| dc.description.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 | en |
