Identifying the Gender of an Oscar Fish

Is it possible to sex Oscar fish? If only I received a £ for every time somebody asked me that question, I'd be holidaying in the South of France every year. Joking aside, of course, there are ways that you can determine the sex of an Oscar fish, however, one way that is absolutely not possible is by looking at them. Oscar fish are what we call monomorphic, this means that the male and female look exactly the same, in size, shape and colouration. Unlike many other species of fish where the female and male are very different, Oscars are absolutely identical and therefore completely impossible to sex by visual means.

Determining the sex of an Oscar fish takes time and patience. It cannot be done when the fish is still a baby. Part of the fun in establishing a pair of breeding Oscars is watching them grow and interact with each other as they form a pair. A lot of patients is required and you will have to wait approximately 12-15 months, well over a year before you will really get the chance to determine the sex.

If you are fortunate enough to have a male and female Oscar that have formed a pair then you can observe them closely when they lay eggs for the first time. It's quite obvious which is the female, she is normally swimming around in a circle with her underside almost touching the surface where she is laying eggs, normally a flat rock or surface. The male Oscar will be doing very much the same but will be following behind depositing milt over the eggs. Milt is the semen of the male fish.

The male and female Oscar's sexual organs are quite different, the females eggs tube will protrude approximately 0.5 cm from around the same area as her anus, you can't miss it. However the males sexual organ is not so obvious and is quite different, again you will definitely be able to tell the difference. The males sexual organ can be best described as resembling a very small spike.

Oscar fish sexual organsThe diagram to the left shows you the difference between a male and female when looking at their sexual organs from underneath. You can see that the male has two holes, one is their sexual organ, the other is his anus, both holes are approximately the same size. On the other hand, the female also has two holes, but one hole is bigger, this is where the eggs tube protrudes from.

Remember that female Oscar's will lay eggs whether there is a male in residence or not. If you observe your Oscar laying eggs but the other Oscar is not exhibiting any obvious breeding behaviour and is not showing its sexual organ then you can either assume that you either have two females or a male Oscar that is basically not interested in fertilising the eggs. I myself have had quite a few female Oscar's that have laid eggs quite happily on their own, however, there has not been a male who has been willing to fertilise them. I would probably say this is more of a common occurrence than you actually having a successful breeding pair.

The other way to determine the sex of an Oscar is to carry out a procedure called "venting" This procedure means that you physically examine the fishes sexual organs to determine the sex. Professional breeders will often use this procedure in order to obtain a male and a female without having to wait months and months. However, unless you are an experienced fish keeper and are going to breed Oscars professionally then this is not something we recommend you trying as you could actually cause the fish harm.