By Donna M. Recktenwalt
After extensive effort, you’ve finally succeeded in spawning and hatching out fry from that “special fish.” You’ve even successfully raised a number of them past the wriggler stage and into juveniles. You’re expecting the more or less normal sex ratio of 50/50.
But when they sex out you have almost all males – or all females.
What happened?
Such occurrences in a hobbyist’s tanks are disappointing and discouraging. But in the commercial aquaculture industry they pose a real problem with major economic effects. This has lead to research, and results that seem to indicate that in most freshwater fish, the combination of lower temperature and higher pH normally results in the production of more females than males.
Other evidence, both anecdotal and from research, seem to indicate that developing fry may be influenced by a number of outside factors – the age of the spawning adults, the temperature, pH, and DH of the water; light levels; and the subtle mix of chemicals in the water, resulting from the presence of other fish, of rotting vegetation, and the like.
Working with the Atlantic silverside (Menida menida), B.E. Kynard noted “The sex ratio in fishes that normally have separate sexes can be influenced by the environment.” He noted that sex determination of the fry was under the control of both genotype and temperature during a specific period of larval development. In addition, the spawn from different females varied in their responsiveness to the variable (temperature).
Uwe Romer, working with Apistogramma species, found that keeping fry at higher temperatures usually resulted in the production of more males, and that higher pH values resulted in more females. His research indicates that the best sex ratio distributions for the study species occurred when water temperatures were maintained at about 78蚌 (26蚓), with pH levels about 6.8, during the first month of life.
Friedrich Bitter, speaking at Killie Revue ’96, observed that among the Rivulus species, the gender of fry is determined not at conception, but some time during the first few days or weeks of life. The most balanced ratios of male to female seemed to result when the fry were hatched and raised in water from the parents’ tank.
The above findings may or may not be valid for all killifish.
However, the next time you find yourself with a badly skewed sex ratio among a batch of developing fry, it might be worthwhile to try adjusting the pH or temperature level in subsequent hatches to see if that has any positive effect on the outcome.
— G.C.K.A. Newsletter, February 1997