Actually, the Guide to fishing says that I think it's the Anglerfish that is the mysterious fish that lurks in the depths, since those are fish found deep in the ocean. I can see how that tricked you, as I had to read it twice myself when I was doing this guide to Ambrosia. Anyone know the sell value of Ambrosia off the top of their heads?
It took me probably 200 catches or more to get an Angelfish, hitting up sylvan glade, the ponds with fountains in that one Oasis Springs neighborhood pictured in the Guide, and a river spot by my Sim's house in Willow Creek. Only at level 10 did I get one. It's not a fish that is listed at level up, so I presume they are catchable even at level 1 if you get very lucky. I was not so lucky and found it to be the hardest part of all the things you need to do to make Ambrosia. Of course, when I did the fishing Guide, angelfish was one of the first uncommon catches my Sim made
However, I can't say I've ever had a lot of them.
I haven't ever caught one and had it mention the bait, so right now assume it's a fish that doesn't have one. If someone sees that notification you caught an Angelfish and it says the bait used, that would be the class of bait we need for this fish. I did try several types, since i knew fishing with bait wouldn't make it uncatchable (mainly small fish - koi, minnow, and perch). Right now I think that fishing with bait might make it even harder given you will be likely to catch something else in that spot that DOES take bait. I would not try organic bait given it would raise the likelihood you catch catfish, which would probably make it harder because they're in the same spots.. so I'm really not sure what would even qualify.