Here is the full list of all 20 Somerholm achievements.
Reach Level 5 In "Animals Discovered"
Reach Level 5 In "Distance Traveled"
Reach Level 5 In "Islands Completed"
Reach Level 5 In "Islands Visited"
Reach Level 5 In "Oddities Discovered"
Reach Level 5 In "Fish Caught"
Reach Level 5 In "Photos Taken"
Reach Level 5 In "POI Discovered"
Reach Level 5 In "Time Explored"
Reach Level 5 In "Times Pet Teo"
Purchase All Camera Film Upgrades
Purchase All Camera Setting Upgrades
Purchase All Map Upgrades
Purchase The Sleeping Bag
Purchase All Items (Except For The Notes)
Discover The First Note
Discover 5 Notes
Discover 10 Notes
Discover All 20 Notes
Obtain All Other Achievements