The writers’ strike may be over but the full brunt of its effect is only now being felt in the ratings. And it will continue to be felt until original scripted series return to lineups, likely by mid April.

Those declines are steep.

The average 18-49 rating for the five major television networks tumbled some 23 percent the last week of January and 28 percent the first week of February, the latest available, according to an analysis of live Nielsen ratings from MediaVest. That's compared to about 15 percent in December and most of January.

In the time since then, they've probably sunk further.

The steep declines are the result of three things, and one was the surge in reruns as the networks finally ran down their stockpiles of original episodes.

That began in late January and by February was in full force. The networks will air just 61 hours of original scripted programs this month, down from 182 hours last February, according to Magna Global.

Another factor was the sudden absence of original episodes of top-rated shows.

“People may have gone away from the broadcast networks when they realized shows like ‘Grey’s Anatomy’ and ‘CSI’ weren’t going to have new episodes for a while,” says Jordan Breslow, director of broadcast research at MediaCom.

“It’s not that there weren’t any originals, but the standbys like ‘ER’ were not there anymore.”

Yet another factor is the poor performance of a lot of the midseason replacement shows, especially the new reality shows, which are not catching on with viewers as network executives had hoped.

But the strike is hardly the only force behind the ratings decline. It may account for about half.

There's the long-running migration of network TV viewers to cable TV and other outlets, which media researchers like Steve Sternberg of Magna figure amount to 5 percent over a given year, on average. Cable ratings are up again this season--4.3 percent among households the week of Jan. 21, for example, according to the Cabletelevision Advertising Bureau.

But another factor, and one's that's growing, is competition from other media, such as video games and the internet.

“There are so many places that viewers can go, including destinations not accounted for by Nielsen,” says Shari Anne Brill, senior vice president and director of programming at Carat. “It’s not just ad-supported cable."

The big unknown for the networks is just how many of these lost viewers won’t come back to the networks even after the original episodes of the top shows return.

One would expect a disturbing number, judging by the lousy season the networks were having even before the strike's full impact set in.

For the season through Feb. 18, the average 18-49 rating, including DVR playback for seven days after a show originally airs, was down 7 percent, to a 13.6 adult 18-49 rating.

Fox is the only network up from last season. It’s up 17 percent when including the Super Bowl and 6 percent when not including it. Fox aired fewer post-season baseball games than in the past, which allowed it to run more original series. That boosted its ratings.

When not counting the Super Bowl, which switches networks each year, ABC is down 14 percent in 18-49s, CBS is down 18 percent, NBC is down 9 percent and the CW is down 21 percent.