What is the water flow rate? Are you sure that you have adequate biofiltration? Does the pond have an accumulation of organic debris from overwintering? This could cause a low oxygen problem.
Based on your figures, you have a stocking density of one fish for approximately every 50 gallons. Without an adequate water flow rate and biofiltration you can expect problems. These problems may only get worse as the fish grow in size.
"Everybody is ignorant, only on different subjects." - Will Rogers