ABC News

Police are hunting down thieves on the streets of San Francisco in a skyrocketing black market: stolen bicycles.

"It's a huge business," Officer Matt Friedman, the head of the bike thief unit, said. "It's this revolving door of bikes that get ripped off, end up in flea markets, end up on Craigslist, and back out on the street again. And they seem to hold their value quite well when they get to that third stage of being resold on Craigslist for almost top dollar again."

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The city has seen a 70 percent spike in bike thefts since 2006.

The boom in bicycle commuting nationwide - up 60 percent in 10 years - has turned expensive bicycles into easy targets.

"I have arrested almost everyone here, and there is a lot more, too," Friedman said as he showed ABC News the mug shots of bike thieves.

Friedman has turned catching them into a kind of sport, posting mug shots on social media like trophies and even baiting suspects.

"[The] Cannondale [bicycle] gets stolen over and over again," he said. "It's worth about $1,500."

He said another bike - a Motobecane Le Champion mountain bike worth about $1,600 - is also stolen quite a bit.

Both the Cannondale and Motobecane bikes are part of a fleet of "bait bikes" that are wired with GPS trackers and then left around town by police, for thieves to steal and then get caught.

To protect your own bike, Friedman suggested staying away from cable locks and opting for U-locks. Small U-locks are better than the larger ones, he said, because the bigger they are, the easier they are to break.

"If people don't take the right precaution, it's only going to get easier," Friedman said. "You really need to know the types of locks you're supposed to use and how to use them.

"Bike theft, again, is about time," he added. "So the more time it takes for someone to get, a thief to get a lock off, the less they are going to be interested in your bike and go on to something else."