Great pond, Dave, Beautiful!
Looks like you have fishing line up to prevent the great blues from getting to the pond. That works pretty well and if you have had some of the KOI for 15 years, it must be doing the job. I have deer netting over my pond, high enough to keep it dry. I use to have loses every year due to Great Blues, but none after I put in the deer netting three years ago and the Potomac River is only a hundred feet away so I have tons of Great Blues nearby.
Where did you get teh rocks and how did you get them into place? ANd what kind of filter do you have on the pond?