My guess is that the main critter we play as represents Jimmy. As for the other fish, the fat one is Swansea, the small one on the hook is Anya, and the sad blue one is Daisuke. The way they're all characterized has to do with how Jimmy views them (Swansea being selfish, Anya being irrelevant and puny, Daisuke being pathetic and naive). Also he literally 'consumes' them all which foreshadows the ending of MW.
(as for the first fish we consume in the beginning, I'm not sure. Could just be the first nod we get to Jimmy's destructive behavior)