quite simple really, you`ll need to take apart a "party popper" put the cardboard roll on a peice of wide masking tape, take the white poder out of a whisteling rocket and pour than on the tape by the roll, seal it all up carefully and put this inside a waterproof container and extend the popper string that should be outside the container and freely pullable.
bury this container under 6 inch of soil and tie the string around the fish, as the cat pulls the fish, the string follows an BANG, the cat gets the scare of its life!!! it`s non lethal and might be worth putting in lemon rind into the container with some garlic powder.

it`s also handy on allotment sites where there are alot of theives!
use chili powder and red food coloring in the place og garlic and lemon, they WON`T come back again and will certainly think twice about entering your greenhouse and steeling tools!