Per Bill, easiest right before spawn for sure, there's really no way to miss a gravid female vs fusiform male, and of course milting is 100% verification. I'd agree 90-95% accuracy with urogenital opening per Cody - I separate them based on initial review regardless of season, but cage them and always check again in March/April right before the spawn to make sure I was right. Again, 90-95% of the time initial estimate was correct. When in doubt, I toss them in the male cage.

Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau

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