First of all, congrats on your first submission!
The movement controls restricted to only mouse are interesting. For me, controlling the speed and direction while running was difficult. Standing still and then jumping felt the best, more so because the speed can be controlled mid-air. Maybe by tweaking the controls a bit you could make a first person jumping platformer, kinda like Jump King, but 3D.
The fish were a bit difficult to find, but not impossible. By the way, the game resets when I grab the fourth one, so I think there are 4 instead of 5?