What you can eat is restricted by the size of your shark. Bigger sharks can eat more bigger things! You can find a list of what you can eat by tapping on the '?' icon next to the shark name.

Some prey will only be eaten after you have grabbed onto them for a short while. Sometimes more challenging prey will get away from you and will have to be grabbed again before being eaten.
If you're unsure as to what you can and can't eat, look out for the on-screen text messages telling you what size shark is required for any given situation.