Downstream drift of invertebrates in Naro Moru river, a tropical river in Central Kenya

Abstract

Downstream drift of invertebrates was investigated in Naro Moru River, a tropical river in central Kenya, from November 1986 to October 1987 to establish the composition and structure of the drift fauna and benthos, the factors that affect drift, diel and seasonal fluctuations of the drift fauna in relation to the dry and wet seasons and, drift fauna as potential food for fish. Prior to embarking on the drift studies, a pilot survey was conducted on seven stations along the river to establish the expected composition of the benthos and to locate a suitable site for the study from among the seven stations. Two 24h. drift samples were collected from the study site every month for one year using six drift nets of 105 ~m. mesh size. The major components of the drift fauna were Ephemeroptera, Diptera, Hemiptera, Hymenoptera, Arachnida and Coleoptera. Baetis spp. (Ephemeroptera) comprised the highest proportIon (50.91%) of the total drift. There was a clear bigeminus diel periodicity in the tot~ drift. Mean river discharge ranged from 0.235~ O.00 I to 3.873-+0.003 m3 .sec.-1 Drift rate increased with increasing river discharge. However, the drift density did not increase with increasing river discharge. Dissolved oxygen concentration was in all cases high (8.32!0.43! 8.97tO.31 mg.l~l) and did not seem to play an important role in drift rate. The roles played by pH and conductivity were difficult to envisage. Colonization experiments showed that maximum colonization of the embedded artificial substrate baskets took place after ten days of exposure. The numbers obtained were used to relate benthos with drift fauna. There was a positive correlation (rs = 0.76, P< 0.01 ). Diptera (64.56%) and Ephemeroptera (24.37%) were found to be the predominant benthic organisms. More animals drifted on the dark nights than on moonlit nights. However, moonlight did not have a depressant effect. There were seasonal fluctuations in the drift fauna and drift rates. Peak drift rates were observed in the wet seasons. The composition of the diet of the resident rainbow trout, Salmo gairdneri Richardson was similar to that of the drift and benthos. The fish was feeding throughout the year and there was no evident seasonality in its feeding. It showed some selectivity in its feeding. There was lack of correlation between trout diet and drift (r = 0.18, P>O.OS). 5