Turtles normally don't impact the population of healthy fish in a pond. I say normally because there is nothing that is 100% when it relates to ponds.

What species of turtle? As far as I know, the only turtle that targets live fish is the Alligator Snapping Turtle, and they aren't very common.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).