This only works on party days.

After catching the current party fish in the shallow and middle area, go inside of the shipwreck interior, sea people mural cave, seaweed collector's house, and or underwater lake cave. Any of the 4 places will work for this, the sea people village and glacier areas will not.
after entering leave the area after swimming around the location for a little bit. ( More testing needed but catching a fish or two may increase the likelihood of increased party fish respawn) This can be done up to 4 times, but the same rooms can't be used twice per dive. There is also a chance of none of the party fish respawning. When no party fish respawns after a entering a room, don't just return as if other rooms have not been used, using that room may respawn them.



In regards to bullet refills, entering and leaving the rooms may also have a small chance of respawning as well.