I'd like to see what Bill and Cecil have to say. In my opinion, (which could very well be totally wrong) the YP on the bottom is closer to dropping it's eggs than the one on top in the following picture. I think the closer they are to dropping eggs, the more pronounced the "step" is right in front of the anal fin.


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[Linked Image from lh6.ggpht.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).