Berg took over from Steve Kean at the start of November, and in 10 games with the 43-year-old at the helm, Rovers won just once.
Fish told Sky Sports News: "He was a fantastic player for the club, but for whatever reason it just didn't work out for him here.
"It's difficult to say what. Perhaps the expectations from the fans, outside influences maybe. Henning Berg just didn't have the nous to get the job done? I don't know.
"He just couldn't get the team playing, he couldn't get them fighting. But someone needs to come out now and hold their hands up.
"There were a lot of people involved in the process. A lot of good, better candidates were overlooked for this position.
"But they (Venkys) seemed determined to bring in an ex (Blackburn) player and it just hasn't worked out the way they had hoped.
"Why didn't Berg bring in his own coaching staff? It was my understanding that he would bring his own people in with him.
"But as it ended up he was left with Steve Kean's men to help out. Someone has to come out and explain why this has happened."
When asked about the remainder of the season, Fish was adamant that Rovers can still achieve their goal of promotion back to the Premier League.
"Do I think we can do it? Yes. Absolutely, we've got the ability in the squad to still get the job done. You've only got to look at Reading last season.
"We need to have one of those runs. Also, the stands are empty. We need to be picking up results and getting people back through the gates again."