The Sundarbans environment and climate change: The Sundarbans mangrove forest is interconnected by an elaborate network of rivers, canals, and creeks, which makes its river salinity very complex. The Sundarbans is a deltaic swamp which receives large volumes of freshwater from inland rivers, and of saline water from the twice-daily tidal inundation from the Bay of Bengal. The climate of the Sundarbans is predominated by the south-west monsoon characterized by high rainfall. The mean annual rainfall varies from about 2000 mm in the east to 1600 mm in the west. Being located at the apex of the Bay of Bengal, the Sundarbans is accompanied with frequent cyclonic storms during May-June and severe cyclones with tidal surges during October-November (Hoq, 2008).
The Bangladesh Sundarbans Reserve Forest (BSRF) classifies the river systems in three different series as Raimanagal-Sibsa, Passur-Sibsa and Passur-Baleswar series. Hoq (2008) reported that mean monthly stream flow varies from 190 m3 /sec in March to 7,650 m3 /sec in August. The water salinity of south-eastern, middle-northern, and western part of the Sundarbans are delineated as 0-5 ppt, 5-18 ppt, and >18 ppt, respectively. The Sundarbans is now victim of excessive sedimentation which has resulted in the rise of forest floor, and in the absence of flushing of the forest floor by tidal water, the habitat condition changes and new species may start establishing themselves (Hoq, 2008). The shifting of the main course of the river Ganges eastward over the past few centuries, and the silting up of some smaller rivers feeding freshwater to western Sundarbans have caused a major reduction in freshwater supply to the western portion of the Sundarbans. This has influenced the changes in the Sundarbans mangrove species.
Two estimates of potential future sea level rise in Bangladesh are 0.30-1.5m for 2030 and 0.30-0.50m for 2050 (DOE, 1993). Majority of the mesohaline areas in the Sundarbans will be transformed into polyhaline areas, while oligohaline areas would be reduced to only a small pocket along the lower- Baleswar river in the eastern part of the forest. (Ahmed and Alam, 1998). The effects of climate change on the Sundarbans would be considerably more critical during dry season extending from November to April. In winter, a different environmental condition is expected with lesser freshwater supply in the rivers facilitating greater saline ingress into the Sundarbans.
Fisheries resources: The Sundarbans waters are highly productive and abound with a good number of fishes, crustaceans, molluscs, lobsters, crocodiles, reptiles, zooplankton, phytoplankton, and aquatic plants. The Sundarbans water-bodies provide homes for 53 species of pelagic fish, 124 species of demersal fish, 24 species of shrimp, 7 species of crab, 2 species of gastropoda, 6 species of pelecypoda, 8 species of locust lobster, and 3 species of turtle (Acharya and Kamal, 1994). Hoq (2008) and Acharya and Kamal (1994) reported that hilsa (Tenualosa ilisha), Asian sea-bass (Lates calcarifer), pangas (Pangasius pangasius), grey eel-catfish (Plotosus canius) and giant riverine prawn (Macrobrachium rosenbergii) are abundant in low salinity zone of the Sundarbans. Major commercial fishes and crustaceans such as hilsa, sea-bass, silver spotted grunt (Pomadasys hasta), Indian threadfin (Polynemus indicus), paradise threadfin (P. paradiseus), silver croaker (Johnius argentatus), black croaker (J. dussumieri), giant tiger shrimp, Indian white shrimp (Penaeus indicus), brown shrimp (Metapenaeus monoceros), Parapenaeopsis sp., and mud crab (Scylla serrata) are found in moderate saltwater zone. In highsaline water zone, Bombay-duck (Harpadon nehereus), Trichiurus haumela, gangetic hairfin anchovy (Setipinna phasa), scaly hairfin anchovy (S. taty), pama croaker (Pama pama), sardine (Sardinella sp.), sharks (Family: Carcharhinidae and Sphyrnidae) and rays (Family: Dasyatidae) are available. Goldspot mullet (Liza parsia), gray mullet (L. tade), flathead grey mullet (Mugil cephalus), corsula (Rhinomugil corsula), milkfish (Chanos chanos) and long whiskers catfish (Mystus gulio) are also common in the Sundarbans estuaries.