100% accuracy in determining gender in sunfish is a tough job. Even with BG who have much better sexual differentiation than RES, people still make mistakes. "Sneaks" and even the occasional female BG with scale tipping are where mistakes are made. Telling the difference between sexually mature male and female BG is fairly straight forward most of the time, even outside of the spawn. Is sexing BG foolproof? Nope.
I will say that examining the urogenital openings in RES, plus looking at other characteristics, some of which clearly overlap, one could divide RES into three groups, males, females, and "tweeners" (a not sure group). One could get close to 99% of the male and female groups right by leaving the "tweeners" out of the mix.
Is that good enough for a single sex pond? Probably not, but it would be helpfully if one wanted to target harvesting one sex or the other to skew the sexual balance in a pond. Like mentioned at the beginning of this thread, RES are one of the most difficult sunfish to sex. Of the 250+ pre-spawn RES I have caught this spring, the slight differences in the urogenital opening appears to be the best indicator of sex that I have seen so far. I will say that seeing it in person is much more clear than any of the pictures I have posted.
My RES are in full spawn mode now, the last three female looking RES that I have caught no longer appear gravid. We will have to wait until next spring to put my urogenital opening hypothesis to a test with a fillet knife. I do currently have a large year class of 6" RES that will be larger and prime for checking for the presence of eggs next spring, a fish fry will follow.
Ryan, if the water is warm enough during spring break next year you are invited to come down and test my urogenital hypothesis out, I would love to have someone with much more expertise in this area looking at the urogenital differences that I am seeing.
