Nothing, I don't want them spawning in my pond.

But, if you are dead set on having reproduction, they are cavity spawners. Take a 30 gallon barrel, cut one end completely out. Sink it in 4' of water so the open end faces deep water.

www.hoosierpondpros.com

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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).