If there's BG of all sizes in the pond, then maybe there's too much cover? If there's lots of hiding places, then the LMB have to work extra hard to find (and catch) food.

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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).