The large, South American tropical fish were probably illegally dumped in the Great Lakes by pet-owners who no longer wanted to keep them in aquariums

Three large vegetarian piranhas with human-like teeth have been discovered in Michigan, amid growing concern among wildlife officials over tropical and invasive fish infiltrating the Great Lakes region.

Over the past month, two red-bellied pacu piranhas have been caught in Lake St Clair and one other in the Port Huron area in Michigan. They were probably illegally dumped by people who didn’t want to keep them in their aquariums any longer.

The tropical fish, imported from South America for home aquariums, prefers not to tear apart carcasses like other piranhas. Instead, it has square, blunt teeth that look much like human dentures, which it uses to nibble upon nuts and seeds.

The fish, which can measure more than 30in, are warm water creatures and are unlikely to survive in the colder environment of Michigan. But wildlife officials are grappling with a growing threat of an invasion of the Great Lakes by introduced fish species.

The teeth on the piaractus brachypomus. Photograph: JJ Photo

“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,” said Nick Popoff, a manager at the Michigan department of natural resources.

“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.”

Asian carp are considered the primary invasive species threat to the Great Lakes, which are a series of five large lakes that together make up the largest freshwater system in the world. Increasingly desperate measures are being taken to keep these newcomers out of the lakes – including the building of a “fish wall”.

Authorities have constructed a 7.5ft wall near Fort Wayne, Indiana, in a bid to halt the onward advance of the Asian carp. The 2-mile-long earth berm, which replaced a temporary chain-link fence, is designed to stop the carp from swimming through Indiana’s rivers into the Great Lakes ecosystem.

The wall divides a marshland area and cuts off one of the most vulnerable approach points to the Great Lakes. The US Army Corp of Engineers is currently working on a more sophisticated “lock and dam” system that would allow water to pass through but keep invasive species from swimming through.

“My dream is that one day we can tear down this berm,” said Betsy Yankowiak, director at Little River Wetlands Project, which co-owns the marsh.

Asian carp were imported to the US in the 1970s to filter pond water in fish farms in Arkansas. Flooding allowed them to escape and spread northward. The species has no natural predators in the US and can gobble up 20% of its bodyweight in plankton each day. Some Asian carp grow to more than 100lbs and now represent the vast majority of biomass in the Illinois and Mississippi rivers.

Careless release of pets and unwitting introduction via international travel has brought a number of harmful species to the US. It’s feared that climate change will help expand the range of some invasive species, which can then out-compete native animals and disrupt ecosystems.

Florida is often viewed as the frontline for invasive species in the US, with animals such as Burmese pythons, feral hogs and lion fish altering the ecology of the region.

The state is now also home to the world’s largest rodent, according to biologist Elizabeth Congdon. There are around 50 capybaras loose in northern Florida, Congdon, of Bethune-Cookman University in Daytona Beach, told a recent conference in Missouri.

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Capybaras are semiaquatic mammals from South America that look like enormous guinea pigs. Congdon said “several sightings suggest they have been breeding”, raising the possibility that they could have a destructive impact upon the environment like nutria, another type of invasive rodent that has caused riverbanks to collapse due to its digging.

Congdon told the Guardian that the animals needed to be studied further but she was confident that they could be contained.

“They aren’t a problem yet as they are confined to a forest area but they will try to eat corn or sugar cane if they get near agricultural land,” she said. “I think they give us the opportunity to study the processes of an invasion and stop it.

“They are very territorial and will spread if the group grows and then splits. The easiest thing to do would be to control the number of females, because they can have nine young in each litter.”

Congdon said capybaras are enjoyed as food in Venezuela and used to make leather products in Argentina. The animals are protected in Brazil but can be an occasional nuisance, as evidenced during the Rio Olympics where they ran amok on the golf course last week.