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Try this article. It helped me understand a little. Sorry the attachment didn't work. So here it is.
Sexual dimorphism – a difference in physical appearance between males and females – is commonplace among fish. True, not all species show this distinction. In some, especially those that spawn en masse, where the girl- and boy-fish never pair off, the two sexes look basically the same. In many other species, sexual dimorphism is delayed until the fish become older. The juveniles typically look like pre-fabricated clones. The only way to sex them is to cut them open and inspect the parts.
And, of course, as I indicated at the first, there can be sexual dimorphisms in body size. In these instances it is normally the female that outgrows the male. Several of our local gamefish show this trait. Muskies and pike do. So do largemouth bass and walleye.
The reason for the size difference is simple. Energetically speaking, it’s the females that bear the greater reproductive burden. Since the vast majority of fish fertilize their eggs externally (i.e. outside the body, in the general water column), both males and females must shed thousands of gametes (sperm or egg cells) into the water in order for reproduction to be successful. But shedding thousands of energy-poor sperm is one thing, shedding the same number of energy-rich eggs is quite another. Reproduction costs the foot-loose and fancy-free males comparatively little. It’s the females that are left holding most of the energy bill.
Usually, the size difference between males and females of a young age is not all that great. However, because the females grow faster than the males, the size difference increases with age. In most cases the females are normally no more than twice the size of males. In a few special cases, however, the size differential can be huge.
Here are some generalizations and interesting facts that may help to identify the sex of a largemouth bass. Male largemouth bass rarely exceed 16 inches, so anything over 16 inches is probably a female. At 5 years of age, females are about twice the weight of males. The male largemouth bass prepares a bed or nest for spawning. Male bass caught during spawning season commonly have abrasions on their lower jaw and frayed fins as a result of this nest building. Also, male largemouth bass guard their nest and fan the eggs with water to provide oxygen and prevent diseases. They even guard their fry (young) until they are about one inch in length. Male largemouth bass rarely feed while guarding a nest, but they will attack anything they think is threatening. This explains why males attack a lure while bedding (on a nest). The female may stay nearby or may swim a short distance away for up to a day. A single nest can contain up to 43,000 largemouth bass eggs. I hope this will help you identify the sex of a largemouth bass next time you catch one
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