MFSc

The present study was carried out in the prawn filtration field of the
College of Fisheries at Panangad adjacent to the Cochin backwater system
to find out the faunal variations of zooplankton and macrobenthos and to
correlate them with the variations in hydro graphic conditions and compare
the results with those obtained from the observations in the adjacent lake.
Monthly samples for hydro graphical and biological studies were collected
during the period from November 2002 to July 2003 from the culture field
and adjacent backwater. Hydrographical features like temperature, salinity,
transparency, pH, total alkalinity, dissolved oxygen, nitrate-N, phosphate-P,
silicate-Si were observed. Crustacean larvae, copepods, tintinnids,
nematodes, rotifers and veliger larvae of molluscs were the zooplankton
groups obtained in the present study. Crustacean larvae and copepods were
the dominant groups in both places and were found during all the months
whereas tintinnids and nematodes were observed during saline period and
their number decreased with decreasing salinity. Rotifers were recorded
during low saline period only. Veliger larvae were recorded during
December and February in the culture field, whereas, they were not present
in the samples form the adjacent backwater. Statistical analysis for the
influence of hydrographical parameters on the zooplankton showed that
none of these had significant influence independently and the variations in
the zooplankton abundance may be due to the combined effect of all these
factors. Even though the seasonal changes in the total zooplankton were not
pronounced the per cent composition of different groups showed
fluctuation.
Tanaidaceans, gastropods, polychaetes, amphipods, bivalves, isopods
and chironomid larvae were the benthic groups obtained. Among these

112250
124
gastropods, tanaidaceans and polychaetes were the dominant forms. In the
culture field benthic populations were maximum during the post-monsoon
months and a showed a decreasing trend during pre-monsoon. With the
onset of the monsoon they showed an increasing trend. In the backwater
high benthic abundance was observed during January and February when
high salinity prevailed and gradually decreased from March onwards
reaching the minimum in July. In the culture field benthic abundance was
mainly influenced by substratum characteristics and pH, whereas in the
adjacent backwater salinity also played a role.
The area of the present study is away from the barmouth and the
nutrient levels especially phosphate is low in the culture field. This may be
limiting the primary productivity of the area leading to low secondary
production in terms of zooplankton and macrobenthos in the culture field.
The high pH noticed in the culture field may be another reason for the low
productivity.