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Abundance and Seasonal Distribution of Penaeus Monodon Postlarvae in the Sundarbans Mangrove, Bangladesh

M. Enamul Hoq
Bangladesh Fisheries Research Institute, Mymensingh 2201, Bangladesh

M. Nazrul Islam
Department of Fisheries Technology, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh

Abstract/ Executive Summary:

We record the decline of Penaeus monodon postlarvae (PL) in five rivers of the world’s largest mangrove ecosystem, the Sundarbans, from 1992 to 1999. Shrimp aquaculture in the coastal belt of Bangladesh is dependent on the collection of P. monodon PL from the coastal rivers, and horizontal expansion of shrimp farming has resulted in a severe decline of this wild resource in the Sundarbans. The abundance of P. monodon PL was significantly (P<0.05)reduced in 1999 compared to the previous two-year studies (1992 and 1995) in the rivers. About 12–551 post-larvae of other shrimps, 5–152 finfish postlarvae and 26–1636 other macro-zooplankters are wasted during the collection of a single P. monodon PL. Water temperature and salinity of the river systems are correlated with P. monodon PL abundance. Besides P. monodon PL, inshore fishery of Hilsa ilisha, catfishes and Scylla serrata is also overexploited. The management practices and conservation of fishery resources of Sundarbans are reviewed in the context of its world heritage status.

Keywords: Mangrove, Penaeus monodon, Abundance, Salinity, Fishery

Location: Sundarbans mangrove, Bangladesh

Start Date:

End Date:

Program Area: Conservation and Biodiversity

Commodity: Vegetation, Plant

Objectives:

This paper examines the present status of fisheries resources and the loss of biodiversity by wild P. monodon PL collection in Sundarbans rivers.

Methodology:

The Bangladesh Sundarbans covers an area of 5772 km² of which 3995 km2is land and 1777 km² is water, sketching between 21⁰30′N and 22⁰30′N, and 80⁰E and 89⁰55′E. Unlike most mangrove forests, the Sundarbans are not dominated by members of the family Rhizophoraceae. Based on species composition and salinity ecological zones, the Sundarbans Reserve Forest (SRF) has been classified into three zones: freshwater, moderate saltwater and saltwater. The zonation of the Sundarbans is defined by the distribution of three forest species; sundri (Heritierafomes, Sterculiaceae), gewa (Excoecaria agallocha, Euphorbiaceae) and goran (Ceriops decandra, Rhizo-phoraceae). All three occur throughout the Sundarbans but in different proportions depending on salinity. Five rivers were selected for studying losses by P. monodon PL collection. Passur, Sibsa and Koyrarivers represent freshwater, Kholpatua river represents semi-saline, and Madar river is saline. Three samplings, each of 2 years, were carried out from 1992to 1999 in these river systems. Samples were collected fortnightly from each river during new and full moons. At least 4 samples were collected per sampling. A rectangular dredge net of nylon netting (mesh size 1 mm) and bamboo split structure (1.6×0.6 m²) was used (these nets, along with fixed nets, prepared with the same materials are widely used for wild shrimp seed collection in nearshore waters, mangrove estuaries and tidal creeks) to assess the damage caused to mollusca fish and finish while collecting P. monodon PL. For each sample, the net was dragged twice, starting from shallow waters and continuing to the adjacent shore, covering a total area of 50 m2. The collected specimens were cleaned of any twigs, leaves, large organisms and preserved in 5% formalin. Shrimp and fish larvae and juveniles were identified by examining external morphological characters, body shape, and body proportions (FAO,1974; Howlader, 1976; CMFRI, 1978; Paul, 1995; Jeyaseelan, 1998). Surface water temperature and salinity were measured using a Salinity Refractometer (ATAGO S/Mill-E) and Alkaline Thermometer. Annual rainfall data was provided by the Department of Metrology. Statistical analysis was performed using the STATGRAPHIC (7.0) package.

Source of Information: Hydrobiologia 457:97–104, 2001.

Article Link (Online Journal):

Funding Source:

1.

Budget:

Total Budget (Tk.) :

Achievement/Findings:

At present, overfishing already occurs in some species and many other species are at risk. There is a need for the introduction of hatcheries and strict regulation on the mesh size of nets, release of unwanted species back to the water, and improved methods of preserving live P. monodon PL to reduce post-harvest mortality. Measures to regulate the SRF fishery by the introduction of closed seasons and protected zones should be considered. The migration of fish and crustaceans from protected areas to the over-fished waters could help the fishery to be sustained. Regulation of size at first capture and mesh size should be enforced by random inspection. The size of the fishery should be controlled by limiting the number of weekly gear licenses issued.

Knowledge Management: Journal