The Condition factor, Feeding and Reproductive Biology of Oreochromis niloticus L. (Pisces: Cichlidae) in Lake Beseka (Metehara) , Ethiopia.
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Date
2005-06
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Addis Ababa University
Abstract
The condition factor , feeding and reproductive biology of O. niloticus from L.
Beseka , Ethiopia , were studied from samples taken during November 2003 to
November 2004. The length - weight relationship of 1028 fish ranging in size
from 4 cm to 24 cm TL and from 6 g to 140 g TW was computed . The
2 g relationship was curvilinear, and described by TW = 0.0546 X TL ! the
regression equation for females was TW = 0.1005 X TL2'3 and that of males was
TW = 0.0742 X TL2'7. Fulton’s condition factor (FCF) was calculated for the total
sample as well as for both sexes separately . The mean monthly FCF was highly
significantly different between sampling months and sexes and the sex by month
interaction was insignificant . The mean . i SE ranged from 1.2 ± 0.06 to 2.0 ±
0.04 for females whereas from 1.3 ± 0.01 to 2.6 ± 0.04 for males. The stomach
contents of 690 fish ( 200 males and 490 females ) were examined . The stomach
contents were found to be composed of diverse items. A total of 22 algal genera
belonging to three families! Cyanophyceae ( blue greens), Chlorophyceae (green
algae ), Bacillariophyceae (diatoms) , were identified . Among the diatoms
Navicula, Cyclotella, Cymbella and Thallasiossira, from the blue greens
Microcysts, Osciiiatoria and Anabena and from the green algae Cosmarium,
Botryococcus and Scendsmus were the most frequently encountered algal genera
in the stomach of the fish. In addition , foods of animal origin , zooplankton,
macrophytes, sand grains and detritus were found in the stomach of the fish.
Generally, diatoms and blue greens as a group contributed the bulk of the diet.
Considering the average importance of algal genera, Microcysts and Navicula are
the most important in the diet of the fish in L. Beseka. The smallest sexually
mature female was 6 cm TL whereas the male was 10 cm TL. However, the 50%
maturity length (Lso) was 12cm TL for females and 14cm TL for males, and all
fish above 20cm TL were sexually mature. The average ± SE fecundity of O,
niloticus ranging in length from 10 to 23cm TL and in weight from 14 to 81g TW
was 154 ± 2.8. The smallest count was 110 eggs and the largest was 235 eggs.
In general, fecundity of O. niloticus in L. Beseka was linearly related to total
length, total weight and gonad weight. The mean monthly gonadosomatic index
(GSI) of females ranged from 0.7 to 3.5 and that of males from 0.6 to 2.1. There
was biannual cycle in which GSI increased from February peaking in April and
from August peaking in September. Ripe females and males also occurred at high
frequencies in April and September. Hence, it was found that the fish in L.
Beseka spawned throughout the year, with a peak of activity between March and
April and also between August and September.
Key words/ phrases: Breeding season, fecundity, Fulton’s condition factor,
L. Beseka, Oreochromis niloticus, stomach contents.
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Biology