Herons and egrets at the edge of a pond create a picture of tranquility.
That is unless these and other fish-eating birds (gulls, terns,
kingfishers, diving ducks, pelicans, cormorants and ospreys) are dining
on your prize koi.

First try scare techniques, then exclusion.

While they may be unsightly, physical barriers can deter most
fish-eating birds. For small ponds, complete screening with bird netting
may effective. Properly spaced monofilament lines suspended over a pond
may exclude gulls (every 4 feet), mergansers (every 2 feet), and herons
(every foot). Perimeter fences provide some protection from wading
birds.