A night that’s up for grabs

Now that Gil Grissom has left the building, we can get acquainted with a simple truth: Thursday nights on network TV are totally up for grabs.

When William L. Petersen left “CSI: Crime Scene Investigation” after more than eight seasons playing the crusty forensics ace Grissom, the dynamics of television’s most important night -- when marketers try to hit viewers planning their weekends -- changed yet again. And no one really knows where things will end up.

If this past week is any guide, “CSI,” long CBS’ top scripted series, will see significantly lower ratings as viewers get accustomed to Petersen’s replacement, Laurence Fishburne. The original “CSI” has always been the most popular of the three-show franchise (the others are “CSI: New York” and “CSI: Miami”), and the Petersen exit is the first major threat it’s seen to its dominance.

This creates an opportunity for rivals. ABC is angling for a ratings and critical renaissance for “Grey’s Anatomy,” the medical drama that viewers have been complaining has been in a creative funk this season. Meanwhile, NBC is set to unplug “ER” this season, apparently with a goodbye pat from former cast member George Clooney (next season, Jay Leno’s new prime-time show will occupy that spot). And Fox, in its umpteenth effort to get some sort of traction on Thursdays, just moved “Bones” to 8 p.m. and scored some decent numbers.

Add it up and you get a picture of a night in flux everywhere. That could have a big effect on what viewers see, and not just on Thursdays.

Thursday is vital to the networks because that’s when movie studios and other big advertisers, hoping to lure consumers making weekend plans, are most willing to part with huge amounts of cash. Also, it’s virtually the only night that can be consistently competitive anymore. The cable networks have increasingly grabbed viewers with original programming on Sundays, while during the second half of the season Fox takes the lion’s share of audience on Tuesdays and Wednesdays with “American Idol.”

The changes taking place now are fraught with history. NBC began asserting control over Thursday in the mid-1980s, with “The Cosby Show” and “Cheers” as well as later series such as “Seinfeld” and “Friends.” Its “must-see” lineup was particularly popular with young adults; during the 1998-99 TV season, NBC’s Thursday schedule drew 30% of the adults ages 18-49 demographic, according to Nielsen Media Research. Its nearest competitor, Fox, had just 12%.

But not long after “Seinfeld” went off the air in 1998, rivals smelled weakness and began closing in. ABC had momentary success with “Who Wants to Be a Millionaire.” Then, in a gutsy move that wound up remaking network TV, CBS in 2001 began kicking off Thursdays with “Survivor,” its reality smash, followed by “CSI.”

NBC held up well at first, but the competition soon exacted a horrendous toll (CBS got a taste of the same medicine in 2006, when ABC moved “Grey’s Anatomy” to the night). The network as recently as the 2003-04 season held a 22% share of the Thursday 18-49 audience; this season, NBC’s Thursday demo share has ebbed to 8%.

Then again, though, current leader CBS has a demo share of only 10%. The point is that all the networks have been hemorrhaging viewers on their key night. True, much of this is because of the natural erosion that networks have experienced across the board as audiences flee to cable and elsewhere. But “Idol” has proved that big audiences can still be captured if networks know how to look for them.

So Thursday is really ripe for a reinvention. But it has been years -- really, since NBC’s glory days -- since a network has seriously tried to develop shows specifically for that night.

ABC, CBS and Fox have all spent the last 10 years trying to chip away at NBC’s dominance by moving some of their most reliable performers on to the night. And their plans succeeded, maybe in ways even those programmers did not imagine.

But now those shows -- the “Survivors” and “CSIs” and “Grey’s” of the world -- are themselves getting old. Time to try something new.

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scott.collins@latimes.com