International Journal of Marine Science2016, Vol.6, No.31, 1-7
5
50 eggs to 2000 eggs. However, the results of the present study are similar with El-Sayed
et al.
(2003)’s findings
which stated that
Oreochromis niloticus
between 42 – 75g spawns eggs ranging from 305- 753 at a time.
Spawning drop-off point
The data for range 3 (treatment 3) was further categorized into classes to determine the spawning drop-off point
since this range had the best egg numbers spawned. The number of eggs for wild strain of
Oreochromis shiranus
had a remarkable drop when the brooders attained an average weight of 132g (18cm) despite the increase in
weight. These results agree with Maluwa (1990) which showed that the number of eggs in the mouth of the
brooders decreased with length of fish above 18cm (170g). The slight difference in weight at which the brooders
in the two studies had their number of eggs decreased might be due to differences in the age of the brooders since
the brooders that were used by Maluwa (1990) were not of the same age with the brooders used in the current
study. The other possible reason that could have caused this difference would be the type of feed given to the
brooders. Watanabe (1985), Finn (1994) and Bhujel
et al
. (2001) reported that spawning performance and seed
production of tilapia are directly related to brood stock nutrition. Despite the difference in weight of the brooders
at the time the number of eggs showed a remarkable drop, the length was similar which shows that length indeed
determines the carrying capacity of eggs by the brooder.
Observations made from the present study show that the number of eggs for both strains increases with the
increase in body weight and length. The number of eggs for wild strain brooders had a remarkable drop at certain
point in time while improved strain brooders continued to spawn at an increasing rate without a drop despite
attaining the same weight at which the wild strain brooders had a remarkable drop. The difference in time of drop
off in number of eggs might be due to differences in age suggesting that the improved strain brooders of
Oreochromis shiranus
attained the same weight as wild strain sooner than the latter, despite having younger age
due to their relative advantage on growth performance as a result of attainment of desirable traits following
selective breeding as reported by Maluwa (2008). The results suggest that the difference in spawning drop- off
might be due to genetic vigor since the improved strain brooders are genetically improved.
Number of eggs between wild and improved strains of
Oreochromis shiranus
The results on eggs spawned between improved and wild strain brooders show that there is no significant
differences (P>0.05). The improved strain brooder with minimum mean body weight ranging from 24g – 200g
(9.5 – 20cm) with minimum mean weight of 40g (12cm) spawned minimum mean number of 257 eggs while,
wild strain brooder with minimum mean body weight 41g (12.cm) spawned 256 eggs. The improved strain
brooders with maximum mean weight of 133g wild (17cm) spawned maximum mean number of 495 eggs and
wild strain brooders with maximum mean weight of 134g (17cm) spawned maximum mean number of 523 eggs.
The number of eggs spawned by wild and improved strain though not significantly different but after the entire
spawning period, improved strain would spawn a lot of eggs because of the delayed drop in number of eggs being
spawned.
The results further show that all the independent variables for improved strain of
Oreochromis shiranus
had more
influence on the number of eggs spawned than predictors for wild strain of
Oreochromis shiranus.
Conclusion
The present study shows that body weight and length have an effect on number of eggs spawned by both wild and
improved strains of
Oreochromis shiranus
. It has been observed that as the brooders increase in body weight and
length, there is an increase in number of eggs spawned which starts to decrease with further increase in weight.
There is a positive correlation among the three variables (standard length, body weight and number of eggs)
within each strain but there is no significant difference in number of eggs spawned between wild and improved
strain brooders of
Oreochromis shiranus
.
The study has also established that wild and improved strain brooders weighing >100g (17cm) is the best category
for selection for breeding programs. Brooders ranging between 124 – 138g for wild strain
Oreochromis shiranus
