Some cats and fish have ‘keys’ above them. What are these for?

When you reach level 5 you'll be able to access the next room by using keys. In order to collect keys you have 2 options:
1. Use the scanner, this will scan everything in the room and locate which fish or cats have these keys, that will be directly visible above the cat or fish pile.
2. You can just try and guess where the keys might be. They are invisible so if you loot a pile of fish or pounces a cat which has a key, you'll get it.