Toby Davison said he initially thought the fish to be a piranha, but his guess was proven wrong by research biologists at the Department of Natural Resources, The Detroit News reported Tuesday.
"I knew it was abnormal as soon as I saw it," Davison said of the 4-pound, 18-inch catch. "That's easily the most interesting thing to come out of these waters since I've been fishing. And I've been at this since I'm 4."
The pacu, unlike the piranha, is more prone to munching on vegetation then meat.
Officials said the fish was likely released into the lake by a former owner.
"Typically, they are released into the waters by people who have them in their aquariums at home," said Michael Thomas, a fisheries biologist with the DNR in Harrison Township, Mich. "They realize that the fish are getting too big for them to keep, but they don't want to do away with them. So they let them go in the lake."