Fishermen are reporting catches they never expected to find in Michigan, a sign that toothy, tropical piranha relatives have come to North America.

State officials announced Tuesday that the recently discovered pacu piranhas are fairly harmless vegetarians, but they illustrate the concerning trend of former tropical pet owners dumping non-native species into US lakes and rivers.

State officials in Michigan note that three pacu piranhas were found in two different lakes inside a week, Mindy Weisberger reported for Live Science. They won't survive long in Michigan, but Lake Erie and Lake St. Claire, for example, have healthy populations of goldfish, all because pet owners keep dumping unwanted fish into the lakes.

"Pacus' temperature requirements are tropical, and Michigan is not a tropical state," Nick Popoff, a biologist and manager of Aquatic Species and Regulatory Affairs Unit for Michigan, told Live Science. "They're not going to be able to survive our winters, so we don't consider them invasive. We're concerned with this because it highlights the issue of pet owners releasing their pets into the wild."

The problem is more common than one might think. In the last two years, piranhas have surfaced in California, Arkansas, and even New York City.

The red-bellied pacus are not actually as dangerous as their appearance and name would suggest. Although the jaws of the non-native fish are reminiscent of their more violent cousins in the Amazon, the pacus are vegetarians. They generally use their square, human-like white teeth to crush nuts, fruit, and other plants. If growing fish are kept in a small tank, however, they may begin to attack the smaller aquarium-dwellers, Michigan Live reported.

They may pose no threat to humans, but they still don't belong in the Lake Michigan, and dumping a non-native species into US waters without a permit is illegal. Officials are already working to keep out another non-native fish – the swiftly moving Asian carp – out of the Great Lakes, and they worry about other seemingly harmless fish creating another intractable aquatic problem.

The Michigan Department of Natural Resources do not technically consider the pacus invasive because they cannot actually adapt to the lake. Although erstwhile pet owners may think they are doing the fish a favor by setting them free in the lake, in reality the tropical fish do not survive the harsh Midwestern winter.

"Pets released from confined, artificial environments are poorly equipped to fend off predators and may be unable to successfully forage for food or find shelter,” Mr. Popoff said in a press release. “Those that do succeed in the wild can spread exotic diseases to native animals. In the worst-case scenario, released animals can thrive and reproduce, upsetting natural ecosystems to the degree that these former pets become invasive species.”

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If pets grow to large for a small, home aquarium, pet owners can trade them with hobbyists, donate to a zoo or aquarium, or even try to return unmanageable fish to a pet shop.

"Invasive species are extremely damaging – to Michigan, the Great Lakes, even globally. But the message here is more about individual responsibility," Mr. Popoff told Live Science. "Releasing that fish into the wild – you're killing it, even if you don't think you are."