Effects of initial age and size on sex reversal of Oreochromis niloticus fry using methyltestosterone

The influence of age and size of Oreochromis niloticus fry at the initiation of hormone treatment on production of phenotypic male populations was investigated. A population of fry was divided into two lots. The first (Age I) was graded into four size categories: 11 mm and less, 12–13 mm, 14–16 mm, and greater than 16 mm total length. The second lot (Age II) was held an additional 10 days then graded into the same size categories. All groups were given methyltestosterone at 60 mg per kg of feed for 28 days. Initial size was a more significant factor than age in affecting the percentage of phenotypic males observed. The duration of hormone treatment while fry were less than 18 mm length was significantly related to the percent males obtained, fish receiving less than 14 days of treatment had a lower percentage of males.