Hydrobiologia 457: 97–104, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

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Abundance and seasonal distribution of Penaeus monodon postlarvaein the Sundarbans mangrove, Bangladesh

M. Enamul Hoq3, M. Nazrul Islam1, M. Kamal1 & M. Abdul Wahab2

1Department of Fisheries Technology, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh2Department of Fisheries Management, BAU, Mymensingh 2202, Bangladesh3Bangladesh Fisheries Research Institute, Mymensingh 2201, Bangladesh

Received 20 June 2000; in revised form 2 April 2001; accepted 20 April 2001

Key words: mangrove, Penaeus monodon, abundance, salinity, fishery

Abstract

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 thecollection of P. monodon PL from the coastal rivers, and horizontal expansion of shrimp farming has resulted in asevere decline of this wild resource in the Sundarbans. 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 postlarvaeof other shrimps, 5–152 finfish postlarvae and 26–1636 other macro-zooplankters are wasted during the collectionof a single P. monodon PL. Water temperature and salinity of the river systems are correlated with P. monodonPL abundance. Besides P. monodon PL, inshore fishery of Hilsa ilisha, catfishes and Scylla serrata are alsooverexploited. The management practices and conservation of fishery resources of Sundarbans are reviewed inthe context of its world heritage status.

Introduction

The Sundarbans, the largest single compact mangroveecosystem in the world, is located in the GangeticDelta, spanning an area of about one million hectaresin South-west Bangladesh and the South-eastern partof the State of West Bengal in India. The Sundar-bans are a complex estuarine ecosystem, dominated bydense forest and classified as a tropical moist forest.The fisheries resources of the Sundarbans constitute2–5% of the total capture fisheries of Bangladesh(Rabbani & Sarker, 1997). Its waters constitute richhabitats for many fish species and serve as fishinggrounds for the local human communities.

Extensive sampling has shown that juvenile pen-aeid prawns are often highly abundant in mangrovehabitats (Boonruang & Janekaru, 1985; Chong et al.,1990; Sasekumar et al., 1992). This nursery roleof mangroves has also been demonstrated by studiesthat compare shrimp populations in mangroves andother habitats (Robertson & Duke, 1987; Chong et

al., 1990). The rapid expansion of coastal aquaculturein Bangladesh, coupled with a trend towards shrimpmonoculture, has resulted in a tremendous demandfor the seed of tiger shrimp, Penaeus monodon. Buta paucity of shrimp hatcheries has attracted thousandsof coastal fisherfolk to collecting P. monodon PL fromSundarbans and nearshore waters, causing destructionof non-target species and damage to nursery grounds.It is estimated that 10 kg of fish and shrimp larvaeare destroyed by the collection of every 1 kg of tigershrimp PL in the Sundarbans of West Bengal, India(Silas, 1987). In Bangladesh, up to 5000 shrimp PLmay be wasted for every 100 marketable shrimp PLcaptured by collectors (BOBP, 1990). This paper ex-amines the present status of fisheries resources and theloss of biodiversity by wild P. monodon PL collectionin Sundarbans rivers.

Materials and methods

The Bangladesh Sundarbans covers an area of 5772

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Figure 1. The ecological zones of Sundarbans and the sampling stations.

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km2 of which 3995 km2 is land and 1777 km2 is wa-ter, sketching between 21◦ 30′ N and 22◦ 30′ N, and80◦ E and 89◦ 55′ E. Unlike most mangrove forests,the Sundarbans is not dominated by members of thefamily Rhizophoraceae. Based on species compositionand salinity ecological zones, the Sundarbans ReserveForest (SRF) has been classified into three zones:freshwater, moderate saltwater and saltwater (Fig. 1).The zonation of the Sundarbans is defined by thedistribution of three forest species; sundri (Heritierafomes, Sterculiaceae), gewa (Excoecaria agallocha,Euphorbiaceae) and goran (Ceriops decandra, Rhizo-phoraceae). All three occur throughout the Sundarbansbut in different proportions depending on salinity.

Five rivers were selected for studying losses byP. monodon PL collection. Passur, Sibsa and Koyrarivers represent freshwater, Kholpatua river representssemi-saline, and Madar river is saline (Fig. 1). Threesamplings, each of 2 years, were carried out from 1992to 1999 in these river systems.

Samples were collected fortnightly from each riverduring new and full moons. At least 4 samples werecollected per sampling. A rectangular dredge net ofnylon netting (mesh size 1 mm) and bamboo spiltstructure (1.6×0.6 m2) was used (these nets, alongwith fixed nets, prepared with the same materialsare widely used for wild shrimp seed collection innearshore waters, mangrove estuaries and tidal creeks)to assess the damage caused to shellfish and finfishwhile collecting P. monodon PL. For each sample, thenet was dragged twice, starting from shallow watersand continuing to the adjacent shore, covering a totalarea of 50 m2. The collected specimens were cleanedof any twigs, leaves, large organisms and preservedin 5% formalin. Shrimp and fish larvae and juvenileswere identified by examining external morphologicalcharacters, body shape, and body proportions (FAO,1974; Howlader, 1976; CMFRI, 1978; Paul, 1995;Jeyaseelan, 1998).

Surface water temperature and salinity were meas-ured using a Salinity Refractometer (ATAGO S/Mill-E) and Alkaline Thermometer. Annual rainfall datawas provided by the Department of Metrology. Statist-ical analysis was performed using the STATGRAPHIC(7.0) package.

Results

Loss of biodiversity

Penaeus monodon PL collection trends between 1992

to 1999 are presented in Table 1. Availability of P.monodon PL was significantly (P<0.05) reduced in1995 and 1998 compared to 1992 in all rivers ex-cept Kholpatua. In Kholpatua river, abundance of P.monodon declined in 1995, but increased in 1999. Theabundance of P. monodon was highest in Kholpatuariver, located in the semi-saline zone, followed bythe rivers of the freshwater zone (Passur, Sibsa andKoyra), and lowest abundance occurred in the salinezone (Madar). Occurrence of other shrimp spp. andfinfishes were comparatively low in Kholpatua river in1995. Composition of macro-zooplankton in differentrivers in 3 years did not differ significantly. Shrimpspecies other than P. monodon included Penaeus in-dicus, Metapenaeus monoceros, M. brevicornis, Pa-laemon styliferus, Macrobrachium rosenbergii, M.villosimanus, M. dyanus, M. dolichodactylus and M.rude. The main finfish were Mugil parsia, Lates cal-carifer, Glosogobius giuris, Hilisha ilisha, Pangasiuspangasius, while macro-zooplankton included Acetesspp., mysids, alima larvae, copepods, isopods andmegalopa larvae.

Postlarvae of other shrimp species and of fish areharvested together with P. monodon in collectors’ net.The collectors pick out the PL of P. monodon, and dis-card the other species. It was estimated that to catcha single P. monodon PL, 12–551 postlarvae of othershrimps, 5–152 finfish postlarvae and 26–1636 othermacro-zooplankton are wasted (Table 2), a high rateof ‘wastage’.

Table 3 represents the correlation of P. monodonpostlarval abundance with water temperature and sa-linity. A significant correlation of PL abundance withtemperature and salinity existed in the Passur andSibsa rivers in 1995–1996, and in Sibsa and Madarrivers in 1992–1993, respectively.

Natural environment

Seasonal variation was observed in the abundance of P.monodon PL. P. monodon PL were found year-roundbut peak abundance was from October to February,linked to the moderate salinity of the river water (Fig.2). Mean yearly rainfall was 1543, 2033 and 2563mm in 1992–1993, 1995–1996 and 1998–1999, re-spectively. The seasonal variation in surface watertemperature ranged from 19.4 to 33.9 ◦C in 1992–1993, a difference of 14.5 ◦C. In 1995–1996, it rangedbetween 17.2 and 33.4 ◦C, a difference of 16.2 ◦Cand in 1989–1999 it was between 21.7 and 33.4 ◦C,a difference of 11.7 ◦C. Annual lows occurred in

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Table 1. Yearly catch composition (per cent individual/unit effort)∗ of P. monodon PL, other shrimp spp., finfish and macro-zooplankton infive rivers of Sundarbans

Mangrove rivers Yearly catch∗∗P. monodon Other shrimp

1992–1993 1995–1996 1998–1999 ±SE 1992–1993 1995–1996 1998–1999 ±SE

Passur 2.30a 0.50b 0.17c 0.66 27.67b 47.76a 16.74b 9.08

Sibsa 0.22a 0.18a 0.06b 0.05 10.51b 9.84b 33.07a 7.63

Koyra 1.13a 0.34b 0.16b 0.30 17.96a 16.76a 24.29a 2.34

Av. 1.22a 0.34b 0.13b 0.33 18.72a 24.79a 24.70a 2.01

Kholpatua 1.79a 0.06b 0.17b 0.56 20.30a 3.70b 17.53a 5.13

Madar 1.09a 0.44b 0.19c 0.27 26.82a 17.16b 9.37c 5.05

Av. 1.37a 0.28b 0.17b 0.38 21.95a 15.22b 17.20b 2.00

Finfish Macro-zooplankton

1992–1993 1995–1996 1998–1999 ±SE 1992–1993 1995–1996 1998–1999 ±SE

Passur 10.82a 9.96a 16.93a 2.19 59.21a 41.78b 66.16a 7.25

Sibsa 3.48b 2.06b 9.13a 2.16 85.79a 87.92a 57.74b 9.72

Koyra 12.48a 5.95a 10.67a 1.95 68.43b 76.95a 64.88b 3.58

Av. 8.93a 5.99a 12.25a 1.81 71.15a 68.89a 62.93a 2.45

Kholpatua 4.30a 1.02b 6.93a 1.71 74.74b 95.23a 75.37b 6.73

Madar 5.34a 5.52a 2.37a 1.02 66.75c 76.89b 88.07a 6.16

Av. 6.19a 4.18a 7.19a 0.89 70.88a 80.34a 75.46a 2.73

∗Operating a dredge net (1.6×0.6 m2) in 50 m2 as a unit effort.∗∗Average sum of 96 samples (4 samples for new and full moon each in a month) from each river.Figures in the same row with same superscript letter did not differ significantly at 5% level.

Table 2. Number of species wasted for each P. monodon PL collected during 1992–1999

Rivers Species 1992–1993 1995–1996 1998–1999

Passur Other shrimp 12 96 98

Finfish 5 20 100

Macro-zooplankton 26 84 389

Sibsa Other shrimp 48 55 551

Finfish 16 11 152

Macro-zooplankton 390 488 963

Koyra Other shrimp 16 49 152

Finfish 11 18 67

Macro-zooplankton 61 226 406

Kholpatua Other shrimp 31 60 103

Finfish 7 17 41

Macro-zooplankton 113 1636 443

Madar Other shrimp 25 33 49

Finfish 5 12 12

Macro-zooplankton 61 242 464

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Table 3. Correlation coefficient of P. monodon PL abundance with temperature and salinity in five river systems of Sundarbans

Parameters Year Rivers

Passur Sibsa Koyra Kholpatua Madar

Temperature (◦C) 1992–1993 −0.461 −0.241 −0.504 −0.022 0.060

1995–1996 −0.712∗ −0.551∗ −0.347 0.071 0.011

1998–1999 −0.212 0.241 −0.269 −0.334 −0.135

Salinity (ppt) 1992–1993 0.420 0.642∗ 0.012 −0.055 0.669∗1995–1996 −0.362 0.432 −0.086 0.302 0.448

1998–1999 0.446 0.160 0.444 0.013 −0.223

∗=P<0.05.

Table 4. ANOVA of the effects of year, month, riverand interaction on temperature and salinity in Sundar-bans mangrove during 1992–1999

Parameters d.f SS F values

Temperature (◦C)

River 4 4.78 0.08 ns

Year 2 33.47 1.00 ns

Month 11 736.45 51.91∗Year×River 14 89.27 0.36 ns

Month×River 59 2395.33 8.20∗Year×Month 11 2209.60 43.29∗

Salinity (ppt)

River 4 612.45 4.41 ns

Year 2 173.05 1.79 ns

Month 11 1820.14 11.32∗Year×River 14 2301.34 4.23∗Month×River 59 7558.61 13.38∗Year×Month 11 5456.36 25.63∗

(d.f=degrees of freedom, SS=sums of squares,MS=SS/d.f, F=MS Group/MS within group).∗=P<0.05, ns=not significant.

January and highs in June with the exception of 1989–1999 when the highest temperature occurred in April.Analysis of variance (ANOVA) showed significant(P<0.05) inter-month variations in temperature in therivers (Table 4).

Salinity ranged from 0.5 to 23‰ in 1992–1993, 0.6to 22‰ and 0.5 to 30‰ in 1995–1996 and 1998–1999.In general, low salinity occurred between August andDecember. Following the monsoon, salinity stead-ily increased to reach its maximum in May. Therefollowed a decline from June to a minimum in themonsoon months (July–September). In the Passur andSibsa rivers, salinity fell below 1‰ from August toNovember. ANOVA indicates a significant (P<0.05)

effect of year and months in the temperature andsalinity regimes.

Discussion

The Bangladesh coast supports a 587 400 ha of nat-ural mangroves and a further 100 000 ha of plantedmangroves. This vast network acts as a habitat ofmany species, particularly young stages of shell andfinfish (Mahmood et al., 1994). A positive correla-tion between nearshore yields of fish and mangrovearea has been documented elsewhere (Martosubroto& Naamin, 1977; Staples et al., 1985; Camacho &Bagarinao, 1987). In recent years, the nearshore fish-eries have been over-exploited along the Bangladeshcoast. Mass shrimp fry collection is a threat to thecoastal ecosystem, causing damage to the nurserygrounds of many species, and to newly planted man-groves as well as to reserve forests (Saikat, 1992).

The Sundarbans waters support 208 species of fishand crustaceans in 84 families (IUCN, 1994), a highertotal than that for other tropical mangroves (Robertson& Blaber, 1992). The total landed harvest, includ-ing the offshore catch, may be as high as 75 000 MT(MARC, 1995). The mean fish biomass is 39 kg/ha,comparable to the Rhizophora-dominated forests ofMalaysia (Sasekumar et al., 1994).

The yield of Sundarbans fisheries have started de-clining for a number of years, except for P. monodonPL and crab (Rabbani & Sarker, 1997). However, P.monodon PL and crab in the tidal waters are also over-fished. Estimation of yield, exploitation rate and MSYof major species in SRF was determined by Chant-arasri (1994) and Smith (1995). The fishery of Hilsailisha, Pangasius pangasius, Plotossus spp., Scyllaserrata were found to be over-exploited at E=0.36–

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Figure 2. Monthly variation of Penaeus monodon PL with salinity during 1998–1999 in different rivers of Sundarbans.

0.42. The exploitation of Lates calcarifer was estim-ated to be at optimum at present. The yield was about150 MT. Increasing the effort would give a lower yield.The fishing of Johnius argentus is nearly fully ex-ploited. The present M. rosenbergii fishery is far fromthe optimum. The exploitation of gastropod and oysterseems to be underutilized. Estimates of sustainableyield require accurate base data, and more data ofgreater reliability will be needed before precise tar-get yields can be set for SRF, and only preliminaryinvestigations have been made.

Hilsa (H. ilisha) accounts for nearly 30% of totalfisheries of Bangladesh. Out of an annual total of120 000 MT, nearly 13 000 MT comes from the

Sundarbans. Hilsa landing in SRF was reduced by38% in recent years from 10 years back. Long-termchanges in biological behavior, particularly in spawn-ing and migratory habits as a result of changes in themorphology and hydrology of the major river systemsof Bangladesh, may be responsible for this. The mudcrab (S. serrata) is harvested on an increasing scalefrom the Sundarbans and is currently a lucrative busi-ness. The quantity of crabs brought to the market hasnearly doubled in the last 5 years. The potential of crabfishery seems to be vast and the prospects of its cul-ture, just as with shrimp, need to be explored. Duringshrimp fry collection crab seed are indiscriminatelydestroyed.

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A prominent feature of the SRF fishery is that, foras long as records have been kept, its size has beenexpanding. This expansion will need to be controlledif fish and crustacean stocks are to be conserved forthe future. Without control, recruitment overfishingwill occur and stocks of the currently most heavilyexploited species will crash. Fishermen will then tar-get other, currently less desirable species, and underincreased fishing pressure, these populations too willcrash. The number of euryhaline fishes in Sundar-bans water is thereby gradually diminishing (Jana &Ghatterjee, 1974).

In the present study, monthly variation in abund-ance of P. monodon PL was observed in its rivers.Although P. monodon PL is available throughout theyear, its density was high in freshwater rivers (Table1, Fig. 2), except in Sibsa, but the sampling stationlocated in Sibsa was further away from the estuarythan other river systems under study. Moreover, riverdischarge in Sibsa was high, due to the presence oflarge numbers of shrimp farms in and around. Gen-erally, P. monodon PL was abundant from October toFebruary, associated with moderate salinity. A majorpeak in abundance did not occur, however, during ourstudy years. This decline of P. monodon PL from 1992to 1999 is a potential threat to their future. Primavera(1998) and Basu et al. (1998) observed two peaks forpenaeid recruitment and settlement, in winter and pre-monsoon, at average salinity and high temperature.A similar trend was found in the present study. Za-far & Mahmood (1994) observed highest density ofpenaeid post larvae in July at high salinity and low-est in March in estuarine waters near Sundarbans. P.monodon PL is available year-round, but its abund-ance is limited when required for stocking in coastalaquaculture ponds.

Water temperature in the five rivers studied wasnegatively correlated with P. monodon PL abundance.Only 4 times was it positively correlated in Sibsa,Kholpatua and Madar rivers. Salinity have positive,in some cases significant correlation, with PL abund-ance. The salinity has been found to determine therecruitment of prawn populations (Easo & Mathew,1989), and at moderate rainfall, extends the nurseryarea (Garcia & Le Reste, 1981). Our results indicatethat the recruitment of other shrimps, finfishes andmacro-zooplankton components of the aquatic foodchain will severely decline within the next few yearsas the consequence of extensive P. monodon shrimpPL collection. It is essential that the shrimp postlar-vae reach the small creeks and brackish waters of the

estuaries to find shelter and food. The recruitmentof shrimp in the deep sea is directly dependent onthe survival of this juveniles in the mangrove nurs-ery grounds. If these are reclaimed and/or juvenilesare captured, this will negatively reflect on the adultpopulation. This scenario will also apply to finfishlarvae, indiscriminately exploited with the targeted P.monodon PL, with disastrous effects on artisanal andcommercial fisheries in the near future.

The IUCN has declared the Sundarbans a WorldHeritage Site in 1999 and the Govt. of Bangladeshhas pledged to refrain from “any deliberate measures-direct or indirect, which might damage the Sundarbansand to take appropriate legal, scientific and financialmeasures to ensure its protection”. However, with over3.5 million people depending directly or indirectly onthe Sundarbans for their livelihood, the forest has beenreduced alarmingly from even 20 years ago (Chaffey,1985). Overfishing and over-exploitation of plant andwildlife species are placing great stress on the viabilityof this ecosystem.

Conclusions

At present, overfishing already occurs in some speciesand many other species are at risk. There is a need forthe introduction of hatcheries and a strict regulation onthe mesh size of nets, release of unwanted species backto the water, and improved methods of preserving liveP. monodon PL to reduce post-harvest mortality.

Measures to regulate the SRF fishery by the intro-duction of closed seasons and protected zones shouldbe considered. The migration of fish and crustaceansfrom protected areas to the overfished waters couldhelp the fishery to be sustained. Regulation of sizeat first capture and mesh size should be enforced byrandom inspection. The size of the fishery shouldbe controlled by limiting the number of weekly gearlicenses issued.

Acknowledgements

The partial funding from World Bank financed ARMP(Agricultural Research Management Project-FRI part)is greatly acknowledged. The authors are thankful tothe scientific staffs of Brackishwater Station, BFRI,Khulna, Dr S.U. Ahmed and M.M. Islam, for provid-ing data, and Mr A. Baki and M.S. Hossain for helpwith field collections.

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