Sex Ratio, Length-Weight Relationship, Condition Factor and the Food Habit of Catfish, Clarias Ganepinus (Burchell) in Lake Langeno, Ethiopia
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Date
2001-06
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Addis Ababa University
Abstract
Sex-ratio, length-weight relationship, condition factor (n = 859) and the food habit (n = 764) of
C. gariepinlls were studied in Lake Langeno from monthly samples collected using hook and
lines between April 2000 and October 2000.
Female to male sex-ratio was in favour offemales in the total sample (1041: 1) and in both seasons
of low water-level (1.29:1) and high water-level (1.53:1). Sex-ratio was not significantly
different from 1: 1 in five of the eight length classes used for the study. ill three of the length
classes (between 25 cm and 54 cm, TL), females were at least 1.53 times more numerous than
males.
The relationship between total length (range: 28 to 104 cm) and total weight (range: 150 to 8000
g) was found to be curvilinear and statistically significant (R2=0.955, P<0.05) with a slope (b =
2.91) very close to the cube.
Monthly mean Fulton condition factor (FCF) ranged from 0.55 to 0.65 for male and from 0.61 ~o
0.66 for female C. gariepinlls. Monthly mean relative condition factor (RCF) ranged from 0.91.
to 1.09 for males and from 0.99 to 1.10 for females. Generally, females had seasonally higher
FCF and RCF than males. On the average, large values ofFCF and RCF were recorded in May,
and low values in June for both sexes. There was no significant difference (ANOVA, P=0.1334)
in FCF and RCF between the low water and high water seasons. Mean FCF and mean RCF of
females were larger than those of males in all length-classes except in the smallest class (25 to 34
cm TL) in which case males had larger values.
Stomach contents of C. gariepinlls were composed of organisms belonging to six taxa or groups,
detritus, sand grains and unidentified plant and animal remains. The identified taxa/groups were
algae (Microcystis sp. and unidentified filamentous species), Macrophytes (shoots, roots, fruits
and seeds), Crustacea, illsecta, Hydracarina and Pisces. Crustaceans were represented by
Ceriodaphnia sp., Diaphallosollla sp. and Mesocyclops sp., and by unidentified Ostracod species.
illsects were represented by various developmental stages of Trichoptera, Hemiptera, Diptera,
Ephemeroptera, Odonota, Coleoptera, Plecoptera and Hymenoptera. O. niloticlls was the only
fish species ingested by C. gariepinlls. The presence of sand and benthic organisms (insect larvae
and pupae) was considered as indicative ofa bottom feeding habit.
The frequency of occurrence, the numerical abundance and gravimetric methods each suggested
that crustaceans, insects and fish were the most important food of C. gariepilllls in Lake Langeno.
Numerically, crustaceans were most important (93%) but in terms of frequency of occurrence
(75%) and weight (93%) insects were most important items in the diet of C. gariepinlls. Fish
were the least important (below 10% by each method) food as compared to crustaceans and
insects.
Cladocera and Copepoda were found to be the dominant Crustacean members in the diet of C.
gariepilllls. Diptera, Hemiptera and to some extent Odonata were the most important whereas
Plecoptera and Hymenoptera were least important insects in the diet of C. gariepilllls. The relative contribution of each food item varied with the size of C. gariepinlls. Thus,
frequency of occurrence was relatively high for Crustacea (about 60%) in C. gariepinlls between
45 and 64 cm TL, for insects (>80%) in those below 54 cm TL, and for fish (20-60%) above 65
cm TL. Numerically, crustaceans contributed more than 90% for each length group but their
contribution was below 90% for C. gariepilllls above 84 cm TL. The numerical contribution of
insects and fish increased for C. gariepinlts above 84 cm. Gravimetric contribution of
crustaceans was relatively high (10-15%) for C. gariepinlls between 45 and 74 cm TL, and that of
fish (about 10%) for large (>84 cm TL) C. gariepilllls. The weight contribution of insects w!!s
about similar for all size groups of C. gariepinlls caught in the study. The results suggestesl that
as C. gariepilllls grows larger it consumes progressively more fish.
The ratio between prey (0. lliloticlIS) length and predator (c. gmiepill11s) length varied greatly.
About 50% of the C. gmiepilllls had ingested tilapia whose length was between 1/1 oth and 1I30tl
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of their own length. A few individuals had ingested tilapia whose length was either above 1I5th
or below 1I30th of their length.
Frequency of C. gariepilllls with empty stomach was quite low (0 to 2.2 %) in samples taken
between April and June, which was coincident with period of low water level. In contrast, the
frequency was high (9.5 to 20%) in July, August and October. In general, about 95% of the total
number of empty stomachs were recorded during the season of high water level. This coincided
with the period when C. gariepilllts ingested more fish as compared to the other period.
The relative importance of fish to the diet of C. gariepilllls was more pronounced during the
season of high water level. Crustaceans and insects, however, were equally important throughout
the study period. .
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Biology