The Condition Factor, Feeding and Reproductive Biology of Oreochromis niloticus Linn. (Pisces: Cichlidae) in Lake Chamo, Ethiopia
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Date
1997-09
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Addis Ababa University
Abstract
The length-weight relationship, condition factor, feeding and reproduction of O. lIiloticus from Lake Chamo,
Ethiopia were studied from March 1996 to March 1997. The length-weight relationship of 1473 fish ranging in size
from 125 to 610 nun Total Length (TL) and in weight from 42.5 to 4800 g Total Weight (TW) was computed. The
relationship between total length and total weight for the total sample was curvilinear, and described by: TW =
0.0258TL'·"'. The regression equation for the females was TW = 0.069TL'·63 and for the males it was TW =
0.022TL'·99. Fulton's and relative condition factor was calculated for both sexes separately. There was a
significant variation in the mean monthiy condition factor of both sexes (ANOY A, P < 0.05). The mean ± SE
Fulton's condition factor of O. ni/oticus in Lake Chamo ranged from 2.10 ± 0.03 to 2.35 ± 0.10 for the males,
from 1.96 ± 0.03 to 2.1 ± 0.04 for the females. The food of O. niloticus in Lake Chamo was studied from the
stomach contents of 449 fish measuring 290-570 mm TL. In addition, a total of 145 juvenile O. ni/oticus, ranging
from 61 to 115 mm caught during three sampling occasions were also examined for diet composition analysis. The
diet of adult fish consists of 10 genera of blue greens, 8 genera of green algae and more than 8 genera of diatoms.
Blue greens as a group contributed the bulk of the diet. The most frequently encountered species from the blue
greens were Allabaena (96-100), Microcystis (82-100) and Oscil/atoria (41-100). In terms of percentage
composition by number, the blue greens contributed over 60% of the total food ingested. Of these, more than 50%
was contributed by Anabaena, Microcystis and Oscillatoria. When the food items were expressed as percentage
volume, the blue greens made up 32.2% whereas the green algae and the diatoms contributed about 32.3 and
35.2%, respectively. The mean percentage dry weight of the green algae, diatoms and blue greens was 53.8, 28.2
and 17.7%, respectively. Among the green algae, Cosmariu/ll alone contributed more than 40% to the total dry
weight. Zooplankters occurred on rare occasions in the stomach contents of adult O. niloticus -w-he-re'a-s -th-es.e. ' -w-"e re
relatively more frequent (1.8%) in the diet of the juveniles. The feeding pattern of O. niloticus was observed to
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have a diel rhythm. O. lIi/oticus in Lake Chamo is a continuous feeder during the day. Daily ingestion of
phytoplankton was estimated to be about 3.75% of its body weight per day at an average water temperature of 26 '
C. Gonads from a total of 1349 O. niloticus were examined in this study. The mean monthly gonado-somatic index
(GSI) of the males ranged from 0.21 to 0.51 whereas that of females ranged from 0.51 to 1.56. Higher values for
the females were observed between March and June, and for the males between March and May, 1996. The pattern
of gonad development for both sexes was more or less similar. Ripe males occurred at high frequencies
(ranged:39-60%) in March, April and May, 1996. The percentage of ripe females ranged from 47 to 53 with
seasonally highest frequencies occurring from March to June, 1996. Thus, it was concluded that O. ni/oticus in
Lake Chamo spawns throughout the year with peak breeding activity in March, April, May and June. The smallest
sexually mature female was 385 mm TL, whereas the male was 395 mm TL. However, the 50% maturity length
was415 mm TL for females while 425 mm TL for males. O. ni/oticlls were all mature above 500 mm TL. The
average fecundity of O. niloticus ranging in Length from 390 to 560 mm TL, and in weight from 1300 to 3900 g
was 2493 ± 300 (N =209). The smallest count was 1047 (fish length=470 mm TL) whereas the largest was 4590
(fish lengtll=510 mrnTL). In general, fecundity of O. lIiloticus in Lake Chamo was linearly related to total length,
total weight and gonad weight.
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Biology