Histology of gonads of Oreochromis karongae was undertaken to study internal cell
characteristics during maturation. This study was necessitated by low spawning output of
the fish species. Several oocyte stages, ranging from primary forms to vitellogenesis,
suggest that the maturation was generally succesfully attained in the fish ponds.
Pre-vitellogenesis oocytes (oogonia to perinuclear stage) and more advanced vitellogenesis
(primary vesicle to tertiary yolk vesicle) oocyte stages were all found in the same gonads.
However, there were some discontinuities observed during stages 3 and 4, suggesting selective
maturation. Failure of gonads to mature normally is attributed to an ecological crunch that
was in a previous study associated with environmental factors. Atretic oocytes were also
recorded in the same gonads, a sign that some oocytes failed to mature normally. This
indicates insufficient stimuli for normal gonad development. Several stages of spermatogenesis
(spermatocytes, spermatid and spermatozoa) were also found in the same gonads. Selective
recrudescence was more pronounced in O. karongae because generally less oocytes attained
final maturation stages compared to Oreochromis niloticus and other tilapias. This could
be the main reason for low natural breeding that has been observed in both wild and captive
stocks, and led to the abandonment of its use in aquaculture. This study corroborates
findings of previous studies that depended solely on external gonad characteristics.
Histology provides conclusive evidence from internal cell characteristics that other
techniques are unable to show.