Fish Behavior After Heron Attack

We have a 500 gallon pond with an assortment of Butterfly Koi,
Shibunkin, Cat Fish, and Gold Fish--all have names. We did not know
until 2 weeks ago that there are herons in Southern California. We
came home from vacation to find Groucho gone; while we did not see the
attack, there were gill remnants beside the pond, and the next day a
large heron was spotted in our yard. Our neighbors then excitedly
reported that they had seen 2 herons in their yard the previous day;
their joy at the sighting was substantially greater than was ours. The
circumstantial evidence is pretty strong that Groucho lost the heron
vs. fish battle. Anyway, 2 scarecrows and an electric fence seem to
have protected us since then.

The problem now, though, is that our fish have been seriously
spooked. Or at least that is what we surmise. Prior to our vacation,
they were quite active and seemingly fearless. Afternoon is feeding
time, and we could anticipate the fish once they saw us to start
circling furiously in anticipation of their food. Now, they are
profoundly less active, and head for the bottom when they see us. If
we sneak up on them at night, and suddenly turn on the flashlight, we
discover that they are more active than during the day, but then
immediately they hunker down on the bottom until we leave.

In hopes that the problem was something other than psychological,
I tested the water for pH, ammonia, and nitrates--high pH (normal for
us), but all other parameters normal. I exchanged about 20% of the
water (using a dechlorinator) to no avail. We have tried withholding
food in hopes of enticing them to the surface, but their food reserves
and the algae in the pond keep them on the bottom. We have tried
feeding them while standing quietly at the pond side in hopes of
operantly reinforcing increased activity, but unsuccessfully so.

For the fish, we would like for their apparent anxiety levels to
decrease so that they could resume their previous carefree existence.
For us, we found them to be much more enjoyable prior to the heron
attack.

If anyone has any suggestions about an approach we can take to
bring harmony back to our situation, we would be most appreciative.
Many Thanks in advance.

BA and Missy Finstead