Rivers have been the major sources of fish production in Bangladesh from time immemorial. It has been reported that inland fisheries composed of 260 fish species belonging to 145 genera, representing 55 families. Fish species diversity and production from rivers have declined within four decades due to natural and anthropogenic disturbances.

Nature of riverine habitat degradation

If the plan is implemented as it is, then the Indian authorities will build 74 large and 34 medium water reservoirs at mountains and hundreds of pumps will be used to store water over there. Water will transferred from reservoirs through surface elevation. Very recently, Indian authority has started to construct a hydroelectric multi-purpose high dam on the river Barak, the origin of the river Meghna, at Teepaimukh in Monipur, India. If these projects are implemented then Bangladesh will tern into a desert. Saline water from Bay of Bengal will reach up to Sylhet-Mymensingh basin, which will create agro-ecological, economic and demographic disasters in Bangladesh.

Siltation: Available information revealed that 187 rivers (28% of total) are dead due to prolong siltation. It is reported that all the rivers of Bangladesh carry 3.8 billion tons of silt every year and 40 45 million tons are deposited only in the rivers Bangalee, Brahmaputra, Dharala, Jamuna, Padma and Teesta. Depth of 98 percent of rivers has decreased and submerged sand bars were found in 87 percent of rivers.

Siltation of river mouth: Mouth of 77 (12%) rivers has silted up due to continuous siltation. Mouth-silted main rivers include the Meghna, Feni, Muhuree, Karnaphulee, Bakkhalee, Tatulia, Ilisha, Andharmanik, Payra, Lohalia, Raymongal, Arpanga, Sheebsha, Pasur and all the rivers and tributaries of the Sundarbans.

River erosion: About 41 percent of rivers suffer from erosion problems and siltation rate has increased in 574 (86%) rivers. During rainy season, erosion is serious in the rivers Arial Khan, Baleshwar, Dhaleshwaree, Dharala, Jamuna, Meghna, Padma and their tributaries.

Obstruction in fish migration routes: Constructing flood control structures have obstructed fish migration routes in many rivers. It is reported that about 35 million ha of marshy land have disconnected from rivers due to implementation of 500 Flood Control Drainage and Irrigation projects during the last 60 years. Important flood control structures are on the rivers Baral, Dhanaguda, Feni, Gorai, Halda, Jamuna, Kapotakkya, Manoo, Meghna, Munuree and Teesta.

Unauthorised encroachment: Result revealed that width of 158 (24%) rivers has decreased due to unauthorised encroachment. Affected major rivers are the Buriganga of Dhaka, Baloo, Turag and Bongshee of Tongi, Kaleeganga of Manikganj, Kapotakkya and Nabaganga of Jessore, Tulshiganga and Pagla of Naogaon, Arial Khan of Faridpur, Narashunda and Kalagasia of Kishoreganj, Surma of Sylhet and Karnaphulee of Chittagong.

Chemical pollution
Industrial wastes and effluents: Results showed that 11 percent of total rivers are polluted due to industrial effluents, agro-chemicals and domestic garbage (Table 2). During winter season, water of the rivers Buriganga and Shitalakkya become poisonous not only to humans but also to birds and animals.

Agro-chemicals: About 1.6 million tons of inorganic fertilizers are used every year in Bangladesh. Moreover, a total of 4000 5000 tons of 242 types of pesticides are used annually for crop production. The flood and rainwater carry residues of these huge agro-chemicals to the river, haor and beel systems for final discharge in the coastal regions. Consequently, riverine habitats became degraded and many fish species have disappeared.

Biological degradation
A total of 15 exotic fish species have been introduced in Bangladesh for aquaculture and 93 fish species for ornamental purposes. Recently, Thai rupchanda / Piranha (Serrasalmus nattereri ?), Thai koi (Anabas sp.), Thai chitol (Notopterus sp.) have also been introduced and cultured in ponds of Bangladesh. All the aliens are carnivorous in feeding habits. During high flood, these species can be spread into the rivers of Bangladesh. Consequently, they may cause biological disaster in the riverine ecosystems of the country. Lake Kaptai is already polluted by the Tilapia (Oreochromis spp) as the species has self-sustaining populations. Self-sustaining populations of Carpio (Cyprinus carpio) and Silver carp (Hypophthalmichthys molitrix) are found in the rivers and haors of Sylhet-Mymensingh basin and in the Chalan beel. Being prolific breeders, Tilapia and carpio are polluting breeding and nursery grounds of many indigenous fish species. Similarly, Silver carp is more efficient feeder than the native Catla (Catla catla) and it is assumed that Catla populations will be declined in the presence of the alien Silver carp.

Discussion
Decrease in water flow is the main cause of physical degradation of riverine habitats of Bangladesh. Water flow in most rivers decreases alarmingly especially in winter season due to water withdrawal and flow diversion from the principal rivers by the Indian authority. Many rivers of Bangladesh flow only in the rainy season (May to October) with surface run-off and discharge from the hills. In winter season many rivers dry up due to low flow.

Number of dead rivers is increasing day by day due to siltation. Increased siltation, submerged sand bars and decreased depth in many rivers are due to high silt load carried by the principal river systems, the Barak-Meghna, Brahmaputra-Jamuna and Ganges-Padma. The principal river systems transport annual sediment loads of between 1.7 and 2.4 billion tons and silt deposition rate is about 35 million t yr-1. If this rate of siltation is continued and rivers are not dredged, then all the tributaries of these rivers will be silted up within next 25 years. Currently, navigation route reduced from 24,100 km to only 3,700 km during dry season. Disrupted navigation system impacts negatively on economy of the country as half of total transported commodities in the country are transported through riverways and more than 40 percent of total passengers depend on the river transport systems.

Erosion is a major problem in many rivers of Bangladesh. During dry season, sluice gates on the transboundary rivers in India are kept close to divert water flow, while in monsoon all the gates are kept open to pass huge water. This sudden high flow creates serious erosion and flooding. In 1974, 1988, 1998 and 2004 Bangladesh experienced severe floods.

All over the world, rivers are used as damping grounds of industrial and municipal wastes and effluents. Pollution from industrial, municipal and domestic discharges is an acute problem of the many rivers of Bangladesh and in the dry/winter season, no fish are found in these rivers. Moreover, residues of many agro-chemicals are washed down through surface run-off to the river systems, which is responsible for riverine habitat degradation, and disappearance of many fish species and decreased fish production. Unauthorised encroachment is a serious problem all over the country.

Conclusion
Most rivers of Bangladesh are going to be dead gradually because of erosion and siltation in the rainy season and low flow in the dry season. This study suggests that these rivers should be dredged to protect and conserve fish habitats and, riverine fish and their fisheries. However, in reality, dredging of river systems is a costly operation, which needs International aid, assistance, technical support and cooperation. Considering the cost of dredging, it is recommended to dredge a stretch of about 5 to 10 km of each river, which will act as a fish sanctuary of that particular river. Moreover, the dredged stretch of the river may be used as a source of domestic water supply for the nearest city / town. Silted mouth of rivers should be dredged to facilitate migration of diadromous fish species.

Natural water flow is a critical factor for the conservation of fish habitats. Therefore, it is necessary to maintain natural water flow to prevent drying up of most rivers and intrusion of saline water. In this connection, it is recommended to implement International Water Policy/Law 2003, UN Water Convention 1997, United Nations Environment Programme 1993, World Bank Water Resources Management 1993, Biodiversity Convention 1992, UNESCO World Heritage Convention 1972, Helsinki Rules 1966 to get natural flow from shared or common rivers originated in India and Myanmar. It is also recommended to take measures to protect and conserve riverine ecosystems from being polluted and encroachment upon.

In this connection, Environment and Industrial Acts, the Territorial Waters and Maritime Zones Acts 1974 should be implemented strictly. Protocols/guidelines should be developed for introduction of exotic fish species. To control over and illegal fishing, Fisheries Act should be applied strictly. It is also recommended to undertake detail studies on geo-morphological and ecological status of the rivers of Bangladesh in order to formulate guidelines for the management, protection and conservation of riverine habitats of Bangladesh.

Dr. Md. Khalilur Rahman is Principal Scientific Officer, Bangladesh Fisheries Research Institute (BFRI) Riverine Station, Chandpur.