Here is the full list of all 11 FISHGUN achievements.
The base game contains 7 achievements, and there are 2 DLC packs containing 4 achievements.
Complete a level in Mirror Mode
Find Aple
Murder the Strawberry Mommy
Complete The Chasm
Complete The Valley
Complete The Temple
Complete A Fishy Beginning
Reach the highest difficulty in MAYHEM
Reach a score of 2000 in MAYHEM
Complete The Hole
Complete The Beach