I just caught a very quick dotty back same problem
I built a fish trap and it worked
Take a plastic bottle 2 liter or pint will work
Cut off the top where it curves down to the side
Invert the top of bottle in main piece
Remove cap, put some food in bottle and fill with water - the damsels favorite
Punch a few small holes in bottle so it will sink
Since damsels are bold they should be first into the trap
They swim in and can't get out
It took about 6 hours for my dotty back to get bold enough
You can let bottle sink to bottom or perch it on some rock
Give it a try won't cost you much
DJB