Three different doses (40, 60 and 90 mgMT/kg feed) of 17α-methyl testosterone (MT) were administered to Oreochromis andersonii. Growth, reproduction and sex ratios were evaluated. The fish raised on 60 mgMT/kg feed had the highest final mean weight and this was significantly different (P<0.05) from other hormone doses. Additionally, the 60 mgMT/kg fed fish had the lowest fecundity and reproductive effort. Histological examination of the gonads did not show any differences between the control and MT fed fish. The gross margin ratio (GMR) of the control group was the highest (P<0.05). The proportion of males in MT treated groups deviated significantly from the control group although the percentage of males obtained from the 40 mgMT/kg feed (93.4%) and 60 mgMT/kg feed (94.4%) fish was insignificant (P>0.05). Furthermore, the highest dose used produced a significantly lower (P<0.05) male proportion (79.3%) than the other treatments. The anabolic effect of androgen MT diminished over time.Growth, Reproduction and Sex Ratios in Oreochromis Andersonii (Castelnau, 1861) The fish were provided with the prepared feed twice a day (10:00 and 15:00 hrs) daily except Sundays calculated at 5% live body weight of the fish. Fortnightly, a total of 160 fish (20 from each tank or 40 from each treatment) were weighed and lengths (standard length (SL) and total length (TL)) measured according to the procedures described by Skelton [13] after the fish were anaesthetized in 5g/l crude clove (Unilever South Africa Foods (Ptv) Ltd) to reduce stress. The experiment lasted for 119 days.At the end of the experiment all the fish were caught and their weights and lengths (SL and TL) determined. In addition, 20 fish (10 of each sex) were taken to the laboratory for gonad and liver extraction and weighing for maturity stage [14] (Table 2) and hepatosomatic index (HSI) calculation. The fish were sexed by checking the genital papillae with the help of the magnifying glass (Figure 2). The gonads were then fixed in 10% formalin in individual vials for later egg counting and diameter determination on an ocular scale mounted to the microscope (Nikon NFX-35) (500x power) after blotting them on paper.Further 20 gonads (10 of each sex) from every treatment were prepared for the histological examination by fixing them 10% formalin solution. They were then embedded in paraffin wax after dehydration in series of methanol (70, 80, 90, 100%) and sectioned in 3 µm and then stained with Haematoxylin and Eosin before they were mounted on slides with DPX mountant. The slides were then examined on the Olympus microscope (150X power) and a picture taken with Kodak camera. Sex ratio determinationOreochromis andersonii breeders were stocked in 250 m 2 semiconcrete pond at the sex ratio of 3:1 (female=71.191 ± 12.093 g: male=174.857 ± 19.828 g; mean ± SD). After twenty one (21) days, the eggs were recovered from the mouth of the four brooding female fish and were immediately taken to the indoor hatchery for incubation in the jars at 28°C with the help ...
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