I was foolish enough to start this credo third in line, and I want to say it's harder than all the other credo's EASILY.
For example, it will ask you to catch 2 sturgeons. I've gotten more than two hundred+ fish and I've still never seen a sturgeon, even with the proper lures and string. The only recourse is abandoning it. Even if I fish all day there's likely little chance of getting one, ignoring the fact that this absurdly hard quest wants two at once.
Please remove the rare fish requirement, or at least make it less common. I would be willing to kill 50 beavers/swans/crabs rather than catch a single impossible-to-find rare fish.
Alternatively, please let us abandon already completed credo's to reduce the amount of quests, because I cannot complete the fisherman credo at all, time is not a factor here, it's simply next to impossible, so it would be more time-efficient to lose my other two credos, then rely on luck, than try to complete this monsterous credo straight up.
Edit: Now that I fully understand the fishing system I guess it's not hard.

(and i got it up to 80%, without fisher hearth magic)