You have two things you can do with the fish. So make a save before doing this. You will pick up the fish when you get past the beach. When you get to the park, there is a seagull. Just as you enter the park, make a save, as you'll need to reload if you want to get this achievement in the same playthrough as the other:
For this one, give the seagull the fish. Pick up the bones. When you get to the pet cemetery, the last but one grave is open on the bottom. Interact with it to bury the fish.