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How to Tell the Gender of Freshwater Fish

In many bird species, the male --- at least during the breeding season --- is more flashily plumaged than the female. Male mammals often display characteristic physical features that immediately set them apart, whether it is the antlers of a bull moose or the mane of a male African lion. With freshwater fish, distinguishing between the sexes is far more difficult: Many species show little exterior difference, and keying out sexual organs is supremely difficult for the layperson. In a number of species, however, a little knowledge of sexual dimorphism --- physical differences between the sexes --- and reproductive behavior is sometimes enough to discern male from female.

Instructions

- 1
  
  The females of a number of gar species are larger than the males.
  
  Compare the sizes of fish within a species. Quite a few freshwater fish differ in size between the genders. For example, the female alligator gar --- a species of enormous, carnivorous fish native to the drainages of the American South --- tends to be larger than the male. The same pattern holds for the freshwater drum, while in many cichlid species it is the male that is the bigger sex.
- 2
  Look for differences in body shape. The males of Pacific salmon species develop hooked jaws as they mature; the curved shape, contrasting with the straighter jaws of the females, becomes more pronounced as the adult salmon head upriver to spawn. In addition, during the same period, the male salmon often become humpbacked in appearance.
- 3
  Watch for gender-distinguishing courtship behavior; the mating ritual, after all, is usually one of the surest ways to identify genders. The males of certain species of sunfish, for example, like the longear sunfish of the Mississippi and Great Lakes drainages, produces grunting sounds during courtship. They also assemble nests and then display in an attempt to convince prospective females to lay their eggs there.
- 4
  
  A number of cichlid species exhibit the parenting technique called mouth-brooding.
  
  Watch for gender-distinguishing parental behavior. You may see an adult fish take up its young inside its maw --- a parenting technique called "mouth-brooding." Depending on the species, the mouth-brooding parent may be the male or female; for example, in many cichlids, it is the female fish that displays this behavior.

Animals & Science