The city's boom in bicycling has had an unfortunate side effect: more stolen bikes.

Reported bike thefts last year reached 4,849 -- 600 more than in 2013 and a nearly 70% increase from 2011 figures, according to NYPD statistics provided to the City Council Wednesday.

"The trend is upward" and likely the result of the surge in bicycle use here, Susan Petito, an NYPD assistant deputy commissioner, told the City Council.

The stats showed a substantial increase in reported bike thefts each of the last four years.

And people staring at an empty space where their bike once stood may as well kiss their ride goodbye for good.

"It's a low-level crime, it's a misdemeanor and it's very hard to track," said Liz Crotty, a former prosecutor and partner at Crotty Saland, a criminal defense firm that handles larceny cases. "So those two things don't lend itself to a lot of enforcement."

Growing up in Stuyvesant Town, Crotty said a pilfered bike would have to be recovered the hard way -- buy it back from the guy selling stolen merchandise on the street.

Meanwhile, new Department of Transportation stats on bike usage showed more New Yorkers are commuting on their bikes.

Last year was a new peak in ridership, with 21,112 riders on average at seven key commuter locations -- the four East River bridges, the Staten Island Ferry, 50th Street at the Hudson River Greenway, and Ninth through 12th avenues -- on a weekday between 7 a.m. and 7 p.m.

That figure is roughly three times more bicyclists than those recorded 10 years ago, when the DOT clocked 7,090 sets of wheels. (Before 2006, DOT counted bicyclists on a single weekday; since 2008, 10 counts have been taken in the peak season, and averaged, while 2014 includes automated counters).

With ridership growth comes more bike lanes. The DOT for the 2014 fiscal year put in nearly 66 miles of bike lanes, up from the 52 miles installed in fiscal year 2013, according to the mayor's recent management report. The DOT is expecting to put in 100 more miles of bike lanes from this year to 2016.

And bikes locked up outside are prey for thieves, whether it's a well-secured $1,000 bike or a $100 junker secured with a flimsy cable.

Chris Wogas, who owns a bike rental company Bike and Roll, has had his share of stolen bikes, which run around $700 each. He estimated a loss of about five to 10 bikes a month, depending on the season, and often when employees are busy or customers take a break from riding.

"We really don't report our bikes stolen unless the theft is large (i.e. a number of bikes stolen at once or from our containers overnight)," Wogas wrote in an email. "We do recover our bikes when we see them on the street or for sale in a bike shop."

The bikes also have a serial tag number, an identifier the NYPD wants for more bike owners.

Petito of the NYPD said police will register a bike's serial number or etch one into it.

"Then if the bad thing happens that the bicycle is stolen," she said, "at least it might be able to be recovered and then the owner identified."

Correction: An earlier version of this story misspelled Liz Crotty's name and the attorney's law firm.