International Journal of Marine Science, 2017, Vol.7, No.16, 141-160
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3.5 Major causes of fish depletion
In Bangladesh, the fish species from the inland and marine water bodies declined gradually due over the last two
decades and the catch of fish declined about 40% compared to the past 20 years (DoF, 2002). A number of studies
indicated that the major cause of declining fish catch from the river are the increased fishing pressure and habitat
destruction (Tsai and Ali, 1987; Siddique, 1990; Hogarth et al., 1999; Graff et al., 2001). Species diversity also
declined due to indiscriminate use of gears, over fishing, destruction of spawning ground and trapping of so called
white fish (i.e., carps and butterfish) during their downstream migration from floodplains to river (Ali, 1997;
Hogarth et al
.
, 1999; Graff et al
.
, 2001). Pollution, soil erosion, siltation, reclamation of land for settlement,
reduction of wetland and biodiversity and proper management problem are the probable causes for the declination
of the fishery resources (Khan, 2005).
4 Impacts of Pollution
The scientists in a recent research on the Karnafully found traces of radioactivity ‘very close to risk level’ on the soil.
If radioactivity of the river soil goes up it will hamper the natural breeding of fish as well as growth of fishes. If the
people eat the affected fish it may spread to their body (Amin, 2015). Ahmed and Reazuddin (2000) reported that
the availability of the heavy metal in river water directly affects the fish physiology and by the consumption
ultimately affects the human health. The presence of heavy metals in the aquatic environment is of major concern
because of their heavy toxicity, bio-accumulating tendency in the biota. Pollution by heavy metals is a threat to
human life and the entire environment as well as the wetland ecosystem (Islam and Tanaka, 2004; Igwe and Abia,
2006).
Oil pollution responsible for loss of productivity, other resources and exert adverse effects on aquatic environment,
sometimes it become carcinogenic to living organisms (Ahmed, 2006). Oil pollution causes severe damage to
fishes and crustaceans.
Contaminated water is not suitable for household uses and possibly hazardous to many aquatic animals and human
health.
Salmonella
spp. concentrations found in water and soil samples exceed the standard level both in water
and soil that indicates contaminated water is unsuitable for drinking or for even washing without appropriate
water treatment for humans (Alam, 2012).
Escherichia coli
and
Staphylococcus
causes abdominal cramping, water
borne diseases like diarrhea, fever, nausea and vomiting (HACCP, 2000).
Long term consumption of fishes may have a negative impact on human health (Bhuyan et al. 2016). Pathogens
(Oligochaetes) are causative agents of different types of diseases of human beings found very high abundance
near sewage outfalls areas in the Karnafully River estuary, which indicates localized pollution in the estuary
(Hossain, 1987). The major channels which carry domestic wastes and spread into the coastal city areas causing
pathogenic microbial pollution and serious health hazards during the rainy season and flood periods severity is
more in the Bay (Mahmood et al.,
1994). 20,000 fishermen in Raujan, Rangunia and Anowara thanas previously
dependent on the Karnafully river have given up their fishing profession because they do not have enough catches
any more (SEHD, 2002). Lower concentrations of methyl mercury may kill aquatic organisms (Hossain, 2004).
Paul (1981) also found the minimum phytoplankton population in the oil polluted Karnafully River Estuary. Most
species of fish cannot survive in insecticides in concentrations greater than about 1-10 ppb. A sub lethal dose for
fish and birds may bring immediate danger (Ali, 1997).
About 23 species of fishes were found in the Karnafully River during 1975-76, which has come down to 6-7
species during 1987-88. Effluents discharged from the large industries have ruined the Hilsha stock of the
Karnafully river (Hossain et al., 2006). Presence of heavy metals in the river water causes perilous impact on the
aquatic organisms (Dey et al
.
, 2015). The fish catch is diminishing in the river year after year due to depletion of
DO (Ali, 1997). The toxic chemical pollutants like Hg, Pb, Cd, COC and DO were found higher than the EQS
value which is dangerous for entire aquatic ecosystem and public health (Akter, 2012). Excessive discharge of
nitrogen, sulphur and phosphorus compounds in the water system can cause eutrophication (Bhouyain, 1981). The
presence of NO
3
-
in natural water associated with ecological and health hazard, excess NO
3
-
in human food and
