Wow! I love it!
I just have two suggestions.
1. I heard someone say that they'd like to see a gulping animation. I second that.
2. Another hidden mode to add even more to gameplay variety. It'd be a pretty dificult mode, though...you see, at the rate little Pengu-san is eating fish, I'd at least think that he'd gain a little weight...so, why not make that a gameplay mode? It would be progressively harder to move the little guy until you got some sort of power-up to, er, release the fish. ^_^; I know, it sounds dumb, but I just thought it would be funny.
Love the game, and may it fuel many more nights of fish-eating madness for other Grounds-goers.