The Sundarbans and coastal fisheries

Mangrove forests are considered a source of primary productivity in the form of litter: reproductive products, twigs and the whole dead tree which directly or indirectly contribute energy to the ecosystem. The mangroves improve soil salinity because they can prevent hypersaline conditions by removing salt from the soil through secretion by glands and leaves that then create a favouable environment for many other organisms.

The mangroves produce prop roots (as in the Rhizophoraceae) and pneumatophores with breathing pores for diffusion of oxygen into the plants. The mangroves can propagate by means of viviparity i.e. germination commences in the fruits while attached to the tree with the seedlings dropping from the tree to be dispersed by water some distance from the parent tree. The mud, suspended particles and other sediments are an important component of mangrove ecosystems. Unicellular algae and blue green bacteria utilise the mud surface for photosynthesis. The mud surface offers both food and shelter for many filter feeders, detritivores, herbivores and predators, including fish and crabs. The mangrove pneumatophores, aerial roots and lower branches often provide shelter for algae, barnacles and oysters that are, in turn, consumed by higher predators.

A network of rivers, canals and creeks intersect this forest, thus creating a different set of habitats to that provided by other forest types in the country. It is also a habitat, nursery ground and refuge for many species of marine and coastal fishes, shrimps, crabs, molluscs and mammals (e.g. dolphin), several species of marine turtles, reptiles such as the estuarine crocodile and different species of monitor lizard. Shrimps and crabs are the most important crustacean and are abundant in mangroves worldwide.

Sundarbans supports total 291 species of fishery resources and constitutes an important commercial and artisanal fishery industry that produces fresh fish, iced fish, sun dried fish, smoked fish, salted fish, fish meal and sharks oil. Thus, Sundarban provides a considerable harvest of whitefish, shrimps, prawns, mud crabs, snails/oysters and billions of shrimp/prawn post-larvae for shrimp aquaculture farms. These industries support about 155,000 fishermen throughout the year.

Mangroves are located in estuarine areas and provide important habitats for fish e.g. nursery functions, shelter for juveniles and food for piscivorous species. The extent of dependence may differ between different species e.g. most species of Mugilidae found in sheltered estuaries or mangroves seldom occur in coastal waters. In addition, tropical clupeoids are estuarine dependent as juveniles. Others such as Asian Tenualosa (ilish) are estuarine dependent with reference to spawning grounds. It is noted that predatory action of some larger fish are hampered due to mangrove structure although some individuals did penetrate these systems in search for food.

Mangroves play a nursery role for estuarine fishes. The mangrove forests are important for healthy coastal ecosystems because the forest detritus, consisting mainly of fallen leaves and branches from the mangroves, provides nutrients for the marine environment. These detritus support varieties of sea life in complex food webs associated directly through detritus or indirectly through the planktonic and epiphytic algal food chains. The plankton and the benthic algae are primary sources of carbon in the mangrove ecosystem, in addition to detritus. The shallow intertidal reaches where there is mangrove wetland provides refuge and nursery grounds for juvenile fish, crabs, shrimp and molluscs. The reasons for these dependencies may be described as follows:

(1) The trophic resources (e.g. convergence of riverine freshwater and tidal currents) produce large volumes of turbid water where organic particles and fragments are concentrated and subjected to strong microbial activity to release nutrients. The nutrient released is used by phytoplankton, at the base of a web including zooplankton and shrimps. Abundant food resources are thus made available to fish and shrimp post-larvae, with a range of planktonic food sizes matching their filtration and capture capabilities.

(2) Water turbidity and shade (e.g. turbidity and shade provided by the mangrove leaves and pneumatophores) reduce the perception distance of predators and increase the escape rate and consequently the survival rate of young fish and shrimps. Estuaries and mangroves are places where less fish predation occurs due to turbid water, absence of larger fish, shallow water and increased hiding places for juveniles in sea grasses or mangrove branch, roots, and pneumatophores.

(3) Structural diversity (e.g. diversity and structural complexity of mangroves and estuaries) offers trophic niches for different species and sizes. The higher concentration of food present in pneumatophore areas supports abundant fish species.

From the above discussion it is suggested that fish depend on mangroves and estuaries as nurseries (food and shelter) and that these habitats are important for juvenile and adult fish for their survival and growth. Hambrey, (1999) reported that economic valuation of the fisheries function of mangroves was estimated to range from US$ 66 to almost $3000/ha. Therefore, the worldwide estimates of the value of the mangroves to commercial fisheries have raised awareness about importance of mangroves. So the Sundarbans mangrove forest needs proper attention, management and political commitment for sustainable coastal fisheries development in the area.

Md Emdadul Haque is Divisional Forest Officer, Aquatic Resources Division, Sundarbans, Khulna.