie T. A., et al. "Advancing Impact Prediction and Hypothesis Testing In Invasion Ecology Using A Comparative Functional Response Approach." Biological Invasions, vol. 16, no. 4, 2013, pp. 735-753., doi:10.1007/s10530-013-0550-8