Heron vs Pond solutions?

Question:

I built my pond late last summer and stocked it with 12 goldfish and 2 koi. Every thing was fine until this spring I found a Blue Herron standing in my pond (I live on a lake and Herron are quite common). Needless to say my fish are gone. Is there anything I could put in the bottom of the pond so that the fish could hide and get away from the Herron? Or do you have any other solution? I bought a net and put it over the pond in the fall to catch the leaves but do not want to leave it up in the summer. THANKS for your help in this matter. Charles