Tech research firm Gartner also says the number of smartwatch shipments will quadruple next year to 40 million, with as many as half coming from Apple.
Apple Watch won't launch until early 2015, but it's already expected to hurt rivals.
The number of fitness wearables shipped next year will drop to 68.1 million units from 70.2 million units in 2014, according to Gartner. The segment includes smart wristbands like the Jawbone Up, sports watches, heart rate monitor chest straps, smart garments and other fitness monitors. Specifically, smart-wristband shipments should drop to 17 million from 20 million in 2014.
The tech research firm attributed the decline, which it called a "temporary dip," to "an overlap in functionality between smart wristbands, other wearable fitness monitors and smartwatches." In particular, Apple Watch -- which includes various health features -- will affect sales of the fitness-centric devices.
"Half the people who would have bought a [fitness] wristband will buy a smartwatch instead next year," Gartner analyst Angela McIntyre told CNET.
She noted that companies will ship 40 million smartwatches in 2015, quadruple the number expected this year. Apple Watch has the potential to make up as much as half of the shipment volume next year, McIntyre added.
Apple in September made its first jump into the hot smartwatch market by showing off three designs for its first wearable -- the stainless-steel-cased Apple Watch, the aluminum-cased Apple Watch Sport, and the 18-karat-gold-cased Apple Watch Edition. The smartwatches go on sale early next year at a starting price of $349. The devices must be paired with an iPhone, and are compatible with the iPhone 5, 5C, 5S, 6, and 6 Plus.
By announcing its first-ever smartwatch more than a quarter before it's available for purchase, Apple accomplished two things. First, it put the rumor-mongering to rest by telling -- and showing -- consumers what they can expect from its wearable. Second, and as important, CEO Tim Cook and his team, led by design chief Jony Ive, got would-be smartwatch buyers to hold off purchasing a rival device from Samsung, Motorola, LG and other competitors trying to nab a share of the emerging market for smart devices.
While fitness-specific wearables will take a hit in 2015, they should rebound in 2016 because of versatile designs, as well as the introduction of models with lower-cost displays, Gartner said. Shipments should total 91.3 million units in 2016.