322
salinity is 32.5 psu during premonsoon, 16.0 psu during monsoon and 28.0
0
/ 00 during postmonsoon (Mitra 2000). A significant lowering of surface water salinity
values is observed in all the areas of Indian Sundarbans during monsoon period
(July to October). In addition to direct precipitation, the run off from the adjoining
landmasses also play a significant role in reducing the salinity to very low values.
The fresh water brought into the estuarine and coastal zone through a large number
of existing riverine systems also increases the dilution factor and subsequently
decreases the salinity. The Hugli and Mooriganga rivers are the major contributors
of freshwater in the Bay of Bengal zone as they are connected to Himalayan glaciers. La Fond (1954) explained that the decline in salinity of the surface waters in
coastal zone is mainly due to the riverine contribution during monsoon. The subsequent eight months from November to June is a period of restoration. This upward
trend of salinity continues till the summer maximum is reached during June.
Considering the salinity profile of the entire deltaic lobe of Indian Sundarbans, the
lower stretch of the Hugli-Matla estuarine complex is favourable for the culture of
edible oyster as salinity below 10‰ hardly reaches in this marine dominated estuarine stretch.
Turbidity
Turbidity in the estuarine and coastal waters is the effect of suspended particulate
matters, which are basically contributed by land drainage and turbulence within the
aquatic ecosystem that churns the bottom sediments and transfer them in suspension. As a result of turbidity, the reduction of light intensity in the vertical column
of seawater occurs due to which the phytoplankton growth is hindered. Thus turbidity often limits the food reservoir of oysters. Loosanoff and Tommers (1948) studied
the effect of suspended solids on feeding rate of oysters and concluded that oysters
are very sensitive to suspended silt and there is an inverse relationship between
concentration of suspended load and rate of pumping. Dead and dying oysters found
in turbid waters invariably contain large amount of silt in their gills (Loosanoff
1962). The aquatic phase of Indian Sundarbans is highly turbid owing to run-off
from the adjacent land masses and nature of underlying substratum and hence offbottom culture is preferred to on-bottom culture technology. Vertical cubical pillars
(around 3 m height) in mangrove-sheltered creeks are also suitable cultch materials
for oyster spat setting in deltaic Sundarbans region.
Wave Action
Waves and currents affect the process of setting oyster larvae on cultch materials or
substratum. In general oyster spats are found at the bottom where the current action
is weak than the surface. In Indian Sundarbans region, the waves and current
9 Mangroves: A Source of Existing and Alternative Livelihood
salinity is 32.5 psu during premonsoon, 16.0 psu during monsoon and 28.0
0
/ 00 during postmonsoon (Mitra 2000). A significant lowering of surface water salinity
values is observed in all the areas of Indian Sundarbans during monsoon period
(July to October). In addition to direct precipitation, the run off from the adjoining
landmasses also play a significant role in reducing the salinity to very low values.
The fresh water brought into the estuarine and coastal zone through a large number
of existing riverine systems also increases the dilution factor and subsequently
decreases the salinity. The Hugli and Mooriganga rivers are the major contributors
of freshwater in the Bay of Bengal zone as they are connected to Himalayan glaciers. La Fond (1954) explained that the decline in salinity of the surface waters in
coastal zone is mainly due to the riverine contribution during monsoon. The subsequent eight months from November to June is a period of restoration. This upward
trend of salinity continues till the summer maximum is reached during June.
Considering the salinity profile of the entire deltaic lobe of Indian Sundarbans, the
lower stretch of the Hugli-Matla estuarine complex is favourable for the culture of
edible oyster as salinity below 10‰ hardly reaches in this marine dominated estuarine stretch.
Turbidity
Turbidity in the estuarine and coastal waters is the effect of suspended particulate
matters, which are basically contributed by land drainage and turbulence within the
aquatic ecosystem that churns the bottom sediments and transfer them in suspension. As a result of turbidity, the reduction of light intensity in the vertical column
of seawater occurs due to which the phytoplankton growth is hindered. Thus turbidity often limits the food reservoir of oysters. Loosanoff and Tommers (1948) studied
the effect of suspended solids on feeding rate of oysters and concluded that oysters
are very sensitive to suspended silt and there is an inverse relationship between
concentration of suspended load and rate of pumping. Dead and dying oysters found
in turbid waters invariably contain large amount of silt in their gills (Loosanoff
1962). The aquatic phase of Indian Sundarbans is highly turbid owing to run-off
from the adjacent land masses and nature of underlying substratum and hence offbottom culture is preferred to on-bottom culture technology. Vertical cubical pillars
(around 3 m height) in mangrove-sheltered creeks are also suitable cultch materials
for oyster spat setting in deltaic Sundarbans region.
Wave Action
Waves and currents affect the process of setting oyster larvae on cultch materials or
substratum. In general oyster spats are found at the bottom where the current action
is weak than the surface. In Indian Sundarbans region, the waves and current
9 Mangroves: A Source of Existing and Alternative Livelihood