How do you tell a boy goldfish from a girl goldfish?

Answers:
This is not easy as these fish do not have distinctively gendered genitals in the way that mammals do. Male gold fish, shubunkin and carp are generally more slender in shape than females. Females tend to be chubbier and almost pear shaped.

However, as fish do not eat in the colder months and survive off their fat reserves their shape will be misleading outside the breeding season. In the warmer, breeding months the males develop distinctive spotting around the gills and on their heads. These little bumps are white and quite noticeable during the breeding season. Watching their behaviour at this time will also offer clues to gender.

The male fish try to stimulate breeding by chasing the females around the pond or aquarium. They deliberately bump into the females to stimulate spawning. However, if you wait until you see these signs it will be a little late to start a selective breeding programme. Once the female has spawned the male will quickly deposit his milt over her eggs. Fertilisation only takes 20 minutes and so, by then, it will be far too late for the fish keeper to intervene.

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the girl always has her mouth open

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Garb it's crotch and see if there are any avocados down there.

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the boy goldfish is the one trying to get away from the girl fish,lol

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I'm no expert on goldfish, but, I think in mating season, the male gets stripes on his dorsalfin.

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Goldfish are one of the fish that are very hard to sex. Here is a web link that has info AND pictures to help you with it.

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Here's a link that describes what to look for and includes photos.

http://www.bristol-aquarists.org.uk/gold...

MM

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MM and Barb both have good websites.

Here's another interesting one: http://www.blurtit.com/q711613.html...