googlefan wrote I guess something
must've frightened them.

Predation by birds leaves frightened fish and no evidence. You can usually tell
when a raccoon visits, they leave a mess and nosh on plants as well as fish.

Herons and kingfishers will leave little or no evidence of a visit (did have
one kingfisher leave a feather and small oil slick behind once).

Netting works. Other devices are motion activated sprinklers, electric fences
that surround the pond or lay over the edge, decoys that lay under the water's
surface.
Some folks lay a tunnel of sorts on the bottom of the pond for the fish to hide
in after the intial strike.



kathy :-)
zone 7, SE WA state