Actually no offense to Ivrugger but it's even easier than that. You don't need bait to supermax. All you need is a sim who likes to be outside and fish. I've caught every fish imaginable from Sunset Valley and it would be the same for Hidden Springs. Be careful of Hidden Springs' issues though.
I think this was mentioned a page or two back but all you need is 20 fish for the supermax challenge. That's 20 out of a possible 37 if you have every single EP. In fact, you can get this challenge only getting the fish from just base game as it doesn't adjust for additional EPs. You just have to know where fish spawn. I've fished in HS so many times so if you ever can't find a fish, just PM me and I'll help you anytime.
I do a lot of what Ivrugger says about jumping spots for fishing. I find a string of them that near each other and I hit fish (I never use bait since the Pets incident - only use bait if you want a specific fish but it's not required for even deathfish) and pull the tab out to fish until you've caught 5 and I do this 8 times. I make sure there is fruit in the pocket to eat and a sleeping bag to take a nap. Voila! Fishing supermaxed. My 2nd generation of Artie's first dynasty had her fishing supermaxed before she was an adult.
Oh wow, haha! That's made me really happy, the challenge doesn't seem so daunting now! Still looks hard, but not entirely daunting!
I'm also aware of Hidden Springs' issues, but I planned the dynasty before I decided on the town and after deciding on my theme, etc. I realised it was the only town that suited.
I'll just have to hope things go right, which they rarely do for me but I have hope! If it doesn't work out on my first go because of town-related problems, I'll just admit defeat and restart in another, less buggy, town.