My outdoor pond contains valuable koi, which are luring herons. I
protect my pond with a net, which is effective but unsightly. I have
heard anecdotes which state that aquatic waterfowl have a phobia of
stepping over strings or wires. It seems to make sense in that herons
stalk their prey by stepping in the shallows rather than diving like a
pelican. My question is: Do herons fear crossing wires? Would I
effectively protect my pond with a zigzag network of wires over the
greater part of my pond?

Dear monroe22-ga,
I've tried the wire method as you have described and the Heron still
pinched some fish. I find the net is the only answer.

You may wish to see this site
http://www.pond-doctor.co.uk/longherons.htm
"a. Invisible lengths of fishing line
stretched around the perimeter of a pond will startle an unsuspecting
heron if it strides across it, causing it to fly off. This method can
complement a pergola and relies on the observation that herons will
not land directly into a pond but walk into the water having landed on
terra firma. However, some bereaved koi keepers will strongly dispute
this theory."

The RSPB also has some advice on the matter.
http://www.rspb.org.uk/gardens/advice/heronsandfishponds/ideal_fishing.asp

answerfinder-ga

Monroe --

Not just are the herons a potential threat, but so too are raccoons. 
They've actively poached the koi from the pond of a neighbor.  A net
(carefully constructed) or something like chickenwire may be required.

Best regards,

Omnivorous-GA

Answerfinder-ga:  Enter your comment as an answer...I will accept it.
By the way, I was thinking of highly visible wiring crossing the pond,
rather than fishing line, hoping that the sharp-eyed herons would see
it as an obstacle.
monroe22