RICHFIELD, Minn. - Wood Lake Nature Center in Richfield is dealing with some unwelcome visitors that have disrupted the ecosystem of the 150-acre park.

Pet goldfish have gotten into the lake, grown and multiplied rapidly, robbing native species of their food sources.

"Maybe they were releasing them to do the fish a favor, but they certainly didn’t do the ecosystem a favor," Karen Shragg, Wood Lake's longtime director, told KARE.

"They eat up everything, toad eggs, little invertebrates that are part of our whole ecosystem, things that the turtles depend on to eat. It unravels the whole ecosystem when you have an invasive species out-eat everything else."

It's not clear if the pet goldfish were dumped directly into Wood Lake or traveled there from nearby Richfield Lake via storm drainage pipes. Either way, they've had a huge impact.

Even the tiny organisms that children traditionally swept up in their nets in pond studies are missing in action.

"The kids lay on their belly and they scoop with nets and they can’t find anything, and those pond studies have been one of our most popular programs," said Shragg, who has been at the center for 26 years.

Wood Lake Nature Center is known for its 70 acres of wetlands in the heart of Richfield, a first ring suburb of Minneapolis. Bird watchers, joggers and walkers are drawn to the park's signature floating walkway. And it's a regular stop for Richfield elementary school students.

But the aquatic invaders aren't easy to see at first glance. Only when they come close to the surface does the sunlight reflect off their golden scales. The juvenile goldfish are even harder to spot, because they're dark brown or black in color.

"Goldfish just lay so many eggs, so even if we catch most of the goldfish, if you leave just one in here it’s going to lay eggs. I’m thinking they’re kind of like the buckthorn of the water," Shragg quipped, referring to the tree brought to Minnesota in the 1850s that wreaked havoc on farms and city yards alike.

All about the bass

The nature center received an aquatic invasive species grant from Hennepin County to combat the goldfish. The first line of defense, traps, haven't been very effective at catching goldfish.

One of the park's consultants, Dr. Peter Sorensen of University of Minnesota, has advised Shragg to introduce bass fingerlings, in hopes they can take a bite out of the gold invaders and their eggs.

Shragg said that seems like a natural solution that would be less risky than other proposals out there, such as partially draining the lake and the wetlands or even using fish poison.

"There are risks to everything you do," Shragg explained. "Invasive species puts you in a position where you have a lot of not-so-fun choices."

The theme of the park's annual Friends of Wood Lake fundraiser April 28 is "Something's fishy at Wood Lake." The invitations feature drawings of goldfish.