Hii! Thank you so much for playing!
You just gotta use the fishing rod a lot on the fishies; they die after a few hits, and if they're all dead, the gate opens.
They have less health in story mode, which may be why you figured it out. :)
Otherwise, there isn't really a good death indication and the enemies are a bit tanky, so that's a bit of a problem. I'll be sure to keep it in mind for my next project! (also sorry for the incredibly late answer- I litterally forgot to click "post reply")