In Defense of Goldfish

Scientists say exotic species may help spread a gruesome amphibian virus, but pesticides should take some blame, too.

July 02, 2015

Photo: James Demetrie/Flickr

“Goldfish in garden ponds may be causing major problems for frogs,” wrote the Tech Times. Over at the Independent, the headline read, “Goldfish helping to kill off frogs by spreading ‘devastating disease,’ scientists warn.”

Egad, you may be thinking, those bubble-cheeked monsters have got to go! Light your torches and grab your pitchforks pool skimmers!

But are pet shop goldfish really going to help wipe amphibians off the face of the earth?

That conclusion might be a bit dramatic, says Alexandra North, lead author of the attention-grabbing research.

In the study, North and her coauthors sought to better understand ranavirosis, a particularly nasty disease brought on by a whole genus of amphibian-attacking viruses. 

When a frog becomes infected with Ranavirus, a few things can happen—none of them good. Some of the amphibians develop ulcers on their skin. Over time, those ulcers can turn into open sores so big that the frog’s limbs actually start to break down. Hands disintegrate. Feet turn to stumps. In extreme cases, systemic hemorrhaging causes bleeding from the mouth and anus. These end stages are what I call the “Red Death.”

A frog infected with Ranavirus. Image courtesy of Amber Griffiths.

In some ponds, mortality rates exceed 90 percent.

“Ranavirus is actually an entire group of closely related viruses, and they occur globally,” says herpetologist Joseph Mendelson, the director of research at Zoo Atlanta.

Mendelson says you can think of ranavirus like a typical infectious disease—sometimes outbreaks occur and lead to large numbers of localized deaths, and sometimes they peter out. (This, of course, differs from the well-known amphibian killer, chytrid fungus, which has been documented to bring entire populations down to zero.)

Scientists with the U.S. Geological Survey have fingered ranavirus for die-offs in over 25 states, involving more than 20 animal species, including salamanders, chorus frogs, and box turtles.

As bad as it’s been in the States, Ranavirus has hit the United Kingdom even harder. Since the 1980s, some species, like the common frog (Rana temporaria), have had infected populations decline by over 80 percent.

Ranavirus isn’t picky about its hosts, and it often jumps between species. Frogs, toads, and newts can all infect each other. “The fact that we know Ranavirus is an infector of whole communities certainly makes things more complicated,” says North.

Photo: H. Krisp/Wikimedia CommonsEuropean common frog, Rana temporaria

Worse still, the virus is perfectly happy chilling for long periods without a host, hanging out in water and sediment until an unsuspecting tadpole swims along.

That the pathogen can survive so long on its own makes it especially easy for humans to unwittingly give it a lift into new ecosystems. When gardening pals swap bulbs, minnows, or even just a bit of mud, they can accidentally bring the Red Death into their backyards.

And this is where the death-dealing goldfish come in.

In her study, North looked at two decades worth of citizen science data on frog deaths in the United Kingdom to see if they could figure out how to better predict when and where the Red Death would strike. They found that the virus was more common in urban areas where Brits do a lot of gardening, and in places that had toads (likely because toads make good hosts, too).

The media, however, grabbed hold of this tidbit: a relationship between the presence of Ranavirus and fish—any fish, mind you. And because 91 percent of the ponds in North’s data held goldfish, this bumbly, doe-eyed exotic has born the brunt of the blame. (Koi, orfe, and tench were some other species implicated, by the way.)

 As for the actual conclusions of the study, North says her research shows that’s there’s something going on between fish and Ranavirus but we don’t know what yet.  

But to be safe, she says, cutting down on the number of exotic species introduced into ponds would help protect native amphibians from disease. After all, it was the pet trade in bullfrogs and goldfish that may have brought the virus to the United Kingdom in the first place.

Photo: Pearl Pirie/Flickr

Another, possibly more effective way to ward off Ranavirus infections would be to curb the use of herbicides and pesticides, says coauthor Amber Griffiths, the director of FoAM Kernow, a nonprofit “cultural laboratory” in the United Kingdom.  

Exposure to atrazine, a widely used weed-killer, has already been proven to make tiger salamanders more susceptible to Ranavirus. Scientists have also shown that pesticides such as DDT can trigger immunosuppression in leopard frogs. North and Griffiths found evidence to support a pesticide connection as well, with a correlation between ranavirosis prevalence and herbicides and slug pellets. These poisons may not bring the ranaviruses to town, but they might be making its amphibian residents more vulnerable to them.

“Pesticides and herbicides are designed to kill,” says Griffiths. “We shouldn't really be surprised that they are harmful to wildlife.”

Many more factors may be at play here, such as temperature, pond depth, and how species, native and exotic, interact with each other. But until we know more, Griffiths says, it’s important to make small changes where we can.

“The picture is pretty grim overall,” she says. “Anthropogenic change is devastating amphibians through climate change, habitat degradation, poisoning the environment, and spreading disease.”

The good news is you don’t have to live in the rainforest to make amphibian-friendly choices. Treat your backyards and gardens like wild places and do away with synthetic chemicals, be they slug pellets or weed-killers. We probably don’t need to annihilate goldfish to save frogs, but it would certainly help to refrain from releasing or nurturing exotic species, and to go native whenever possible.  

Besides, you don’t want to be the house in the neighborhood who helped accelerate the sixth mass extinction because you thought goldfish would be fun companions for your lawn gnomes. Seriously, don’t be that guy. 

Photo: Jason Jacobs/Flickr

onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.

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