06-bioresourcesstatu.. - M. S. Swaminathan Research Foundation

Vembanad Lake
cyprinoides (Grey). As mentioned in the
preceding columns with a length of 60 km north to
south from Cochin to alleppy and an area of 21,053
ha. with the commission of salt-water barriers, the
lake is divided into two entirely different
ecosystems. The northern sector downstream is
estuarine while the southern upstream is
freshwater.
The main demand for lime from clam resources is
for brick laying, plastering and white washing. At
present, number of industries such as the
Govetnment owned Travancore Cements and
Travllllcore Electrochemicals use clam shells
(Mammen, 1984). According to a rough estimate
they consume nearly one lakh tonnes of clam
shells annually. The clam resources of the lake
have been estimated by Rasalam and Sebastian
(1976) and Achary (1987) based on the data
gathered from co-operative societies. The clam
beds were located by Kurup et at. (1990). This
study is important as it pertains to more than a
decade after the commissioning of the barrier ..
The species Vellorita cyprinoides supports
sustenance fisheries in the estuaries and
backwaters of Kerala, Karnataka, Goa, Tamilnadu
and Andhra Pradesh and contributes 67% to the
clam fishery. The meat is locally consumed and it
is also used in supplementing various aquacultue
feeds (Kizhakundan and Narasimhan, 1995). This
species plays an important role in the rural
economy of Kerala.
Exploitation Pattern : The Department of Mining
and Geology controls the clam collection and
periodically leases out areas for dredging and also
issues licenses to divers to collect surface deposits
of live (black) and dead (white) clam shells. There
has been lot of agitation from the divers who feel
that dredging leads to depletion of live clams. The
fisherfolk operating stake and Chinese dip net also
feel that dredging has lead to depletion of their
catch. These allegations are not entirely without
basis (Mammen, 1984).
Cutter suction dredgers are used in Vembanad
Lake. As the dredger cuts through the soil, the
90
suction pump takes the soil and clam shells along
with water to the floating washery, where the
shells are separated and delivered to a barge
while the clay water slurry is allowed to flow into
the lake. Initially the dredger used to collect about
50 tonnes of sub fossil clam deposit in two hours
later as the resource declined due to
overexploitation, it took upto six hours to get 50 t
of clam shell (Mammen, 1984). However, even at
this rate the factories are finding it economical to
dredge the shell rather than buy from the divers.
The factories claim that they are dredging in
deeper waters and thus do not come in the way of
collection of clams by the divers. According to
them, if the divers get the sole licence for
collection, they are likely to dictate the terms.
Estimates of the exploited clam population: The
population of clam in Vernban ad lake has been
estimated by Kurup et at. (1990). They had
divided the regions into ten zones comprising five
zones each on the northern and southern side of
the Thanneermukkam barrier. They reported the
presence of extensive clam beds in the regions
where the lake bottom is sandy. The clams avoid
the areas' where high percentage of silt is present.
The important live clam beds of different zones as
reported by Kurup et at. (1990), are presented in
Tables 1 & 2.
Size distribution of clam : The size of the
exploited clams varies from 9 to 59 mm. However,
size below 10 mm and above 39 mm were rarely
represented. The dominant modal groups were 15
- 19, 20 - 24 & 10- 14 mm, in that order. The sizes
prevalent in different zones of the lakes were
analysed. It appears that the dominant mode in
zone I was that of 30-34 mm. Here the clam is
present in semi-enclosed lake with low salinity. In
zone II and III where higher salinity is prevalent
the modal groups 15 - 19 and 10 - 14 mm were
dominant. Zone IV had almost all the three groups
15 - 19, 20 - 24 and 25 - 29 mm equally
represented. Zone V had modal group 20 - 24 mm,
which was not apart from all other zones with a
dominant modal group of 30 - 34 mm.