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

Rangers in Boulder County last month detected teeming shoals 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.