Thanks, a sensible interpretation. I would translate that as:-

'Big fish hides in the depths waiting for unsuspecting prey, (in order to be able) to swim up from the shallow edge of the turquoise abyss.'

Implying that as the school of fish approach the deep hiding place, the big fish gets ready to attack by preemptively moving itself to the edge of the deep bit? Dunno!

But then why not 'с мелкого края' instead of 'с отмели края'?

I may drop this one and try a video with (for me) less ambiguous speech.