For most of the year distinguishing between male and female common carp is not straightforward, but there are a number of visible clues which can be used to make an educated guess.
As a general rule males are slimmer than females, but the actual body shape varies between genetic strains, and to an extent is a function of diet and exercise regime, so this can be difficult to ascertain with only a couple of fish in the bank at any one time.
The nature of the genital opening can also offer some clues as to the sex of a fish, as in males this is generally concave and not particularly noticeable, while females it is more noticeable, and even slightly protruding. While these two features combined can allow an educated guess, it is not generally possible to be certain about the sex of a carp outside the breeding season.
In the run up to spawning however, there are a number of physiological changes which result in externally visible differences between the sexes, for much of the year the males are generally slimmer, but in the weeks leading up to spawning the abdomen of the female carp becomes noticeably distended as the eegs are swelled ready for release. The genital opening also becomes distended, protruding from the body and looking like a small fleshy tube. Meanwhile, male carp usually develop tubercles, small raised lumps, most noticeable on the head and gill plates. At this time the sexes even feel different, with the females being slippery and the male feeling noticeably rough to the touch.
The final, and conclusive, piece of evidence is the release of the reproductive products, with females releasing eegs and the male releasing sperm.
After spawning the tubercles on the males quickly disappear, but the genital opening in the female can remain visibly distended for a short period, and having released all of her eegs have a flaccid, saggy abdomen for a few weeks.
But very soon they regain condition, and it becomes very difficult to tell them apart again .. until next year
