That is a fairly shallow pond of only 4'. It is possible that it experienced drought at some point where the surface area got too small to support healthy fish. There is also the possibility of long-term freeze killing them also.
My pond drops almost 3' in the summer which is partly evaporation and some leakage, and if it were only 4' I would not expect the fish to survive.
I don't want to alarm you, so maybe observe for the first year or two before making changes, but you certainly could stock some new fish this year.
does it look like there is an easy way to raise the depth, perhaps raise a dam height or overflow?
