International Journal of Marine Science 2015, Vol.5, No.23 1-13
9
(southern zone) was little lower (av. 6.7 mg/l),
compared to the mesohaline zone (northern zone) that
exhibited a higher oxygen concentration (av. 7.3 mg/l).
This may be due to oxygen rich fresh water from the
Muvattuppuzha river and also due to the wind and
tidal action from the Arabian sea. All the season, the
lower estuary (northern region) always showed the
highest salinity, whereas the upper estuary (southern
region) depicted lower values. During monsoon
season a near limnetic condition prevailed in the
southern part of estuary (av. 0.66 ppt), whereas an
oligohaline condition prevailed in the northern part (av.
2.4 ppt). During the premonsoon season the average
salinity pattern in the southern part of estuary showed
oligohaline condition (av. 3.1 ppt) and mesohaline
condition in northern part (av. 14.3 ppt) of the
backwater.
Sabo et al., (1991) suggests that larval fish density
was positively correlated with dissolved oxygen,
conductivity and turbidity. The effect of temperature
on tropical species in low water temperatures (<14 ºC)
have occasionally resulted in mass mortalities of
fishes in St Lucia (Cyrus and McLean, 1996) and Kosi
Bay (Kyle, 1989) estuaries on the subtropical
KwaZulu-Natal coast. Temperature plays an important
role in the difference in estuarine fish assemblages on
the northeast coast of the United States where the
fauna south of Cape Cod was represented by
warm-temperate species with cold-temperate taxa
dominating ichthyofauna to the north (Ayvazian et al.,
1992). The distribution of tropical fish species in
South African estuaries is strongly linked to the
decrease in sea temperatures along the southeast coast
(Day et al., 1981). The endemic species such as
Horabagrus
brachysoma
and
Hyporhamphus
xanthopterus
in Vembanad backwater were influenced
by the prevailing temperature regime. Fluctuations in
temperature could affect the sudden or possible
changes in benthic and column feeding fishes such as
Horabagrus
brachysoma
and
Hyporhamphus
xanthopterus
respectively. The mass mortality of
species such as
Diplodus capensis
,
Galeichthys
feliceps
,
Lithognathus lithognathus
and
Liza
richardsonii
occurred in the Bot estuary on the
southwest coast of South Africa, when salinity
declined to 2-3 ppt (Bennett, 1985). Temperature and
salinity formed major factors affecting the distribution
and abundance of fishes in estuaries in many parts of
the world. The fish species assemblages in the North
American West Coast estuaries were structured mainly
by salinity and temperature (Monaco et al., 1992;
Emmett et al., 2000). The assemblage structure of
fishes in estuaries depend upon salinity (Marshall and
Elliott 1998., Plavan et al., 2010; Neves et al., 2011)
and the effects of salinity appear to be regulating the
distribution of fish and in the attraction of larvae,
post- larvae and juveniles into the estuaries (Elliott
and Hemingway, 2002).
Among the environmental variables, salinity,
temperature, turbidity, dissolved oxygen and their
regular or irregular fluctuations at different time scales
affect the estuarine fish ecology (Whitefield, 1999;
Blaber, 2000). Individual fish populations and
communities have strong physiological and
behavioural responses to environmental changes
(Boesch and Turner, 1984). CCA analysis showed that
salinity, water temperature, depth and pH are
important in relation to fish assemblages (Monte Carlo
permutation test, p<0.05). The first CCA axis
positively correlated with salinity (r = 0.74), water
temperature (0.73), depth (0.7) and pH (0.68). Large
vector length of these parameters shows their
importance in making assemblage structure of fish
species in Vembanad estuary. The vector length of a
given variable showed significant correlation with
stations. Smaller the angle of an environmental vector
relative to another environmental vector representing
higher correlation between the two environmental
factors. This indicates their importance in shaping
assemblage structure of fish species across the study
site. The longest vector of salinity, pH and depth
showed significant correlation with station 10. The
high value of salinity, alkaline pH and depth were
associated with fish groups such as Mugilidae,
Channidae and Sciaenidae. The meso-polyhaline
environment in the northern zone supported the
marine-estuarine and marine-dependent fish species
such as Mugilidae, Channidae, Sciaenidae, Carangidae,
Leiognathidae, Engraulidae and cat fishes
(Tachysuridae). Where, Mugilidae and Channidae
were catadromous, Sciaenidae and Engraulidae were
oceanodromous and Leiognathidae was Amphidromous
fishes. This observation is supported by Akin et al.,
(2005). According to Smith and Parrish (2002) most of
the species in the upper reaches of estuaries were
estuarine dependent species, while the species in the
