Nancy, welcome to PB. It is unlikely that the turtles are a problem. They are about 90% scavengers and/or vegetarians. On rare occasions, a turtle might be able to catch a healthy fish.

I'd spend some time identifying the balance of fish. Do a lot of fishing and keep a log.
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It's not about the fish. It's about the pond. Take care of the pond and the fish will be fine. PB subscriber since before it was in color.

Without a sense of urgency, Nothing ever gets done.

Boy, if I say "sic em", you'd better look for something to bite. Sam Shelley Rancher and Farmer Muleshoe Texas 1892-1985 RIP