Lake Muhazi, a small lake of Rwanda (East Africa) was studied from 1986 to 1990. A dramatic decrease of the catch of Oreochromis niloticus (350 T y-' in the fifties vs 30 T y-' in 1982) suggested a loss of productivity or overfishing. In the same period, other ecological changes occurred: the submerged macrophytes regressed and there was a decrease in Secchi depth (0.65 m in 1987 vs 1.5 m in the fifties). Compared to other lakes of the same area, the plankton production seemed low. The results of the present study characterize lake Muhazi as a shallow lake with a rather unstable diurnal stratification and with slight differences in mixing regime between its eastern, deepest part and its western, shallowest part. Secchi disk depth does not vary seasonally to a large extent. The water has a rather high mineral content (conductivity of about 500 PS cm -' at 25 "C) and low concentrations of dissolved N and P, except in the hypolimnion, where NH,'-N can be high. Two species, Microcystis aeruginosa and Ceratium hirundinella, account for most of the phytoplankton biomass, which is about 50-80 mg chlorophyll a rnd2 in the euphotic zone, usually with little seasonal variation. Daily gross production estimates amount to about 6 to 9.5 g O2 m-2 d-' with a significant difference between the two parts of the lake. Data on C:N and C:P ratio in the phytoplankton suggest that some N deficiency might occur in the eastern part. Moreover, the Zm:Zc ratio could also lead to rather low net production rates (0.21-0.25 d -i for a mixed layer of 4 m)In conclusion, the primary production of lake Muhazi is medium for African lakes and the hypothesis that decreased planktonic production could account for a reduced fish production should be discarded.
Le régime alimentaire de Tilapia, Oreochromis niloticus, principale espèce exploitée commercialement au Lac Muhazi, a été étudié de décembre 1989 à septembre 1990, en fonction des saisons, des stations et de la taille des individus capturés dans les filets de pêche. Tous les indices considérés montrent que O. niloticus du Lac Muhazi consomme bien le phytoplancton et que les Cyanobactéries sont les plus représentées dans les contenus stomacaux comme c'est le cas dans le lac. Pour les 118 contenus stomacaux analysés, l'indice d'occurence arrive à 100 % pour certaines espèces de Cyanobactéries (Merismopedia tenuissima). Pour ce groupe dominant, l'indice d'abondance est en moyenne de 64,3 %, l'indice volumétrique de 83 % et l'indice de dominance de Lauzanne de 58,1. Deux espèces, Microcystis aeruginosa et Ceratium hirundinella représentent à elles seules plus de 90 % de la biomasse contenue dans les estomacs des poissons dont la taille est supérieure à 90 mm quelles que soient la station et la saison de prélèvement. Pour la période d'étude, nous avons constaté que O. niloticus a une activité essentiellement diurne (il se nourrit entre 6 h et 21 h 30), avec une ration journalière élevée (26,5 g.j 1 )-Ceci explique sans doute la bonne croissance de cette espèce dans les milieux où le phytoplancton est abondant. Cependant, au Lac Muhazi, la population d'O. niloticus n'atteint qu'une biomasse relativement faible et ne consomme qu'une faible fraction de la production primaire nette (6 %). Diet of the Nile Tilapia, Oreochromis niloticus, in lake Muhazi (Rwanda)Keywords : Tilapia, Oreochromis niloticus, diet, Cyanobacteria, phytoplankton, Muhazi lake, Rwanda.The diet of Oreochromis niloticus, the mainly exploited species in Lake Muhazi, was studied from December 1989 to September 1990. The diet was investigated in terms of sites, seasons and size of the specimens caught. O. niloticus feeds on phytoplankton, and Cyanobacteria make up an important fraction in the stomach contents analyzed. In the 118 stomach contents, belonging to fishes larger than 90 mm, the index of occurrence reaches 100 % for some Cyanobacteria (Merismopedia tenuissima). For this dominant group, the mean abundance index is 64,3 %, the mean volumetric index is 83 % and the dominance index of Lauzanne is 58,1. Two species, Microcystis aeruginosa and Ceratium hirundinella represent more than 90 % of the biomass in the stomach contents analyzed from the two sites and in all seasons. For the period of study, we established that O. niloticus feeds only during the day (from 6 h to 21 h 30) and has a high daily ration (26,5 g.d 1 )-This may explain the good growth of this species in an environment where phytoplankton is abundant. However, the population of O. niloticus of Lake Muhazi has a relatively small biomass and feeds on a small fraction of the phytoplankton primary production (6 %).
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