DYSMANTLE

For this achievement you need to invent the Fishing Pole in the right side of the skill tree. Once you have it, you can fish in any patch of water, it doesn't have to be spot with a fish icon near it, but the ones that do have a fish icon guarantee fish to be caught. Equip the fishing pole tool and press near the water to start fishing. You can also invent the Tackle which is right next to the fishing pole to improve your fishing speed.

When this achievement unlocked for me, I had caught 18 fish so I think the requirement is slightly off but I'm not sure.