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International Journal of Marine Science 2013, Vol.3, No.2, 4
-1
6
http://ijms.sophiapublisher.com
5
assemblages in estuaries has shown that salinity plays
a major role in shaping assemblage structure
(Whitfield, 1999).
Fisheries have a vital importance in contributing
beneficial nutrition for human beings, providing raw
material for the industrial sector, creating employment
possibilities and high potential for export (Can and
Demirci, 2012). Various estuarine wetland systems
spreading over three lakh hectare form an important
component of the inland fisheries resources of India
(Sugunan, 2010). India produces an average of 4.6 million
tonnes of fish annually from inland water bodies. The
average yield of estuarine fish production in India was
estimated to vary from 45 to 75 kg/ha (Jhingran, 1982).
Thirty major backwaters of Kerala forming the crux
of the coastal wetlands form an abode for over 200 resident
and migratory fish and shellfish species and fishing
activities in these water bodies provide the livelihood
to about 200 000 fishers and also provide full time
employment to over 50 000 fishermen (Bijoy Nandan,
2008). These are indispensable habitat to a variety of
biologically and economically important aquatic fauna;
moreover, the inter dependence of the adjoining marine
and estuarine zones in completion of the life cycle of
the finfish and shell fish species (Jhingran, 1982; Chao
et al., 1982; Muelbert and Weiss, 1991; Vieira and
Castello, 1997).
The Vembanad backwater has been extensively studied
on the composition, distribution and gear wise catch of
major fishery (Shetty, 1965; Kurup, 1982; Kurup and
Samuel, 1985a; Kurup and Samuel, 1985b; Anon, 2001;
Bijoy Nandan, 2008; Harikrishnan et al., 2011; Bijoy
Nandan et al., 2012). Annual average fish production in
the Vembanad lake and including other back waters of
Kerala was estimated at 14 000 t ~ 17 000 t (Sugunan,
2010). Kodungallur-Azhikode estuary (KAE) is a
northern extremity of Vembanad wetland ecosystem, is
an ideal habitat for several species fin fish and shellfish
species (Anon, 2001).
A comprehensive study in the fish diversity and
abundance in relation to environmental variability in
the Kodungallur-Azhikode estuary (KAE) in particular
was lacking. This study discussed the diversity and
abundance of fishes in relation to environmental
quality and use of fishes as Indicators of ecological
change and estuarine health.
2 Results
2.1 Environmental variables
The study area covered a both marine side (EMZ) and
low saline side (EUZ) of estuarine transition zone.
Wide range of variations in environmental variable
was monitored during the study, which may
potentially affect fish assemblage. Annual mean water
column temperature in the KAE was 28.9
℃
and it
showed a clear vertical stratification especially during
post monsoon season. Temporal variation was also
noticed in the water column in both zones and it was
lowest during south west monsoon (EMZ, 27.5
℃
;
EUZ, 27.6
℃
) compared to pre monsoon and post
monsoon seasons. The ANOVA of water temperature
showed the variation between months were significant
(p<0.01). The mean dissolved oxygen (DO) content of
5.1 mg/L was noticed in the KAE and monsoon period
showed highest concentration (av. 5.8 mg/L) as
compared to post monsoon period (5 mg/L) and pre
monsoon period (5 mg/L). A noticeable trend was
observed in the DO regime in the estuary, where
surface water was higher than bottom waters. Surface
water DO (5.6/mg) displayed comparatively higher
values than that of bottom waters (4.7 mg/L). The
ANOVA of DO showed that the variation between
months were significant at 1% level (F=7.113). Carbon
dioxide (CO
2
) values displayed highest mean in Station
5 (7.1 mg/L) and minimum in Station 7 (5.3 mg/L);
temporarily the values were high in post monsoon
(6.9 mg/L) as compared to monsoon period (6 mg/L)
and pre monsoon period (6.3 mg/L) in the KAE. A
remarkably high CO
2
value of 14 mg/L was recorded in
the bottom water in Station 2 (EMZ) during September
and also comparatively high values were observed in
the most of the stations particularly in stations 1
(7±3 mg/L) and 5 (7.1 mg/L). The ANOVA of CO
2
between months showed variation and were significant
at 1% level (F=18.324). The average biological oxygen
demand (BOD) during the present study was 2.6 mg/L;
it was high in the station 1 (3.1 mg/L) and
temporarily it was high during monsoon (3.1 mg/L) as
compared to the post monsoon (2.2 mg/L) and pre
monsoon (2.3 mg/L) periods. Transparency values were