So here's my list

phytoplankton
Zooplankton (the little animals that eat plankton)
small fry (or a specific fish that would be small fry)
Tuna (or any fish that eat small fry or that are bigger)
Shark
Fishermen
Bear
Wild man
Werewolf (And now I'm just making things up)
Vampire

The idea is that the lower rank would be eatten or killed be te higher rank. If you got something better by all means...