A METHOD OF QUALITATIVE BIOLOGICAL RIVER SURVEY 253
Fig. 3. A typical sampling station (diagrammatic').
The sampling methods used are: —Standard Sweep Transect,
Stone Transect, Transverse Stone Quadrat, Gravel Square, and
the Sparganium Sweep.
Standard Sweep Transect (Figs. 3 and 5)
Before commencing to sample, the dissolved oxygen of the
water which has passed over the riffle should be determined and
a sample taken for chemical analysis. Sweeping should begin from
the right bank when facing upstream (point one). Otherwise if
(point 6) were swept first then subsequent sampling would be
spoiled (Fig. 4).
Sweeping should be against the flow and the net passed slowly
through an arc of about 6 ft., noting the depth of immersion of the
net, with each sweep. The average population density can then
be determined from the rate of flow of the river and the volume
of water displaced through the net. The best net for standard
sweeps of this kind was found to be a circular one about 1 ft. in
diameter and having a mesh size of 20/square inch.
The contents of each sweep is emptied into a bucket and the
total catch examined in a large white tray. Those organisms
which cannot be classified are pipetted into specimen tubes and
later examined in the laboratory. The reason for sampling down-
stream is to see how this habitat might reflect that of the riffle.
A sample of any fish caught can be dissected for their gut contents.
The stomach is removed by slitting the fish median-ventrally
from anus to mouth. The gut is then cut at the oesophagus and
pyloric sphincter, the stomach opened using scissors and the con-
tents washed with water into a petri-dish and examined with a