NEWBERG, Ore. -- Thieves stole tens of thousands of dollars worth of koi fish from a farm in Newberg, deputies said.

The thefts happened over the past two weeks and at least 75 of the expensive fish are now gone.

The fish are valued for their beautiful color and bloodline. Just one of the stolen fish is worth as much as $14,000, and today its owner is offering a reward.

"They just came in through this back door and literally cleaned out these two tanks," owner Michelle Stewart said.

Stewart is at a loss. Who would steal $100,000 of her most expensive koi?

"I don't know. Somebody who had just been out here, who knows the property very well and knows what's here," Stewart said.

Stewart said thieves first broke into her retail store two and a half weeks ago taking fish supplies, then returned days later and stole fish.

She said she'd never dreamed they'd come back again but they did when she was home in the middle of the night.

"They're very bold. That's what scares me," Stewart said.

All together, the crooks stole nearly three quarters of Stewart's best breeding stock.

They're the fish that keep the farm running year after year.

"They were 20 inches to 30 inches each and anywhere from $1,000 to the most expensive was $14,000 for one fish," Stewart said. "So it's a very traumatic impact I guess."

Raising and collecting expensive koi is a specialized market.

Only a small number of people would know how to catch, transport and care for the fish, Stewart said.

And selling them would be difficult unless the thieves already have a koi farm set up.

She wants the public to be on the lookout for her colorful fish, she said.

Be careful if someone tries to sell you koi but doesn't want you come and see where they're kept "or if they already have a large quantity of cheap big nice-looking fish that's a red flag," Stewart said.

The koi farm is offering a $5,000 reward in the case.

If you have any information, call the Yamhill County Sheriff's Department.