In case you didn’t know, goldfish aren’t meant to survive in saltwater.

Yet that’s what James Tweedley and researchers from the Centre of Fish and Fisheries at Murdoch University discovered in the Vasse and Wonnerup estuaries, located in south-western Australia. 

They’ve been surveying the invasion of goldfish in these waterways, the results of which have been published in an article in the journal, International Aquatic Research. And well, it’s not looking good.

“If you have a goldfish at home, you’d just put it in a tank with tap water which has a salinity of zero,” Tweedley explained. “By comparison the ocean has a salinity of about 35, and we found [the goldfish] in about 17 — which is halfway between the two, but a lot more than we’d expect.”

An estuary is a body of water, next to the ocean, where freshwater from rivers and saltwater from the seas mix. Its salinity is higher than a river, but less than the ocean. 

The big problem is if goldfish use these Vasse and Wonnerup estuaries as what Tweedley calls a “saltbridge.” With these goldfish swimming down to an estuary, the concern is if these goldfish manage to use the estuary to cross into other connected rivers — just like a bridge.

If the goldfish spread, they’ll cause even more havoc than they are already.

“If they can tolerate salt for a short period of time — enough to make the swim — from one river to another, they can spread,” he explained.

If the goldfish spread, they’ll cause even more havoc than they are already. So far they’ve been accelerating algae blooms, stirring up sediment, and consuming the eggs of native fish species. 

And as you may have seen, the size and plentiful food sources of a large waterway mean they can get so big that they have no natural predators, like birds or any other species in the water. Researchers found 526 goldfish over three surveys in these estuaries.

“By the age of one, they’ll grow to 18 centimetres (7 in) in length … and they actually get to 40 centimetres (15.75 in) in length and 2 kilograms (4.41 pounds) in that river. Some of the biggest in the world,” Tweedley said.

There has been a goldfish control program in place since 2003, but when it comes to stopping it, prevention is key. That means people not flushing goldfish down the toilet, or releasing it into an open waterway.

“With any invasive species, once they’re out there, the horse has already bolted,” Tweedley explains. “You only need a couple to survive an eradication attempt before they spawn and produce more.”

If you have unwanted goldfish, you can try and return it to an aquarium. Or you can euthanise it by putting them in the freezer — it’s better than unleashing havoc on the environment.

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