We have had a large number of fish in our farm's pond die. What can be done...
Can you share more detail? I'm not certain what you mean by "improve the fish population." Are you looking for a way to change the population structure of the fishes that remain in the pond or add new fish? Or are you looking for ways to reduce the likelihood of future kills and thus improve the habitat for fish?
Almost all substantial, rapid fish kills are caused a sudden loss of dissolved oxygen. That can be caused by a sudden inversion (i.e., "flipping") of stratified pond water in the summer and often follows a sudden rain storm. Did your fish kill follow a big rain event? An oxygen crash can also be caused by applying herbicides too aggressively once the water becomes warm. Did you recently treat the site to kill algae or aquatic plants?
There are several ways to reduce the likelihood of summer fish kills. One is to simply manage aquatic plants early and conservatively, only using small spot treatments once your surface temperatures enter the low to mid 70s°F. Perhaps the most effective is to initiate a program of diffuser aeration in the spring (it's too late to do so practically for this season).
Seasonal fish kills, especially summer kills, rarely kill everything. The typical approach to improving fish populations themselves is to allow them to recover on their own over time, tweaking/shaping the population using selective harvest or adding fish through some supplemental stocking if you feel it necessary.
Check out the Ohio Pond Management Handbook that can be downloaded using links on this page: http://wildlife.ohiodnr.gov/species-and-habitats/pond-management. It has a nice discussion of fish-kill issues beginning on page 29. It discusses supplemental fish stockings on page 10. You can find a summary of appropriate fisheries management strategies beginning on page 14 and a table to help you shape the fishery with some simple harvest policies on page 16.
Please feel free to contact me directly with any additional, specific questions related to this issue.
. . . And good luck out there!