Well you can put a little fish house in the bottom, just made from a large enough flat rock, and two bricks. If your fish are small enough to fit under there. If they are large, you can get an even larger flat rock, and use cinder blocks instead.
However, I am not entirely sure that goldfish/koi will recognize the threat and run and hide. I am not sure they can tell the difference between a big bird, and a person. Even if they can tell the difference between the big bird and you yourself, they may still see it as someone that might throw some food in the water.
And then come right up to him.
So the best thing you can do is build one of those PVC based nets, or build a nice enclosure over the top of the pond. Seems to me like if the birds couldn't see it from the air, they wouldn't come to it.