International Journal of Marine Science 2015, Vol.5, No.23 1-13
10
lower reaches of estuary was related to other factors
such as migration of marine – dependent species, rather
than to salinity and temperature. Around 90% of the
fish species must require brackish water environment
to complete their life cycle. Freshwater-dominated
estuaries and deltas are well known as important
nursery areas for large numbers of marine fish species
(McHugh, 1967; Haedrich, 1983). The
juveniles of
these fishes in the near shore nurseries are facing
lower predation pressure and increased food supply
(Miller et
al., 1985). The ability of fishes to adjust to
the changes in salinity may be gradual, as in a
temporarily closed estuary or sudden as in tidal
estuaries. This magnitude of change in salinity
depends mainly upon the balance between the fresh
water inflow and tidal regime (Whitfield, 1999). In the
estuarine environment, the ability to adjust the
changes in salinity was considered the most essential
adaptation for fish and other organisms (Panikkar,
1960). But among fishes the ability for adjusting the
salinity variation differed from species to species and
it affects its distribution (Blaber, 2000). Temporal shift
in the normal discharge pattern due to regulation or an
overall decrease in the water body alters the migration
of fish species. The construction of dam’s reduced
upstream migrations of shad (
Hilsa ilisha
)
in several
rivers in southern India through reductions of high
freshwater flow rates was reported by Ganapati,
(1973).
One fifty species of fishes belonging to hundred
genera and fifty six families were identified in
Vembanad estuary by Kurup (1982) and later Kurup et
al., (1993) reported 115 species of fishes belonging to
84 genera, 6 species of penaeid prawns, 4 species of
palaemonid prawns and 3 species of crabs. Bijoy
Nandan et al., (2012) and Jayachandran et al., (2013)
observed 78 species of fishes from the Kodungallor –
Azhikode Estuary, the northern most part of Vembanad
estuarine ecosystem. Comparing the pre and post
barrage phases a significant change in the fish faunal
composition was noted by Anon., (2001). Atree, (2009)
identified 65 species of finfish and 14 species of shell
fish in the southern part. Asha et al., (2014) identified
80 of fin fishes, 5 species of penaeid shrimps, 3
species of palaemonid prawns and 2 species of crabs
during 2012-2013 period, in which three were
vulnerable and one was near threatened from the
Vembanad estuarine system. All these noted that in the
Vembanad estuarine system, the fish species are facing
various threats due to anthropogenic interference and
other disturbances in the habitat. During closure of
barrage,
the continuity of the estuarine system was
lost especially in the pre monsoon season. This led to
limited flushing of the system with heavy
accumulation of agricultural waste – fertilizers, oil
residues from house boat tourism and other toxic
contaminants in the southern part of the backwater
that
triggered the proliferation of weeds, deterioration
in water quality and related ecological impacts.
So the
periodic tidal inflow was completely prevented, along
with the interruption in migration routes of marine and
estuarine dependent fish fauna. Moreover, the
southern zone of barrage was inaccessible to majority
of the euryhaline migratory fishes.
Increased fishing activities, habitat loss, human
intervention also affected the fishery of Vembanad
estuarine system. A maximum of 56 species of
finfishes were observed in northern region (station 10),
which was due to the mesohaline condition (average
salinity10 ppt) that prevailed throughout the year and
also the optimum environmental conditions which
provide suitable habitat for the species. According to
Barletta et al., (2005) numerical abundance of fishes
was normally highest in the upper part of estuary
mainly due to the migration of a few fresh water and
marine species and the presence of the estuarine
species. But in Vembanad estuary the upper part was
less diverse and only 36 numbers of finfish species
was observed in southern part. The operation of
Thaneermukkom barrage
could negatively affect the
species diversity; reduction of upstream migration of
marine fish and prawns in Vembanad estuary
(Revichandran, 2012).
The opening/closing of the
Thaneermukkom barrage limited the numerical
abundance of finfish species.
Average Shannon –
Wiener diversity index, Margalef index and evenness
index of southern zone of the estuary was less as
compared to northern part (Table 3). The number of
fishes increased during pre monsoon season, in which
the whole estuarine system displayed a mesohaline
condition. The juvenile estuarine fish populations
were strongly affected by climatic variability, which
may affect fish production potential through changes
in either growth or abundance (Martin and Michael,
2002).
Atree, (2009) reported threats to fishery of
Vembanad Lake system from Invasive Alien Species
