The fish has three states, it can either wander around, head towards the sub, or flee. Each fish has its own 'curiosity' stat which decides how likely it is to react to sound.
When a fish is heading towards the sub, there's a small chance for them to lose interest in the sub. In my playtests I found most of the time I had to press the button 2-3x and then there'd be some fish. Fish which weren't at the sub were more than likely nearby.
The intention is that you track them with the radar at the bottom left of the screen. When you scare them, whilst they do gain some distance, once they exit a radius of 2.2km from the ship, they'll soon return to within 2.2km to assure they don't get too far.
I'll likely update the game a bit post-jam to make the monster go away faster.
Were you able to finish the game, or were you unable to catch/scan all fish?