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 init iation of hormone treatment on production of phenotypic male populatio ns was investigated. A population of fry was divided into two lots. Th e 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 lo t (Age II) was held an additional 10 days then graded into the same si ze categories. All groups were given methyltestosterone at 60 mg per k g of feed for 28 days. Initial size was a more significant factor than age in affecting the percentage of phenotypic males observed. The dur ation of hormone treatment while fry were less than 18 mm length was s ignificantly related to the percent males obtained, fish receiving les s than 14 days of treatment had a lower percentage of males.