Fish Survival Rate

We built a pond in northern VA last year, about 15' x 22', 2 1/2 feet deep.
Kept at least an opening in the pond through the winter but only 13 of the
17 goldfish survived the winter. No idea what happened to the 4 that didn't
make it, didn't find any remains, etc. when we cleaned the pond a few weeks
ago. Is this unusal for the first year, the fish range in size from about 1
1/2 inches to about 5 inches, the bigger ones are still with us? We had a
lot of string algae all through the winter so took that as a decent sign
that there was plenty of oxygen in the water, didn't feed the fish once the
water dropped below about 55 degrees? Ideas.