2015 could be a high watermark for the global audience for linear TV, if recent projections from media agency network ZenithOptimedia are anything to go by.

Starting from next year, the number of regular viewers of linear TV will shrink for the first time, by 1.9% ZenithOptimedia forecasts, after a 3.1% rise this year.

This decline has already started in some big markets: in France and Russia since 2013, and the US and UK since 2014. Asia’s biggest TV market, China, is expected to join them this year.

Blame OTT. More people are watching online as both the quantity and quality of available video increases, Zenith suggests in its latest Online Video Forecasts report.

Some viewers are opting for subscription services, but ad-supported options are also attracting plenty of eyeballs.

Online video is the fastest growing category of internet advertising, although it still has a long way to catch up with linear TV.

In the US, online video will make up 16.5% of internet ad spend this year, Zenith estimates – the biggest proportion worldwide, equal with Italy.

Taiwan is in third place, with 15.8% of online ad spend going on video this year, according to Zenith’s numbers.