Mika, lovely springtime shot of your garden there, except for the heron of course. I have a pond too, and when we first moved in, a white heron came by 3 days in a row and wiped out all the fish before I knew what was happening. (we had not had a pond before) Very upsetting as it turned out the fish were a fancy exotic type and would have been pricey to replace.
I re-stocked the pond with inexpensive goldfish, and I have a heron sculpture (cheap tinny thing, looking for something better but they're expensive!) that keeps the other herons away - they are evidently territorial birds and if they see another heron at a pond they will not land. I don't think I've had any more herons since putting out the sculpture and my goldfish are now at least 5 years old, and 9 inches long.
One other trick is to never feed the fish any food that floats. They learn to come to the surface when they see someone/something and this makes them more vulnerable to hunting birds. I'm lucky because between the insects and the algae in my pond I don't have to feed mine at all.
