First off, this game is beautiful.

Secondly...

I've always felt that (and could never confirm for sure) that rarer fish have shorter bob windows.
Not sure tho.
No, you're right. It's been a while since I hunted truly rare fish in AC. Usually I apply a "close my eyes/look at my character and hear the bob" rule but with rare fish like this, you can't do that because you need to use reflex. The problem is that because it has the same size as a sea bass you don't know what you're going to get. Usually ultra rare fish are much bigger than bass so you know what to look for. Here, you don't. In NL it's easy to tell what an ultra rare fish is: by the fin protruding from the water (this doesn't apply to all rare fish obviously). But in NH, I think the sizes are more subtle. So you have to be actively looking for the bob and hit A asap.
I got it as soon as making that post once I realized that I needed to look at the fish rather than away to get it due to its rarity.
Lesson learned. Or relearned, I should say.