Here is the full list of all 12 Rule with an Iron Fish achievements.
Caught 150 fish
Unlocked the Sky
Caught 108 fish
Defeated the Kraken
Unlocked the Deep
Caught 50 fish
Unlocked the Shivering Timbers
Unlocked the Lava Cake
Caught 30 fish
Caught 20 fish
Unlocked the Sump of Despair
Caught 10 fish