Shared electric scooters are disappearing from city streets in high numbers as thieves target the portable vehicles.

“In the first two weeks, we lost 200 vehicles to theft, which was just shocking,” said Michael Keating, CEO and founder of San Francisco’s Scoot Networks. “People were grabbing them all the time. We realized that if we kept losing them at that rate, we wouldn’t have any left.”

In response, Scoot scaled back its vehicle numbers and expedited its plans to include locks on its two-wheelers.

San Francisco’s Skip has also experienced thefts but declined to specify how many. However, its vehicles have a wheel-lock mechanism so they cannot be ridden unless they’re unlocked with an app. Scoot’s two-wheelers can be ridden in manual “kick” mode without using the app — albeit in an unsatisfying experience, as they are not designed for that. Their electric motors require an app unlock to work. Both Skip and Scoot have alarms that go off if scooters are moved without being reserved in their apps.

City-sanctioned e-scooter rentals started Oct. 15 under a pilot program in which Scoot and Skip each have permission to rent out up to 625 scooters.

Scoot started out offering about 300 scooters but halved that to about 150 a day in recent weeks, something residents have complained about on social media.

“We’re not trying to hide that,” Keating said. “The reason is the theft. Ramping up more scooters depends on the success of the lock.”

Scoot this week will add combination locks and cables to scooters as an interim measure. Users will get the combination from the app after a scooter is reserved.

In early December, Scoot will switch to its long-term solution: built-in electronic locks with 32-inch cables. The smartphone app will unlock the devices. At the end of a ride, the user must lock the scooter to “street furniture,” such as bike rack or pole, and snap a photo of it.

Skip has been renting its full complement of 625 scooters and said it has gotten positive rider feedback. Skip also plans to add built-in locks to its fleet and is now testing them out on about a dozen scooters.

In addition to thwarting thieves, the locks should help keep more of the scooters out of the flow of pedestrian traffic and prevent people from knocking them over.

Skip said it’s working on new technology to detect when scooters are ridden on sidewalks, which is prohibited.

“We want people to learn that when they get to the sidewalk, they should pull up the scooter and walk it to the ‘furniture zone,’” said Julie Supan, Skip chief marketing officer, referring to the sidewalk edge where scooters are supposed to be parked.

In March, three other scooter companies — Lime, Bird and Spin — burst onto the streets in San Francisco. The scooter deluge led to a temporary ban and then creation of the pilot program.

The San Francisco Municipal Transportation Agency, which oversees scooters, said the number of complaints is much lower now than in the spring. The agency’s 311 line logged 336 scooter-related complaints from Oct. 15 to Nov. 14, or 11.2 a day. In the spring, it received complaints at the rate of 44.6 a day.

Scoot’s scooters average two or three rides a day, Keating said. By contrast, Santiago, Chile, where Scoot rolled out scooters around the same time, sees six to 10 rides a day per scooter. That’s because the vehicles there are concentrated in a central business district, while in San Francisco they’re dispersed throughout neighborhoods, he said.

Skip declined to say how many rentals it facilitates, but it’s “pleased with usage,” Supan said. Riders use them for first-mile, last-mile connections to transit, as well as for scenic rides such as along the Embarcadero or in Golden Gate Park, she said.

Constant theft is clearly not sustainable, Keating said. He declined to say what Scoot pays for its two-wheelers, but industry sources say e-scooters are bought wholesale from China for around $300.

Scooter thefts and vandalism are issues in many of the hundreds of cities worldwide that have the two-wheelers. Bird Graveyard, an Instagram account with 60,000 followers, chronicles the sometimes-brutal ways scooters meet their end, such as burning, being thrown off buildings or being tossed in the ocean.

Carolyn Said is a San Francisco Chronicle staff writer. Email: csaid@sfchronicle.com Twitter: @csaid