NEW YORK — Deepak Jayasimha’s fitness tracker is now with his father-in-law in India, where it sits unused. Annabel Kelly foisted hers off on the kids. Virginia Atkinson took hers off to charge the battery and hasn’t picked it up since February.

Although sales of Fitbit and other fitness trackers are strong, many of their owners lose enthusiasm for them once the novelty of knowing how many steps they’ve taken wears off. One research firm, Endeavour Partners, estimates that about a third of these trackers get abandoned after six months. A health care investment fund, Rock Health, says Fitbit’s regulatory filings suggest that only half of Fitbit’s nearly 20 million registered users were still active as of the first quarter of 2015.

“The question for investors is how long the market will continue to grow at this rate, and whether Fitbit can execute on growing engagement before ... the number of devices sold per year reaches saturation,” Malay Gandhi, a managing director at Rock Health, wrote on a blog.

Abandonment affects all manufacturers of fitness trackers, which are relatively cheap at about $100 and are commonly given as gifts. Fitbit gets the spotlight because it started trading publicly last month and has 76 percent of the U.S. market share by revenue, according to the NPD Group.

Fitbit now has competition from Apple Watch and other smartwatches that do what fitness trackers do and more, such as showing news updates and boarding passes for flights.

In a statement, Fitbit said it intends to remain a market leader through new features and services. The company said it keeps users motivated by offering ways to compete with friends and family and awarding virtual badges for hitting fitness milestones. The statement didn’t address Fitbit owners who’ve stopped using the device.