Quagga and zebra mussels, Eurasian watermilfoil and mysis shrimp are among the rapidly reproducing aquatic invaders that keep Montana’s natural resource managers up at night.

In the Flathead Valley, add goldfish to that list.

Mark Deleray, a fisheries manager with Fish, Wildlife and Parks, said well-meaning pet owners can often complicate the already difficult job of keeping native fisheries healthy.

“People have them as pets, and then they move or need to dispose of them and are attached to the animals and don’t want to kill them,” Deleray said. “And that can create a problem.”

Goldfish are one of three common aquarium dwellers — the others are koi and rosy-red minnows — that can survive harsh Montana winters. Goldfish also can quickly reproduce and begin depleting food sources by gobbling up aquatic invertebrates and native minnows.

According to the U.S. Geological Survey’s “Nonindigenous Aquatic Species” database, goldfish are native to Eastern Asia, but reports of their presence in the wild have occurred in all 50 states.

Kenny Breidinger, a state fisheries biologist, is investigating potential infestations of goldfish at Foy’s Lake near Kalispell and Riverside Park pond in Whitefish.

At Foy’s Lake last week, he netted several goldfish in three distinct age classes, a major indicator that they may be reproducing. State officials have caught goldfish — close relatives of the carp family — from Foy’s Lake that measured up to 6 inches and observed others at least 10 inches long.

“If conditions are right, they can reach really high densities and out-compete other fish,” Breidinger said, adding that because they are bottom feeders, goldfish potentially could affect water quality by increasing turbidity — stirring up sediment and reducing the water’s clarity.

Within the next couple of weeks he’s planning a trip to the Riverside Park pond, from which a reported goldfish population could potentially access the larger aquatic system via the Whitefish River.

When an infestation is confirmed, Breidinger said the protocol is to learn as much as possible about the population, including size, habitat, age classes, whether it is reproducing and the potential for it to spill over into other bodies of water. However, management actions are limited and not necessarily feasible.

“In the simpler, smaller systems, it’s easier and there’s more options for us,” he said.

Deleray added, “What we’d look at is, is there an Achilles’ heel, are they spawning in one place and can we attack the population there and remove them?”

Such removals usually rely on nets or electrofishing, which stimulates the muscular system of fish and compels them to swim toward the source of the electric current. In other cases, a toxicant might be used, eradicating all the fish in the system, but, according to Deleray, leaving organisms lower in the food chain relatively unscathed.

In the 1990s that approach was used on a goldfish population that had invaded a pond with access to the Swan River.

Breidinger and Deleray are still weighing their options and continuing to collect data, but both agreed the most effective management strategy is increasing awareness.

“It’s a different motive than people doing it with sportfish, who are looking to establish sport fisheries,” Breidinger said. “But anytime you introduce a new fish, it can come at the expense of an existing fishery,”

Anyone with information on an illegal fish introduction can give an anonymous report to Fish, Wildlife and Park by calling 1-800-TIP-MONT. The agency offers awards of up to $5,400.

Reporter Samuel Wilson can be reached at 758-4407 or by email at swilson@dailyinterlake.com.