DICKINSON, N.D. (AP) - A woman fishing in Patterson Lake near here thought she caught a "nice-sized bluegill," until she and her husband saw the fish's big teeth.

Pam Zastoupil found out her catch was a pacu, a species of South American freshwater fish that is a cousin of the piranha.

"It's a popular aquarium fish that grows fast and large," says Emil Berard, the southwest fisheries district supervisor for the North Dakota Game and Fish Department. "It obviously outgrew somebody's aquarium."

Zastoupil said she was fishing for walleyes when she caught the unusual fish on a floating jighead tipped with a minnow last Sunday.

"My husband and I had been fishing for two hours or more, and I thought I'd throw it out one more time," she said. "All of a sudden it felt like a bullhead bite. It was bouncing a bit and then my line took off to the right. It came to the surface and I thought it was a nice-sized bluegill.

"It came off the hook when we got it in the net," Zastoupil said. "We thought it was kind of strange."

After the fish was dumped into a bucket, she and her husband saw its teeth.

"I thought it was a really weird fish. My husband looked at it and said 'Only you,"' Zastoupil said with a laugh.

The Zastoupils took the fish to the Dickinson office of the Game and Fish Department, where Berard identified it as a pacu. It weighed 3 pounds, 4 ounces and was 16 inches long.

In June, a woman caught what she thought was a piranha at the Casselton Reservoir. Biologists believe that fish, about 4 inches long, also came from someone's aquarium.

"It's got to be almost a broken record this summer," state fisheries chief Greg Power said. "It is a cousin to a piranha. The one caught at Casselton earlier was a lot smaller. This one you might worry about if you were swimming."

Pacu are known as vegetarians, and use their jaws primarily used for cutting vegetation and eating seeds and nuts, officials say. They are tropical fish not expected to survive North Dakota winters, and biologists say dumping them in North Dakota lakes is illegal.

"It's always been illegal," Berard said. "We need to protect the resources we have. Some of these undesirables will reproduce and compete with the fish we are managing for."

Zastoupil, a teacher at Dickinson's Roosevelt Elementary school, said she plans to mount her fish and take it to "show and tell" time at school.