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

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iii)Pollution hazards from coconut husks retting
ground.
iv)Pollution from industries.
it. Prawn resources
a. Penaenid prawns
The other main resource of the lake was the
penaeid prawns. They were dominant in zones 1 &
2. Highest catch was recorded from zone 2
followed by zone 1. No penaeid fishery was located
in zone 10. Metapenaeus dobsoni, M.monoceros
and Penaeus indicus were available throughout
the year. M.dobsoni formed 72.2% of the total
penaeid yield while P.indicus and M.monoceros
contributed 14.88 and 10.31% respectively.
b. Palaemonid prawns :
Four species of Macrobrachium contributed 1.83%
to the total production of the lake. Of these
M.rosenbergii and M.idella contributed 0.55 and
0.99%respectively. Zone 7 followed by zone 5 gave
the maximum yield of these two species.
c. Crabs:
Highest contribution of crabs came from Zone 1
followed by zones 3 and 2.
The penaeid prawn resources are biologically
related to the brackish water ecological system.
Adults and larvae of six species of penaeids are
found to inhabit the lake. Available information
suggests that the post larvae of Mefapenaeus
dobsoni, M. monoceros and Penaeus indicus
remain in the backwaters for 5, 10 and 6 month
whence they attain the sizes of 50, 85 and 80 mm
respectively (Mohamed and Rao, 1972). Larval
stages of M. dobsoni and M. monoceros are
available in the inshore and estuarine areas on
year round basis. The juveniles of P.indicus and
P.semisulcafus do not penetrate the estuarine
envionment as deeply as in the case of M.dobsoni,
M. monoceros and P. monodon. The major part of
the post larval prawn migration into the estuary
takes place during the premonsoon period
followed by lesser magnitude in the post-monsoon.
The migration pattern is strongly influenced by
lunar phases and tidal rhythm.
Palaemonid prawns of commercial importance
available in the lake are the giant freshwater
prawn Macrobrachium rosenbergii and M idella.
With the onset of monsoon, these species descend
into the lake from the adjoining rivers. The down
stream breeding migration is observed during
Vembanad Lake
August to November when the estuary has
mesohaline characteristics (8- 15 ppt. of salinity).
This enables the larvae to complete its
metamorphosis. The zoea is planktonic and as it
transforms into the post larval stage, it sinks to
the bottom and grows rapidly to the size of 20-30
mm. The return migration into the rivers begins by
December or early January. The commercial size
group of M. rosenbergii and M. idella are 18-32
cm and 7-12 em respectively
Of all the penaeid and palaemonid prawns
available in Vembanad, the one that has received
maximum attention is the giant freshwater prawn,
Macrobrachium rosenbergii. The catch of this
species was reported to be around 400 tonnes in
sixties (Raman, 1967) has dwindled to a mere 39 t
in 1988-89 (Kurup ef al., 1992). The main reason
for the decline is that enroute the downward
breeding migration, the berried females are
subjected to indiscriminate exploitation. A wide
array of indigenous fishing gear and methods were
employed for the exploitation of M. rosenbergii,
including cast net, gill net and stake net.
93
The fishery of M. rosenbergii in the downstream
part of the lake is seasonal and therefore it could
be inferred that the migratory stocks descending
from the upstream part of the lake and confluent
rivers constitute the fishery.
Prior to the construction of the Thanneermukkam
barrier, Kumarakom and adjacent regions of the
upstream part of the lake offered ideal
brackishwater conditions for hatching and
completion of larval metamorphosis. Therefore,
the fishery was well restricted to the upper
reaches of the lake (Raman 1967), as the stock
could not perform the downward migration for the
purpose of breeding. However, after the
commissioning of the barrier, a shift in the
breeding ground was found effected by 40 Km
downward (Kurup et.al., 1992) and the berried
females M. rosenbergii are compelled to
undertake lengthy spawning migration in order to
reach the breeding ground. With the
intensification of fishing activity in, the lake for
freshwater prawn during July to December, the
slow moving ovigerous females are most vulnerable
to heavy fishing pressure and this may be the
reason for their heavy exploitation in the
downstream part of the lake.
The result of the studies carried out by
Harikrishnan and Kurup (1997) indicates that on
an annual basis 23.4 t of berried females are