U.S. television viewership declined by 12 per cent in January compared to the same month a year earlier, the eighth consecutive double-digit drop, according to new data from ratings firm Nielsen.

The main reason for the drop-off is that viewers are switching to streaming video services like Netflix, Hulu and Amazon Video, say analysts Anthony DiClemente and Benjamin Black at investment bank Nomura.

There's also little evidence that January's data is a blip.

On an annual basis, overall television viewership has declined by more than 10 per cent annualized every month since last July. Even before that, the numbers were down by single digits for several months.

"The viewership declines that we observed throughout most of 2014 accelerated for most media companies in January," the analysts said. "This does not bode well for … domestic cable TV ad revenue trends."

Declines across the board

Every media company that Nomura tracks saw viewership declines in January, according to the Nielsen data, but none fared worse than Viacom, which owns channels such as BET, MTV, CMT, Nickelodeon and Comedy Central. Overall ratings across Viacom properties were down 23 per cent in January compared to the same month a year earlier.

Others fared only slightly better:

Twentieth Century Fox saw its ratings decline by about 10 per cent.

Disney, which owns ABC, was off 7.5 per cent.

Time Warner, one of the brighter lights, was still down by three per cent.

Specialty network AMC saw its ratings decline by almost 19 per cent. But the analysts noted the channel had no new original programming during January, so AMC's number is expected to change in February with a new season of The Walking Dead starting, and the premiere of the Breaking Bad spinoff Better Call Saul.

Of Scripps, owners of several specialty channels, the analysts said: "Food Network and the Travel Channel ratings remained soft, somewhat offset by modest viewership growth at HGTV," but overall Scripps's ratings were down by a little over six per cent.

Add it all up and it's a worrisome trend for the industry that's facing exponential growth from online streaming services.

Lobby group The Internet Association, which counts Google, Netflix, Amazon and Yahoo among its members, revenue from online streaming video has increased by 175 per cent between 2010 and 2013, from $1.86 billion to $5.12 billion, the group told the New York Times this week.

In a nod to the shift towards digital, Nielsen said late last year it plans to start including data on online video streaming in its monthly ratings in the near future. That development should give a clearer picture of what people are watching and how much, on a head-to-head basis.