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

Evaluating the annual landing trends of different
zones studies by Kurup et at. (1993) indicates
downstream sectors - Thanneermukkam barriers
to Cochin were the most lucrative sector. Highest
catch was from zone 1 and it started declining
from zone 2. However, the rate of decline in the
catches between zone 2 and 4 and also between 5
and 6 was very sharp. Fishery of zone 6 showed
more resemblance to zone 8 than to zone 5. The
drastic ecological changes (Kurup et al., 1989)
encountered in zone 6 and the adjoining
downstream areas due to man made changes can
be taken as the reason for the observed depletion
of the resources.
Post larval recruitment into brackish water
environment can be considered as a causative
factor in bringing about the fluctuations in the
prawn yield (Blaxter, 1962). Natural immigration
of the penaeid prawn larvae into Vembanad Lake
during high alien periods have been reported and
certainly the environment at the lake offers a
better refuge as a nursery than the marine
habitat. In recent years 16,000 ha were reclaimed
from the lake (Anon, 1988). The salinity of the
estuary has also presently decreased. Prior to the
construction of the barrier, salinity incursion was
noticed upto Pulinkizh, about 90 Km away from
Cochin and salinity upstream reached upto 23 ppt
in Kumarakom - Muhamma region (Josanto, 1971).
However, in the present condition, the salinity of
the region did not go beyond 6 ppt. Similarly in the
downstream region from the salinity barrier, in
zone 6 salinity did not go beyond 10 ppt (Kurup et
al., 1980) against 22 - 26 ppt (Josanto, 1971).This
. may be either due to Periyar diversions into
Muvatupuzha or commissioning of the Edamalayar
Hydroelectic Project. Thus a reduction of nursery
area has affected the production of penaeid
larvae.
d. Salinity tolerance
The Post larvae, which arrive in the lake, can
tolerate a wide range of salinity (Kuttyamma,
1975) and hence invade the best feeding grounds
even those with moderate salinity values, for rapid
growih. As they grow bigger in size, they become
more sensitive to low salinity and hence are forced
to descend into more favourable downstream
areas. The maximum growth rate of penaeid
prawns. is also associated with optimum saline
.conditions (Kuttyamma, 1975). The shrinkage of
nursery grounds may have resulted in over
crowding of juveniles in the lower reaches viz:
Vembanad Lake
zone 1 and 2. These areas where the shrimp
juveniles become concentrated are subjected to
heavy fishing pressure due to indiscriminate
operation of stake nets. The incidences of size
growth over fishing are very much discernible in
case of M. dobsoni.
Prior to construction of the barrier, the fishery ill
the upstream region was also sustained by
Penaeus indicus, P.monodon, Mctapenaeus
dobsoni, M. monoceros, Mugil cephalus, Liza
macrolepis, Chanos chanos etc. But after the
construction of barrier, very little catch is
obtained from the southern part. Some of the
freshwater fishes like Horabagrus brachysoma ,
Clarias batrachus, and Ompak bimaculatus are
on the wane and need protection and artificial
propagation. The production of Etroplus
suratansis however has increased especially from
zone 8 during December - May when the barrier
remained closed. The stagnant oligohaline
conditions prevalent in the upstream regions of
the lake may be very congenial for the survival of
this species.
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e. Depletion of resources :
It is obvious that salt-water exclusion project is
responsible for the remarkable difference in the
yield pattern in the up stream and down stream
regions of the lake. Further, the depletion of the
resources in the up stream is not adequately
compensated either by natural propagation of
freshwater species or by ranching using fast
growing freshwater fishes like the carps.
f. Variation in fishing intensity and gears
and their catch in Vembanad Lake:
The fishing methods employed for exploitation of
fin fishes and crustaceans of lake has been
described by Gopinath (1953), Shetty (1965),
Kurien and Sebastian (1982) and Kurup and
Sammuel (1985) and Kurup et al. (1993). The
fishing methods of the lake can be classified into
seven categories namely stake nets, dip nets, gill
nets, seine nets, cast nets lines and indigenous
methods.
Out of the stake nets studied and recorded, only a
small percentage was found to be operative in the
upstream (southern) region. Nearly 90% of the
stake nets have cod end mesh size of 13 mm and
about 47% have these below 8mm. They are not in
the operation during monsoon season. With the
building up of barrier, the intensity of tidal