Freshwater Aquatic Ecosystems of Bangladesh
Rivers: There are about 700 rivers, with a total length of over 24,000 Kms. These include two of the world's largest
rivers, the Ganges and the Brahmaputra which form an extensive estuary of 552,000 hectares.
Riverzone and Khals (Canals) cover an estimated area of 480,000 hectares.
Open Waters: Beels and Haors: Although their ecology is not identical, they are grouped as one category with a
combined area estimated to be about 118,000 hectares, and classed as open waters. They become part of the flowing
water system when inundated, during monsoon. Beels are swamp lands formed in the dead channels of former rivers
which may contain water during part of the year only. Baoys are tectonic depressions filled with water for part of the year,
with the deepest part containing water throughout the year. Baoys or oxbow lakes are formed in the meandering parts of
old rivers which became cut off from the river system.
These are standing freshwater bodies and mainly situated in Jessore and Kushtia districts. The area of Baors is
estimated at 5,500 hectares.
Ponds: Used for fish culture, they are mainly burrow pits, i.e. closed water bodies, excavated and filled with water, a
rough estimate of pond area would be close to 200,000 hectares.
Burrowpits consist of a variety of innumerable excavated pits along with roads, highways, irrigation channels etc. Their
area and length are unknown.
Lakes are large freshwater reservoirs, either constructed or natural. The three natural lakes in Bangladesh are the
Rainkhyngkine lake, the Bogakine lake, and the Ahsula lake. The two artificial lakes at Kaptai and Feni have significantly
different ecosystems because of different discharge system.
There is a dynamic equilibrium among physical environment, biodiverse aquatic resources and livelihood strategies that rural
households have developed. The cycle of reproduction and growth of aquatic food resources is closely geared to the cycle of
seasonal and physical change of the floodplains. The process of migration, the onset of breeding and reproduction are
triggered by environmental signals, which bring a complex array of genetic, sexual, hormonal and behavioural factors into
play. Environmental parameters include day length, turbidity, water temperature, oxygen and carbon dioxide levels, pH,
currents, rain fall, wave action, availability of food etc. Other factors include intra and inter-specific competition (Hossain,
1989) between species.
Some SIS breed throughout the year, other species breed more than one between March and October, whilst others
maintain more confined breeding seasons. A few prefer clean water and low temperature for breeding.
Many SIS breed in baors throughout the year. Haque et at (1996) identified 50 species in two oxbow lakes in southwest
Bangladesh with a yield ranging from 78 kg/ha/month to 32 kg/ha/month.
In the case of small prawns in the Rajshahi area, Dutta et al (1996) found that they were present in the catch throughout the
year, .put" with two peak seasons (December to February and June to early August). In beels and small rivers in Rajshahi,
Dutta et al (1996) found relatively high densities of small prawns. They estimated the total prawn production for greater
Rajshahi area s 8,391. 16 maunds (37 tonnes) per day.
Breeding behaviour can be broadly categorized into two types: Flood plain (and beel) breeders, and fish that breed in rivers.
For many species breeding begins during the pre-monsoon floods. The optimal conditions are during the storms when flash
floods, continuous rain and thunder storms stimulate fish breeding. Fish begin to move against the pre-monsoon flood waters
as they enter the beels in search of a suitable spawning substrate. Thus the Ghollia, Boal, Foli, Pabda, Shoal, Gazar, Lati,
Cheng, Koi, and Laso prefer to breed on freshly inundated grassy areas, where current is slow, water depth is shallow, and
bush and reeds are present (FAP 6, 1993).
Miukin (1989) describes the up-river migration of fish, and their lateral rillgration into the flood plain~ during the rainy season.
In rivers the timing of the spawning migration is between April and June. The survival of fish in beels is correlated to surface
area of the wafer, and this determines how many spawns will survive to the following season (CIDA 1989). During the dry
season food supply is limited, fishing pressure is high and growth is slow (Welcome 1985).
Hossain et al (1997) described the onset and progress of breeding cycle of SIS according to six parameters: