Fun idea and it's impressive you did it with no engine. Not sure if it's something with my PC but even in fullscreen mode there seemed to be almost no fish, when I did see them they were almost always too big to eat. Also ran into some kind of bug where the kelp at the bottom was really small, not sure if that's related.
Fits the theme super well though and I think if the spawning was changed a little it would be really fun. Also liked the art a lot and the audio was good.