| dc.description.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 | en |
