On Sexing our Fish


     In deciding what fish to move to our 300 gallon tank I needed to know the sex of each fish. I wanted to keep a mix of males and females so that they could reproduce and continue the process of setting up a fully integrated aquaponics system here at Windward.


      I had tried to sex the fish by using morphological cues such as coloration and fin shape to no avail. This method is very ineffective as Tilapias are not sexually dimorphic, meaning that each sex looks similar both in coloration and body structure. The only real way to tell is by examining the genitalia of the fish, which is also quite difficult. Males will have 2 openings, the anus and the urogenital opening, where urine and semen are released. The females have 3 openings, the anus, the oviduct, and the urinary pore. For more details on tilapia genitalia, Click Here.

     I found resources online that had pictures and tips to sexing the fish, although most said that even trained professionals still get about 5% wrong. The one tip I found very helpful was using a dye of some sort that would collect in the openings and make identification easier.


      With these resources handy I took an amateur go at identifying the sex of our fish. I enlisted the help of Lindsay and Ryan as extra pairs of eyes so that they could catch something I might have missed.

      As we worked through the tank it became clear, unless of course we misidentified some fish, that we had a group of 10 males. This struck me as quite odd because one fish had exhibited a female behavior of holding what we thought at the time were eggs in its mouth. It wouldn't eat when feed was offered, had changed to a darker color and remained under the rock structures in the tank.


     Then in a couple of days when we would expect fry she would be swimming around and eating again. This was most likely due to the water chemistry of the tank being in a state of heavy flux.

     In talking to Garrett, who got us the fish, he said that it was most likely a misidentification of sex. This isn't much of a surprise to me as this was the first time I had tried to determine the sex of fish who had their genitalia inside their bodies. Based on behaviour, which is my strong suit, I strongly believed that we had a mix of males and females because of how the different fish interacted. Two males would lock jaws and have a pushing contest, where a mixed sex pair would have the male chasing the female. Both of these behaviors were exhibited at different times.

      With all the fish identified, or misidentified, I could then choose which ones to keep in the 50 gallon tank to continue as breeding males and which ones would be moved to the 300 gallon tank to grow into fish to harvest. I chose to remove the alpha male as it was becoming very aggressive to the other fish to the point where it was becoming dangerous for those fish. In total I moved 7 fish up to the 300 gallon tank and kept 3 in the 50 gallon tank so that we could acquire some females and keep them in the 50 gallon tank with our breeding males.


     It has been interesting to watch the new power dynamics that have happened since removing the clear alpha male. Although the power struggle is nowhere near as bad as before, which was most likely due to overcrowding, there still has been some fighting. However, all the fish now have their own rock structure territory and are much less aggressive. The alpha male in the 300 gallon tank has been much less aggressive, which is directly due to the fact that it has plenty of room.

     After further behavioral observations I decided to move back the alpha male back to the 50 gallon tank as I know that it had definitely exhibited mating behaviors and is a good candidate along with the large female to be the strong mating pair to overwinter here at Windward. Although we were concerned about the aggressive behavior of the fish I feel it should be a lot less because the tank isn't overcrowded as before.

Notes From Windward - Index - Vol. 71