DENVER  | Sat Apr 11, 2015 1:02am BST

A handful of goldfish dumped into a Colorado lake, evidently by a pet owner years ago, have reproduced and thousands of the non-native fish now threaten indigenous aquatic species, state wildlife officials said on Friday.

Rangers in Boulder County last month detected teeming schools of the goldfish in a semi-rural lake, Colorado Parks and Wildlife spokeswoman Jennifer Churchill said.

The pet fish, a form of carp not native to North America, now number between 3,000 and 4,000 and state biologists are reviewing the best way to remove the invasive species, she said.

"We could use electrofishing or end up draining the lake," Churchill said.

Electrofishing entails dangling a live wire from a boat into the water. An electric current emitted by the device stuns the fish and forces them to the surface where they can be netted alive.

The shock method was used to remove nearly 2,300 goldfish from a nearby lake three years ago, Churchill said.

If the invasive goldfish escape into waterways downstream, they pose a threat to native species such as channel catfish, pumpkinseed and blue gill sun fish, wildlife officials said.

"Non-native species can be devastating to native populations by causing disease outbreaks and creating competition unbalance," said Ken Kehmeier, a senior aquatic biologist with Colorado Parks and Wildlife.

The illegal introduction of non-native aquatic animals into Colorado waters is an ongoing management problem, state wildlife officials said.

In addition to unwanted pet fish, some anglers dump sport fish of their own choosing into lakes, rivers and streams.

While some Colorado waters are stocked with non-native fish, they are only done so after rigorous testing to assure the fish not upset the local ecosystem, state biologist Ben Swiggle said.

Churchill said authorities are seeking the public's help in finding out who dumped the goldfish into the Boulder lake.

Whichever method is used to remove the fish, she said, they will likely be fed to raptors at a local wildlife rehabilitation facility.