Abstract:

The histology of the gonad of Nile perch, Lates niloticus was examined in order to study
the early gonadal development and sexual maturation in a natural population from the
Ugandan waters of Lake Victoria. The study describes the anatomical and histological
morphology of L. niloticus gonad as was observed in the males and females. The samples
examined were collected over a period of four months (October 2005 - January 2006).
All the fish individuals (178) analyzed with histological, microscopic and macroscopic
techniques were found to be either undifferentiated, males with testis or female with
ovaries. It was found that the Nile perch gonad did not exhibit any form of intersexuality
within the size range sampled (4 cm -110 cm, SL) and period of the year when the study
was undertaken. The gonadal development pattern concluded was differentiated
gonochoristic where individuals develop from an ovarian or testicular juvenile pathway.
The size at which the first indication of sex differentiation for females and males was
defined at 9.7±3.3cm and 11.13±2.7 cm, SL respectively. The oocyte development
followed through six distinct stages from oogonia (9.8 ± 3.0pm), chromatin nucleolar
oocyte (16.2 ± 3.3 pm), perinucleolar oocyte (47.86± 18.3 pm), primary yolk vesicle
oocyte (122.6 ± 38.8 pm), secondary oocyte (260.9 ± 61.2pm) and the tertiary oocyte
stage (475.5 ± 70.7pm). Broodstock collection from the wild for induced spawning in
aquaculture should therefore aim at females with egg size of 475.5 ± 70.7pm. In the
males the type of spermatogenesis was cystic and the testicular structure was lobular.
Ripe males exuded copious amounts of sperm and for induced spawning it would not be
necessary to sacrifice the males in order to obtain milt as is done in other species under
culture like the African catfish, Clarias gariepinus and the American channel catfish,
Ictarulus punctatus. Macroscopically the male had only the anal and urogenital openings
just anterior to the anal fin, whereas the female had a genital orifice separate from the
urinary opening. The sexual dimorphism exhibited by the Nile perch is key in broodstock
management.