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NFC: sexing sunfish and alternative male mating strategies
This is not entirely practical, but rather academic. Enjoy the natural history if you are inclined...
In many sunfish, bluegill in particular, there are alternative mating strategies among males. These alternative strategies confuse the sexing issue because one male mating strategy is to mimic female behavior and color.
There seem to be two genetically coded male breeding strategies: parental versus sneaker males. The parental strategy is to be a large, territorial, nest-building, brood-tending male. It is the strategy most of us would consider the "normal" strategy, and it requires males to delay sexual maturity for many (7-9) years.
The sneaker shirks all the energetic investment of territory and brood defense. Instead, sneakers obtains fertilizations by rushing in on "legitimate" fertilizations taking place in the paternal male's nest, and they add their "sneaky" sperm into the mix.
Furthermore, there is a developmental switch in the exact nature of the sneaker's method. Young sneaky males 2.5-5 years, I think it is, simply hang out near paternal male nests and dash in as described above. Older sneakers are even more sneaky: they mimic female behavior and color pattern, and even school with roving bands of reproductive females. The deception of looking female-ish allows them to enter male nests with little effort and then they have much less contended access to paternal males nests during otherwise "legitimate" fertilization events.
Thus while parental males are easy to tell by color and nesting behavior, females may be less easy to discriminate from the sneaky males who act as female mimics. Girth is probably the best guide in this case.