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

Vembanad Lake
Estimates of live clam production: While the dead
shells of the clam cater to the lime industry, the
live clam serves as a food. According to Kurup et
al. (1990), the estimated catch of live clam for
1988-89was 7026 t and the catch per hectare of
the lake was computed as 333.7 kg. The total area
of the live bed of clam is estimated as 1565
hectares. The maximum catch is from zone VIII
followed by zone IV and the lowest in zone 1.
Hornell (1921) indicated that the black clam
(Vellorita sp.) is not so clearly abundant in the
lakes as the backwater clam, its habitat being
restricted to regions usually more distant from the
sea than that of the latter. However, over the years
drastic changes have occurred in the distribution
and abundance of these species. Though it has
been observed that Meretrix meretrix (Linnaeus )
and M. casta (Hanley) have been recorded from
the northern part of the lake in addition to
Vellorita cyprinoides ( Gray) as observed by
Rasalam and Sebastian (1976), their distribution is
now confined to very narrow stretches along the
high saline bar mouth area. Probably the black
clam has taken up their niche and the fishery of
the lake is excessively supported by this (Kurup et
al. 1990).
There have been reports that black clam can
tolerate extreme salinity variations from 0.8 to
29.5 %0 (Nair and Shynamma, 1975). Kurup et al.
(1990) noticed clam beds in the regions where the
salinity varied from 0 to 23%0.However, they have
also noticed the perennial abundance of this
species only from regions where the bottom salinity
varied from 0- 13%0.More than 80% of the clam
beds were found to be situated in areas where the
bottom salinity was less than 13%0.The large size
groups were found to be flourishing in the lower
Kuttunad area where the salinity was 0-2%0.This
observation corroborates with the' observation of
Hornell (1921) that Vellorita cyprinoides is
purely a freshwater species and made mark~\!changes
in the habits and acquired tolerance for
the saline conditions (Kurup et al., 1990).
Nearly 5.5 % of the total area of the northern
sector supports a regular fishery of live clam as
compared to 10.2 % of the southern sector. The
catch 5392.45kglha and_3786.39 kglha. in the
northern' and southern sectors respectively. The
zone IV gives marginally higher values of the clam
resources as more people.come here for collection
(Kurup et al.; '1990). With the commissioning of
92
the ldukki hydropower project the flow of
freshwater from the Muvathupuzha river is
discharged into this region which makes the ideal
requirement of salinity for V.cyprinoides i.e. 0.5 -
13%0. Thus a conducive environment for the
growth and reproduction of V. cyprinoides is
provided even before the monsoon period. During
the period January-May, the shutters of the
Thanneermukkam barrier are closed. Hence the
water level in the upstream increases making
collection of the clams difficult. But in the
downstream region, the water level is substantially
lower during their period, making collection
easier. Because of this, there is difference between
collection of clam in the pre-monsoon season
between northern and southern sectors.
Growth studies of this species have been made by
Kizhakundan and Narasimhan (1995), which
indicate that this species grows to 24.6, 39.09,
47.19 and 51.83 mm at the end of I-IV years of its
life span. The asymptotic length was estimated as
58 mm and growth coefficient, K as 0.56. The
decline in the clam resources has been very
drastic. Rasalam and Sebastian (1976) reported
that about 26, 858 t of live clams were collected
from Vembanad Lake in 1968. Acharya (1987)
observed that the average clam productions
during the period from 1979-1984was 21,4901.The
data analyzed by him shows a steady decline in
production from 1983 onwards reaching a value of
13, 804 t in 1984.Kurup et al. (1990) estimated the
catch of this resource around 7202 t in 1987- 89.
The estimated catch from the northern sector was
3693.8t, which was only 35.8% of that of estimated
by Achary (1987) for 1984. The southern region has
not shown much variation and the replenishment
rate is stable. It is thus inferred that construction
of the salt water barrier has not made any impact
on the black clam resources in the southern
sector-Thanneermukkam to Alleppy. The low
quantity of catch obtained in the northern sector
illustrates the declining trends in catch from the
lake over the years and it is independent of the
existence of the barrier. The reasons for the
decline as cited by Kurup et ai. (1990) are listed
below:
i) Increased fishing pressure and resultant
removal of undersized clam of 10-14 mm.
il) Indiscriminate fishing practice using toothed
iron rakers for combing the clam beds and,
disturbing spat settlement.