Turtles did not cause the fish loss. Turtles basically only eat dead, sick, weak or almost dead fish. Turtles are basically clean up or cleaner crews and not basically predator killers.

Do the old original plans specify a maximum depth and depth of the upper areas? Compare those depths to the current depths. This might allow you to calculate how much muck accumulated each year.

Without knowing or seeing the pond, I suspect that summer fish kills or winter fish kills or both types of fish kills have occurred because the pond is surrounded heavily by trees and the annual deposits of leaves from those trees takes it toll and contributes way too many leaves that decay and fill the pond basin with decomposition that robs oxygen from the water and periodically suffocated the fish during the last several years. Lots of trees closely surrounding a small pond causes it to age, degrade, and fill in much faster.

If your goal for the pond is to do what the previous owner did as do nothing to the pond other than enjoy it, then all is well. If you want it to produce a quality fishery then there will be numerous challenges ahead some will likely be pretty costly. Firstly the maximum depth should be determined. 2. see if you can probe the bottom in the upper areas to determine the depth of the sediment. 3. All the trees surrounding the pond are heavily contributing to its current degraded condition. It will no doubt soon begin to produce huge amounts of plant growth. The previous owner could have had a pond company come in before the sale and chemicalize all the excessive plant growth. Be prepared do deal with that possibility next spring. A consultant will want to come in and install costly aeration to minimize fish kills. Aeration system will slow the aging but IMO the damage has been measureable and should be significantly mediated.

IF it were my pond I would start investigating cost of draining, removing all the muck and some trees and restating the entire pond. 60 years surrounded by trees causes lots of damage to a shallow smaller pond. In lakes the trees are not as big of a problem due to water surface water volume ratio. Maybe resize current pond to a smaller pond what would be less expensive than to maintain than the original size. A rebirth. The aeration is an investment and will be beneficial in the old or rebuilt pond. Building a new smaller pond away from tree leaf deposition zone may be a cheaper better option that has a high quality fishery than rebuilding the current 1.5 acres. The old pond could be used to produce lots of forage foods for the new smaller pond. Fish kills might only happen occasionally in the old pond which is not a bad thing for the purpose of a forage fish pond.