The combined audience for ABC,
The drop is a result of more viewers watching their favorite shows through DVR playback, video on demand services and streaming platforms.
"It's a continuation of a trend of what we saw last season," said Brian Hughes, senior vice president of audience intelligence and strategy for the media buying firm Magna Global.
The number of people using television in prime time has declined steadily over the last five years with the emergence of streaming video, especially among viewers under 35.
Overall, the percentage of 18-to-49-year-old viewers who watched prime-time television during premiere week fell to 25.5%, down 8% from a year ago. In the 2011-12 TV season, the figure was 36.3%, according to Nielsen.
Streaming data for network shows are not included in Nielsen totals, although the audience measurement company is moving toward providing data that does incorporate it.
Many of the network programs gain a significant amount of audience when delayed viewing on DVR playback and video on demand services are added into the ratings totals. A majority of the deals with advertisers are based on how many people watched the commercials in the shows within three or seven days of their initial airing. Networks are also selling more commercials on their online viewing platforms for their programs.
Even with the drop in traditional TV viewing, network executives note that they are still reaching large audiences through time shifting and generating more revenue from digital platforms.
Despite ratings declines last season, the five major English-speaking broadcast networks sold $9.1 billion in advertising for the 2017-18 season, a 4.1% increase over the previous year, according to the research firm Media Dynamics.
CBS won premiere week with an average of 9.5 million viewers per night, according to Nielsen, but that was down 15% from last year's season opener. NBC, which came in second with 7.8 million viewers, was down 11% from a year ago. ABC ranked third with 5.8 million viewers, down 1%, while Fox saw a 14% drop with 3.1 million viewers.
NBC was the leader in the 18-to-49 age group that advertisers covet most with 2.7 million viewers, but that was down 16% from last season, It was followed by CBS (2.4 million, down 24%), ABC (1.7 million, even with last year), Fox (1.3 million, an 18% decline) and CW (292,000, down 10%).
The weekly averages for CBS and NBC were hurt by year-to-year comparisons for both prime-time NFL games, which were down from 2016 (the CBS game on Thursday was likely depressed by a 47-minute weather delay). But even with those numbers excluded, the networks had significant declines.
Nonetheless, the "live" viewing numbers for premiere week offer a glimpse into what is clicking with audiences so far.
"There were actually more bright spots than we thought there would be," Hughes said.
ABC appears to have its first big hit drama in several seasons with "The Good Doctor," which stars Freddie Highmore as a young surgeon with autism. The program was picked up for a full season on Tuesday.
"The Good Doctor," premiered with 11.4 million viewers on Sept. 25 and held that number in its second airing on Monday. When delayed viewers were added, the premiere's total gained another 5.5 million viewers.
CBS had a strong premiere from "Young Sheldon," a warm comedy prequel to "The Big Bang Theory." The 17.2 million viewers who watched on Sept. 25 were strong enough to get CBS to pick up the show a for a full season.
While the original "Will & Grace" was an edgy comedy during its original run from 1998 to 2007, the new revival with the original cast intact was comfortably familiar and drew 10.2 million viewers for its Sept. 28 premiere. Three days of delayed viewing boosted that total to 14.8 million.
"This Is Us," NBC's breakout family drama of last year, delivered an all-time high in its second season premiere with 12. 9 million viewers on Sept. 27.
Early successes indicate viewers are looking for uplifting entertainment, said Preston Beckman, a former network executive and TV consultant.
"My gut is that people are wanting shows that make them feel good or are aspirational," he said. "There is enough darkness in the world where you don't have to watch it on TV right now."
The gritty military shows and realistic crime dramas that the networks have added saw mixed results, another sign that viewers are moving to content that is less bleak.
CBS’s “SEAL Team” with
But NBC was less successful with the "The Brave," another special ops-themed drama that opened with 6 million viewers in its first week, dropping significantly from its lead-in from the network's top-rated reality competition show "The Voice" on Monday at 10 p.m.
ABC's "Ten Days in the Valley," a serialized show about a kidnapped child, drew a modest 3.4 million viewers in its Sunday premiere.
1:50 p.m.: This article was updated with additional ratings details.