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International Journal of Marine Science 2013, Vol.3, No.2, 4
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conditions have direct relationship with growth,
reproduction, abundance and distribution of fishes they
were mainly temperature, salinity, dissolved oxygen,
nutrients, pollutants, water current, tide and turbidity of
water column (Vivekanandan and Sivakami, 2007).
Fish assemblages in estuaries are largely structured by
abiotic gradients (Kupschus and Tremain, 2001) that
include salinity (Martino and Able, 2003), temperature
(Maes et al., 2006) and dissolved oxygen (Weisberg et
al., 1996; Eby and Crowder, 2004). The significant
seasonal variation was observed in the fish assemblage
in the KAE during present study. South west monsoon
and associated river discharge; which finally resulted in
the decline in distribution and abundance of fish
community, fin fishes were suddenly responded and
avoided stressful conditions by migration and most of
the sessile organisms like molluscs died due sudden
environmental changes and heavy fresh water
discharge.
High rate of primary production was observed during
pre-monsoon period in the KAE and it has been support
fish assemblage in the estuary by providing suitable
energy sources. According to (Day, 1889) high rate of
coastal primary productivity, which suggest high
availability of pelagic and or benthic prey; this prey
availability could be the one of reason for observed
maximum fish production in the KAE during Pre
monsoon period. Similar trend was also reported from
the same estuary by (Harikrishnan et al., 2011) during
2005 to 2007 period. Whereas, fish production in the
estuarine mouth zone (EMZ) decreased to 348.5 t
during the present study as compared to earlier studies
(Harikrishnan et al., 2011) where 369 t (2005-2006)
and 424.8 t (2006-2007) were recorded. Total fish
production in the estuary also declined to 908.6 t, when
compared to the earlier studies (Anon, 2001) where
2 747 t was observed. Average fish catch yield in the
KAE (5.4 kg ha day
-1
) was well below the average
yield of estuarine fish production in India where it was
45 to 75 kg/ha (Jhingran, 1982). However, compared to
total annual average fish production in the whole
Vembanad wetland and including other back waters of
Kerala (14 000~17000 t; Sugunan, 2010) fish production
in the KAE was moderately good.
Both fisheries exploitation and increased nutrient
loadings strongly affect fish and shellfish abundance
and production in estuaries (Breitburg et al., 2009).
Turbidity level enormously increased particularly
during monsoon could be due to high influx of silt
content, agricultural runoff, sewages and other
allochthonous organic matters. Clam fishery in the
KAE drastically reduced from earlier reports and it
could be due to increased turbidity level and bottom
disturbances by intense sand and clam mining and
these stresses could be also affected larval settlement
pattern; similar situation was also reported from the
southern part of Vembanad Lake (Menon et al., 2000).
However, high turbidity level in the estuarine
environment generally offering increased partitioning
of resources and shelter from predators (Day, 1889).
Heavy fresh water discharges from the rivers during
monsoon and later caused to depletion of oxygen
content by the degradation of organic matter
accumulated in the basin of the estuary. The combined
effect of eutrophication and fisheries exploitation is to
simultaneously increase algal production, degrade
habitat and remove fish and shellfish biomass
(Breitburg et al., 2009). As well as, clam population
structure in the KAE altered on a temporal and seasonal
scale. Omnivore type of fishes in the estuary increased
during post monsoon season due to organic detritus
enrichment, high level of habitat diversity in the
estuary. In contrast, nutrient enrichment can reduce
sustainable harvest by reducing the growth, survival
and reproduction of target species where negative
effect of eutrophication (Breitburg et al., 2009).
Seasonal mean fish catch in the KAE was significantly
reduced during monsoon period and highest observed
during pre-monsoon period and it could be due various
stressors in the ecosystem. The observed reduction in
the fish landing during south west monsoon season was
not only due to environmental stress but also due to
decreased fishing days; marked by heavy rain. In this
ever changing environment salinity has an important
role, not only in determining the distribution of fishes
within an estuary, but also the abundance and diversity
of ichthyofauna. Salinity was comparatively high
during pre-monsoon months and it substantially reduced