Photographs by Paul Winters
Firstly, sexing young and juvenile goldfish is virtually impossible, you have to wait until the fish reach breeding age (usually one year old).
Secondly, sexing adult goldfish out of the breeding season is difficult, because the sexually distinguishing features only develop during the breeding season (in the spring time). That said, females may be deeper in the body than males.
During the breeding season, the following physical features develop:
Male breeding tubercles
Illustrated above are the small white breeding tubercles that develop on the gill covers and the leading edges of the pectoral fins of males. It has been suggested that these may play a role in stimulating the female to spawn during chasing.
The female vent enlarges slightly and appears slightly out-turned (convex), as shown above.
The male vent remains normal (slightly concave) in appearance, as shown above. (The vent is the common/single orifice for reproduction and excretion).
Put quite simply - in the breeding season, males chase after females to induce them to spawn. Easy? No. Why? Because males will chase each other in the absence of females. Use the physical features as well as the chasing behaviour to tell the sexes apart!
© Bristol Aquarists' Society