My guess is strictly age. They are spawned by man mixing eggs and milt, and I'd venture to guess that happens at least once a week. There is a high demand for them as food fish, and they can be grown in an aquaculture setting, so just like tilapia, they want to harvest them every week.

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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).