But some invasive species start out as beloved pets. In Australia, 20 million feral cats — descended from animals brought in by European settlers — have been fingered as direct threats to at least 124 of the country's threatened species. According to a new study, there's at least one other pet that could be wreaking havoc on Oz: goldfish.
Left to thrive in waterways, these gluttonous fish are growing to weigh as much as four pounds, researchers say.
"Perhaps they were kids' pets where the family have been moving house, and their parents, not wanting to take the aquarium, have dumped them in the local wetlands," study author Stephen Beatty of the school of Veterinary and Life Sciences at Perth's Murdoch University told the Australian Broadcast Corporation (ABC).
"Unfortunately, a lot of people don't understand that wetlands connect up to river systems, and introduced fish, once they get in there, can do a lot of damage to native freshwater fish and the aquatic habitat," he added.
This isn't a new problem or one unique to Australia: In 2015, the Canadian government put out a plea for pet owners to stop dumping goldfish into local waterways. They've also caused trouble in ponds and lakes across the United States.
What's so dangerous about a goldfish? As we mentioned before, they can reach monstrous sizes in the wild. Like all species of carp, the domestic goldfish, Carassius auratus, grows to be as large as its resources will allow (within reason, anyway).
And they don't just eat fish flakes, either. In the wild, goldfish are carnivorous. At best, their feeding habits — trawling along the bottom of the body of water — disrupt sediment and make it harder for other fish to eat. At worst, goldfish will fatten up on the eggs of native species. Goldfish may also be bringing new diseases to the wild fish population.
Goldfish are also surprisingly tenacious: One of the fish tracked in the new Australian study traveled 142 miles in the year researchers followed it.
So no matter where you live — and no matter how sentimental you are over your unwanted pet — don't set Nemo free when it's time to say goodbye.
"The key thing is if you've got unwanted pets, you can see if the pet shops will take them back. But if you're going to euthanize them, putting them in the freezer is the most humane way," Beatty told Mashable.
A quick death in the freezer may seem harsh, but, hey — you're the one getting rid of the fish in the first place.