Hmmm I wonder if it'll even be possible to wait for the Museum then unless you don't sell any fish until then. Like it's not certain it'll be an option to donate or if he will just automatically keep the first of every fish species you sell.
Either way, I think it's good because people could accidentally sell off a rare fish or one that goes out of season before the museum opens and that'd feel like the one that got away.
iirc, Blathers didn't have anything unique to say about bugs and fish in NL unlike previous games, so I don't really care who I donate it to if he's not going to say anything special. [SUB]Honestly it made him kinda useless as an NPC to me and I was disappointed he had nothing to say[/SUB]