International Journal of Marine Science 2015, Vol.5, No.23 1-13
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Table 4 Results of CCA for fish assemblages between selected environmental parameters and the first two CCA axes
Axis
1
2
Eigenvalue
0.604
0.075
Constrained inertia (%)
80.248
9.971
Cumulative %
80.248
90.219
Total inertia
71.533
8.888
Cumulative % (%)
71.533
80.420
Canonical axis
Environmental parameters
CCAAxis 1
CCAAxis 2
pH
0.677675
0.099365
Depth
0.704023
0.08974
Water temperature
0.726389
-
0.05726
Dissolved oxygen
-
0.45776
-
0.12314
Salinity
0.73969
0.071924
Transparency
-
0.3749
0.045261
3 Discussion
Environmental factors are considered as essential to
determine the composition, distribution and
assemblage of fish species in an aquatic environment
(Mansor et al., 2012). In Vembanad estuarine system
depth was associated with the occurrence of species of
Mugilidae,
Chanidae,
Sciaenidae,
Cichilidae,
Cyprinidae and Hemiramphidae. Depth in tropical
waters formed an important factor in structuring the
species assemblages (Bianchi, 1992). Depth was
generally low in the present study where the northern
part was higher as compared to the southern zone that
also influenced the distribution of species as evident
from the CCA analysis. Fish in estuaries appears to
show a preference for relatively shallow waters
(Blaber, 1985); this provide shelter and refuge from
larger fish predators, which are normally confined to
deeper waters (Whitfield and Blaber, 1978). Water
depths control the habitats for fishes from biotic and
abiotic threats. Shallow water provides shelter from
piscivorous fishes, and deeper water shelters fishes
from avian and terrestrial predation (Power, 1987;
Gorman, 1988). During the study period an alkaline
condition was predominant in all stations of
Vembanad estuarine system. Joseph and Ouseph (2010)
and Jayachandran et al., (2013) also observed an
alkaline condition in Vembanad wetland system.
Generally the pH increases towards the northern zone
of estuary where the influence of sea water was higher.
Rainfall in Vembanad is mainly concentrated from
June to September (southwest monsoon) and October
to December (northeast monsoon); this is reflected by
higher river discharge into the estuary. During
monsoon season, the upper area of Vembanad estuary
is flushed with annual monsoonal flood water which
carries the river fish species that move down stream
concentrating in the estuarine area (Anon., 2007).
During this period the southern zone was dominated
by fresh water species such as
Amblypharyngodon
microlepis, Puntius sarana, Labeo dussumieri
and
Channa marulius.
Seasonal variations in rainfall create
or eliminate micro-habitats which are important for
fishery abundance (Olukolajo and Oluwaseum, 2008).
This indicates that rainfall has a direct relationship
with the species present in the water body. Seasonal
variations in rainfall act as a main factor, which affects
the strategies of the life cycle of fish, such as their
movement, feeding, growth and spawning (Roessig et
al., 2004). The dissolved oxygen was consistently
higher in the present study and it varied spatially from
4 to 9.6 mg/l. Martin, (2013) reported dissolved
oxygen concentration of Cochin backwater that varied
from 4-7 mg/l. The higher oxygen values are an
indication of higher photosynthetic activity. So it
should be able to support good fauna and flora
especially the fish species. Usually dense macrophyte
(
Eichornia
crassipes
) cover on the water surface that
shaded the water column and decreased the turbulent
mixing and transfer of oxygen from the atmosphere
happened in the Vembanad estuarine system. During
the study period macrophytes from the agriculture
field are trapped and so the weed infestation is
reduced in the estuary (Anon, 2012). The average
oxygen concentration in the oligohaline zone
