Sex Reversal, Growth And Aspects Of The Biology Of Oreochromis Niloticus (L.)

Abstract

Some aspects of the reproductive biology, sex reversal and growth of sex-reversed O. niloticus are reported. typical O. niloticus displays reproductive behaviour of oreochromine tilapias. The male is territorial and polygamous, and is responsible for the construction, cleaning and protection of the nest. Egg-laying is sequential and each batch of eggs is fertilized before the next one is laid; fertilization takes place before, during or after egg-collection into the mouth of the female for brooding. There was a direct positive correlation between incubation and ambient temperature. No relationship period was found between brooder size and number of young brooded. A positive and significant relationship was, however, between size of brooder and size of young brooded. The fish began breeding at the age of about SlX months and- at a small size' (smallest brooder was 9.4 em (TL) and 13.6 g). synthetic androgens: 17 alphaalphalevels ethynyltestosterone methyltestosterone (ET) and 17 (MT), were used at two dose each: l.e. ET at 40 and 60 and MT at 15 and 30 mg

kg of feed. Fry were fed-with each of the hormone ln hormone-forti fled diet for 21 days ln one Series of experiments, and for 28 days in another, at body weight per day.

ET at 40 mg kg of feed for 28 days resulted in 1001. males, while the same hormone at the same fed for 21 days yielded 97.41. males. 97 . 3i'. ma 1es obtained -1 when the hormone was used at 60 mg kg feed for 28 days, and 88.11. males during the lasting 21 days. On the other hand, MT at dose of 15 and 30 mg -1 kg of feed administered dosage were of treatment levels both in for 28 and 21 days, respectively, 86.71. males. -1 kg resulted 77.11. males were obtained in the 30 mg treated for 28 days while only 60.9:<' males realized the fry treated with 15 mg k -1 'g hormone for 21 days. The controls recorded 64.5:<' day) and 61.51. (28-day) males. All treatments - 15 21-day and the controls, had sex ratios significantly differed from the expected 1: 1 0.05) . group were of the (21- except which ( p < same hormones, it was realized that ET - 40 and 60 Comparing the effects of different doses of the -1 kg were equally efficacious in O. niloticus. mg -1 kg however. gave better results than MT -1 kg Incre.asing the duration of treatment from 21 28 days resulted In ins i9 n~.fii,.can t lncreases per-cen tages of males. At the" dose levels ethynyltestosterone was more potent methyltestosterone -~s all experiments conducted the former hormone ~ieided sex ratios mg 15 mg to used, than uSlng higher those of the controls (p < 0.001 ) sigrlificantly than while only one MT experiment (MT - 30 21-day) had sex ratio significantly higher than those of controls (p < 0.05). Studies on the effects of the two hormor,es on growth revealed that, with the exception of MT - 30 mg -1 kg of feed-treated fry which grew significantly less than the controls (p < 0.05), the growth rates of the hormone-treated fry and fingerlings were generally not different from those of the controls throughout the course of the treatment and also during the growth phase. High survival rates were recorded during treatment and no differential mortality occurred either hormone treatment or dosage. During the recovery period, however, mortality was high both In treated and control fish. During the growth phase there very low, accidental mortality