"The main difference is (that) a piranha has an underbite whereas the pacu has almost more of an overbite," DWR outreach manager Scott Root said.
Pacu are traditionally vegetarians and use their teeth to cut through vegetation, Root said, but they occasionally may eat other fish.
The tropical fish probably dropped into Utah Lake by an aquarium owner who became overwhelmed when the fish, which can be purchased, started growing from 3 inches toward its fully grown size of 30 inches, Root said.
"This is the second time I've heard of (pacu) in Utah Lake," Root said. "What happens is people don't know what to do with them. They can't kill them - they don't have the heart. So they do something even worse and put them in some water somewhere."
Root said there aren't any reports of anyone being bitten.
Scott Clements, 62, of Provo caught the fish Thursday near the Provo boat harbor and turned it over to the DWR.