I think the game is quite slow in the beginning, i.e. it's very simple to avoid the rocks and collect all fish, but then it rapidly gets faster and more difficult, and I'd say eventually you get to a point when it might not be possible to *both* avoid all rocks and collect all food onscreen. I died after letting too much food pass by in the span of ~2 seconds, because the food was also being somewhat blocked by rocks.
I don't know the solution to this, but from my point of view, the slow build-up is the more important problem (it's common in arcades to get to the 'humanely impossible' bit, but the game should be fun and engaging from the beginning). You could try adding a little more variety, perhaps powerups; just to give the player something more to do. For example temporarily slowed-down time, a shield (which you can use to crash through rocks), a magnet for nearby collectibles, etc.
All in all, getting and maintaining a high multiplier is more important for your score than being able to last for a long time – but that's perhaps a good thing.