Thus the use of the word "likely" in my picture descriptions. I'll be the first to say there is some overlap in general characteristics between male and female RES. The urogenital opening appears to be the most consistent difference that I am seeing along with other general stuff, one really needs to look at the whole fish to get a good picture. I plan on putting this to the fillet knife test next spring to see how accurate this really is.
In general male RES have wider and more brightly colored margins on their ear tabs, slightly longer pelvic fins, and the urogenital opening is roughly the same size as the anal opening.
In general female RES have a narrow and much more dull colored margins on the ear tab, slightly shorter pelvic fins, and a much larger urogenital opening size relative to the anal opening.
I have seen both what I would consider male and female RES to have brightly colored bellies, and creamy colored bellies. I would not use that as a sexual differentiation characteristic.
On the length of the pelvic fins, in general with what I believe are males the pelvic fins will completely cover the urogenital opening most of the time when laid flat against the body. With females the pelvic fins will not completely cover the urogenital opening when laid flat against the body most of the time. Body condition will affect how the pelvic fins appear when laid flat against the body and should always be considered. This did play a role in what I needed to do in order to get pictures of the urogenital openings, some I had to spread the pelvic fins apart, others I didn't.
