The NCIS franchise underwent a reboot of sorts this calendar year — some of it voluntary, some of it not so much. And the changes have led to mixed results.

Most notably, the mothership lost one of its main stars, Michael Weatherly, who departed at the end of last season to star in his own show on CBS, Bull. To replace him, the show added three series regulars, played by Jennifer Esposito, Wilmer Valderrama and Duane Henry, as new members of the team.

Then in September, NCIS showrunner Gary Glasberg died suddenly and unexpectedly at age 50, leaving the ship without a captain. In November, it was announced that NCIS executive producers and writing partners George Schenck and Frank Cardea would take over showrunner duties.

Unfortunately, the new blood hasn't translated into viewership numbers for NCIS, whose ratings have inched downwards this season, with a few spikes; and whose most recently holiday episode notched a 1.7 rating in the adults 18-49 demo (down from 2.2 for the season premiere, which was itself down more than 10 percent). That episode tied a Voice clip show and just barely beat ABC's The Middle in the same time slot.

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This could be attributed to a number of things, but the biggest red flag is the loss of Weatherly, whose Tony DiNozzo was a huge draw for the show. In addition, a large portion of the show's fanbase was comprised of "Tiva" shippers (Tony and Cote de Pablo's Ziva), and with Ziva being killed off (off-screen) in Tony's exit episode, the will-they-or-won't-they of that relationship has pretty much been answered once and for all.

It's also possible that viewers simply aren't into the newbies, who still don't seem to be totally clicking with the rest of the gang. To be fair, viewership was down before we even met those characters — as previously mentioned, the Season 14 premiere showed a decline of more than 10 percent.

However, at the same time as NCIS (which, it should be noted, is still the No. 1 scripted drama on television in terms of viewers, and has already been renewed for Season 15) has been slipping, NCIS: Los Angeles' stats have been climbing; and the latter show even topped AMC's juggernaut The Walking Dead a couple of times in November. Though it still doesn't draw as many overall viewers as NCIS, it's notable that the Los Angeles-based spin-off is drawing a bigger audience in its eighth season (up more than 20 percent in the demo some weeks), after being shuffled around on CBS' schedule for the past few years.

This season, NCIS: LA has focused heavily on the popular 'shipper couple Kensi (Daniela Ruah) and Deeks (Eric Christian Olsen), with a compelling storyline about Kensi being gravely injured and struggling to recover, so it's possible that grieving Tiva fans are simply looking for a new pair to root for.

It should also be noted that the flagship wasn't the only show in the NCIS franchise to lose an original cast member: NCIS: New Orleans, which has held mostly steady in the ratings, said goodbye to Meredith Brody (Zoe McLellan), who was replaced by Vanessa Ferlito's Tammy Gregorio. Now in its third season, the youngest of the NCIS shows is starting to find a solid audience. But while its rating in the demo has remained relatively consistent, the show's overall viewership numbers have inched downward since the start of the season, with a few upticks, including the most recent Dec. 13 episode.

While overall television viewership is down in this era of #PeakTV (So much to watch! So little time!), the trend is certainly not the most positive one for NCIS. Perhaps it will just take audiences time to adjust to the changes. But should NCIS continue to shed viewers and relinquish its title of TV's No. 1 drama, it's possible one of its offshoots — N.C.I.S.: LA in particular — could be in a position to take its place.

(Full disclosure: TVGuide.com is owned by CBS.)