LOS ANGELES — When the highly anticipated “Marvel’s Agents of S.H.I.E.L.D.” premiered this fall on ABC, its ratings looked solid, if not downright heroic.

Just as important, the network had reason to believe it had finally found a show that young men would watch and its own superhero franchise.

Now it seems the helicarrier is losing some power.

The heavily marketed “S.H.I.E.L.D.” — Marvel’s first foray into network television since Walt Disney Co. bought the comic book brand in 2009 — drew more than 12 million viewers to its original telecast, according to Nielsen.

That figure grew to 15.8 million in the next three days as people caught up via digital video recorders and video on demand.

But the second episode fell sharply, and the show has posted ratings declines every week since, causing concern among advertisers and glee among rival networks.

Counting only people who watch on the day of first airing, “S.H.I.E.L.D.” lost about 40 percent of its total audience and half of its viewers ages 18 to 49, the demographic most cherished by advertisers.

The network has played up the fact that a large portion of the show’s audience watches the episodes on DVRs and video on demand. Factoring that in, the trends look a little better. But those numbers also are on the decline.

“We hope that there is no more erosion of the current audience,” said Roy Currlin, a strategist at Carat, a media buying firm whose clients include General Motors Co.

ABC did not provide an executive to comment for this story. But a spokeswoman for the network said the series is performing well against its time-slot competition.

In addition, “S.H.I.E.L.D.” is helping ABC by drawing men, who have generally not shown interest in shows such as “Grey’s Anatomy” and “Scandal.”

This comes after a string of hits for Marvel at the box office. Its “Iron Man 3” is the top-grossing movie this year, with domestic ticket sales of $409 million. Marvel’s “The Avengers” took in more than $623 million last year.

Its latest film effort, “Thor: The Dark World,” has brought in $145 million in less than two weeks in release.

Tenuous connection

Many TV critics and comic book bloggers have been scratching their heads at “S.H.I.E.L.D.’s” creative direction. The show, created by “Avengers” director and geek hero Joss Whedon, appears to have only a tenuous connection to the greater Marvel franchise, despite a brief cameo from the character of Nick Fury, played — as in the recent films — by Samuel L. Jackson.

For example, the character Skye, played by Chloe Bennet, did not exist in the comic books.

In addition, the series tends to follow a procedural format rather than having an overarching plot that would provide momentum.

Viewers are also bothered by the series’ biggest unanswered question: Just how is main protagonist Agent Coulson, played by Clark Gregg, alive after his fate in the “Avengers” movie?

Despite the ratings slide, ABC ordered a full season of “S.H.I.E.L.D.” The show is doing well overseas, so it may already be a financial success for Disney. But the numbers probably will have to rise, or at least stabilize, for the network to go forward with a second season.

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