I have had this pond for 15 yrs. and have been wag... (
Having tried all the methods you mention above to protect my pond from Herons, I have found that:
a) the netting is effective, but, as you say, unaesthetic. However a Heron managed to stick his beak through it and kill one of my largest Koi (maybe 5-6 lbs), but couldn’t get him out to eat him.
b) the monofilament works pretty well - visible, but not as bad as the netting.
c) the sprinkler with the infrared sensor (called a scarecrow) is effective for some bids, but not others, but excellent at soaking you or you friends when you forget it’s on. Unfortunately the ones I’ve bought which are $70-$80 only seem to last a year or two before dying.
d) my local fish store suggested a floating alligator head on a monofilament tether. They’re made of thick styrofoam, painted to look exactly like a gator, but obscenely expensive (~$40-$50) for a piece of styrofoam. The story is that a Heron recognizes the gator head and won’t go near it. Amazingly, it has worked very well except for one recent “visitor”, who apparently hadn’t gotten the message.
e) my latest plan, which I’m still considering, is to string a regular electric fence single wire around the perimeter about a foot off the ground, powered by a regular electric fence charger. Not pretty, but I’m betting it may be effective.
And even if you were willing to shoot such a majestic creature (I’m not) after he’s eaten your prize fish, don’t do it! There’s a huge Federal fine. A man I used to buy aquatic plants from had a neighbor that had outside ponds and raised Koi commercially. As the story goes, one day, having reached his limit, he shot a marauding Heron, who was able to fly up into a tall tree before expiring and whose dead carcass was hanging over a limb - too high to get down, but plainly visible if you looked up. Apparently, the shooter lived in mortal fear of the charge and the fine for a year or two, every time a wildlife officer came onto his property, until the carcass finally fell from the tree.