Sex Ratio, Length-Weight Relationship, Condition Factor and the Food Habit of Catfish, Clarias Ganepinus (Burchell) in Lake Langeno, Ethiopia

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Addis Ababa University


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 • 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. .