International Journal of Marine Science 2015, Vol.5, No.23 1-13
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A clear seasonality was observed in the salinity pattern.
During the southwest monsoon the salinity declined
nearly zero mainly due to the heavy rainfall and river
discharge. In Alappuzha district the annual rainfall
was observed as 2113.6 mm and in Ernakulam district
it was 2780.7 mm. The maximum salinity of 33 ppt
was observed in Station 10 and minimum 0.1 ppt in
Station 2. The average salinity of 1.37 ±2.1ppt was
observed during the monsoon and followed by pre
monsoon (7.58 ±8.2 ppt) and post monsoon (7.89 ±
1.15 ppt) periods. ANOVA showed that station wise
and seasonal variations of salinity were significant (F=
11.74, p < 0.01).The annual average salinity showed
that the southern region of the estuary was oligohaline
(2.45 ppt) whereas northern region was mesohaline
(11.85 ppt) in nature. According to salinity gradient
and general morphology, the estuarine system was
divided into two zones (upper and lower). The upper
part is located between Alappuzha to Thaneermukkom
barrage and lower part extends from north of
Thaneermukkom barrage to Thevara which is
influenced by coastal waters.
2.2 Fishery abundance
Seventy four species of finfishes belonging to forty
families were recorded during the study period (Table 2).
The finfish species,
Ambassis ambassis
(15.16%),
Etroplus maculatus
(11.14%),
Amblypharyngodon
microlepis
(9.7%),
Mugil cephalus
(4.65%),
Hyporhamphus xanthopterus
(4.02%),
Liza parsia
(3.95%),
Puntius filamentosus
(3.53%) and
Liza
microlepis
(3.5%) were the major contributing groups
in Vembanad estuarine system. Annual average landing
was estimated at 4774.46t from the Vembanad
estuarine system, of which 4292.59t was contributed
by northern zone and 481.87t by southern zone.
Maximum number of species was observed during
March 2012 and minimum during August 2012.
Highest Shannon diversity index (Hˈ) was found in
March 2012 (4.14) whereas it was low during July
2012 (2.79) (Figure 2). The maximum diversity index
was observed in Station 8 (4.70) and minimum in
Station 1 (2.84). The highest Margalef’s richness (d)
values were observed during March 2012 (5.27) and
lowest in August 2012 (2.91). Maximum richness
index was observed in station 8 (5.34) and minimum in
station 3 (2.34). The highest Pielou’s evenness index
(J’) was observed during November 2012 (0.84) and
lowest in July 2012 (0.60). Compared to the southern
region the species diversity index, richness index and
evenness were always higher in the northern region
(Table 3). MDS analysis indicated that fish
assemblages significantly varied within the estuarine
system. This was however due to clear differences in
the presence of finfish communities at each site. In
MDS plot, samples showing similarities were grouped
together and dissimilar ones, far away. In the case of
months such as June, July and March, May were
grouped closely and showed 80% similarity in fin fish
abundance (Figure 3).The stress coefficient in the case
of monthly distribution pattern was 0.06. CCA biplot
showed the eigen values of the first two axes as 0.604
and 0.075 respectively (Table 4). The first two CCA
axes explained 90.22% variability of environmental
parameters in which the first axis that explained
80.25% of the total variance and the second axis
represented 9.97% of the total variance (Figure. 4).
Figure 2 Temporal variation of species diversity of fish species
[Margalef’s species richness (d), Pielou’s evenness (J’),
Shannon index (H)] in the Vembanad estuarine system
Figure 3 MDS plot based on the temporal variation of fish
assemblage in Vembanad estuarine system
