Bangladesh J. Fish. Res., 3(1), 1999 : 107-112 Scientific Notes

Colossal loss of shell and fin-fish larvae during collection of Penaeus monodon (Fab.) fry in the rivers of Satkhira

M.S. Islam*, M.M. Islam and S.U. Ahmed Brackishwater Station, Fisheries research Institute, Khulna 9280, Bangladesh *Corresponding author

Abstract This study was conducted in five river-estuaries of Satkhira from January to December'96. It was fotmd that during the collection of each Peneaus monodon) postlarva (PL), about 45 larvae of other shrimps, 12 individuals of fin-fishes and 530 macro-zooplankters were mercilessly destroyed. It was also recorded that about 11.6 million of P. monodon PLs were harvested annually from the study area. The sh1dy implies that colossal loss of shell and fin-fishes and other plankton resources is done by tiger shrimp fry collectors, and such massive destruction adversely affect the natural productivity and ecological balance of the coastal environment. Key words : Colossal loss, P. monodon fry, Finfish larvae, Shellfish larvae

Traditional or extensive shrimp farming being practiced in the greater Khulna region of Bangladesh mainly depends on the availability of wild tiger shrimp fry. The common culturable shrimps in the vicinity of the Stmdarbans are Penaeus monodon, P. indicus, Metapenaeus monoceros and Macrobmchium rosenbergii etc. (Nasker and Chakraborty 1984) but particularly in Khulna region, P. monodon, locally known as "bagda" is considered to be the most important culturable species. Due to horizontal expansion of shrimp cultivation and shortage of hatchery produced shrimp fry in the cotmtry, wild P. monodon postlarvae (PL) are collected extensively from the natural stock throughout the coastal area. High demand of tiger shrimp fry, low investment but high income have stimulated thousands of poor coastal people to be engaged in shrimp fry collection. Push net and set bag net are extensively used in shrimp fry collection, after every haul the whole catch along with debris is taken in bowls made of plastic, aluminium or clay by splashing water on the net. The P. monodon PLs are sorted out quickly from the catch by the collectors themselves or their family members and the rest is discarded and thrown along the shore. M.S. Islam eta/. Some interesting information on zooplankton with emphasis on shrimp and fin fish larvae were given by various authors (Funegaard 1986, Begum 1984, Hossain 1984, Elias 1983 and Ahmed 1981). Mahmood (1990) followed by others (Islam et al. 1996 and Rahman et al. 1997) gave the first infonnation on the quanhnn of damage caused to zooplankton while fishing P. m.onodon fry in the estuarine waters of Chakaria, Stmdarbans, Satkhira, Khepupara, Bhola and Barguna. Considering the importance of this topic of research, a year-rotmd shtdy was tmdertaken to assess the distribution of P. monodon PL and to quantify the damage done to different shrimp and fin-fish larvae and macro­ zooplankton during collection of P. nwnodon PL in the five rivers of . Samples were collected from three selected spots of each of the five rivers of Satkhira district from January to December 1996. Fortnightly sampling was done by a fine meshed (1.0 mm) push net made of nylon with bamboo split frame (1.6m x 0.6 m) locally called "Tana jal". The net was manually operated in the shallow water against the current for about 10 minutes (for each haul). Sampling was done twice per day during low and high tides. Samples were immediately stored in a plastic pot and preserved with 5% buffered formalin solution after collection. Laboratory analysis was done within two weeks from the date of collection, penaeid shrimp larvae were identified upto species level following Muthu (1978) and Motoh and Buri (1980). Macro-zooplankters including other shrimps and fin-fishes were identified as major taxonomic groups following George (1969) and Fischer and Witchead (1974). Salinity of water was recorded with the help of a hand refractometer (model, Atago, S/Mill, 0-100%o) and temperahrre of water was measured by an alcohol thermometer.

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Fig. 1. Water temperature and salinity of five rivers of Satkhira. Average monthly values of water temperahtre and salinity are presented in Figtrre 1. Water temperature were fotmd to vary between 17.3 and 34.2oC in the five rivers. The highest temperature (34.2oC) was recorded in the Mother river in July and the lowest (17.3°C) in the Kholpatua in January. Salinity varied from 0 to 20 ppt in different rivers having different patterns, possibly due to monsoon effect, river run-off and land drainage. However, salinity was fotmd to increase gradually during post monsoon. 108 Colossal/ass of/aroal population duringfnJ collection Monthly distribution (individual/unit effort) of P. monodon PL, larvae of other shrimps, fin-fishes and macro-zooplankters in different rivers of the Satkhira district has been shown in Table 1. Data reveal that during freshwater regime in the Ichamati and Coxali rivers P. monodon PL was absent became poorly available during December through Jtme in other rivers except in Mother. C>-n the other hand, abtmdance of other shrimp species were found to increase during the months of August through December and maximum was recorded in December. But in March no larvae of other shrimp was fotmd in Kholpatua river. During postmonsoon comparatively higher abundance of fin­ fishes and other zooplankton were observed than the other part of the year which is in agreement with the finding of Rahman et al. (1997) in the Barguna area where the authors indicated that low salinity and temperature are probably the vital factors influencing greatly the larval distribution of aquatic organisms.

Table 1. Monthly distribution (Individual/unit effort) of Penaeus monodon, other shrimp seeds, fin fishes and other macrozooplankton in different rivers of Satkhira region'96 Month !Yearly total % Maj_or_gi'oups I T F M A M T T A s 0 N D I chamati river P.monodon 1 - - - 1 1 ------3 0.06 Other shrimp 11 6 2 1 4 7 5 13 17 76 60 61 264 5.18 Finfishes 1 9 2 5 1 4 2 10 1 1 23 21 80 1.57 Other rnacrozoo 951 2 10 6 19 112 25 384 44 23 1385 1788 4749 93.19 plankton Total number 964 17 14 12 25 124 32 407 62 100 1468 1870 5096 100.00 Coxali river P. monodon - 1 1 ------2 0.33 Other shrimp 2 4 2 2 2 30 4 10 15 8 15 3 97 16.03 Fin fishes 3 1 1 2 - 1 2 4 1 1 2 6 24 3.97 Other rnacrozoo 85 3 3 7 12 23 19 91 6 11 105 117 482 79.67 plankton Total number 90 9 7 11 14 54 25 105 22 20 122 126 605 100.00 Kalindi river P.monodon 1 ------1 - 1 - 1 4 0.18 Other shrimp 11 10 15 2 3 8 4 15 21 33 43 55 220 9.84 Fin fishes 2 1 - 8 3 4 3 4 2 2 7 10 46 2.06 Other rnacrozoo 593 5 6 5 38 58 31 102 102 30 45 950 1965 87.92 plankton Total number 607 16 21 15 44 70 38 122 125 66 95 1016 2235 100.00 Kholpatua P.monodon 1 1 ------1 3 0.05 Other shrimp 11 10 15 2 3 8 4 15 21 33 43 55 220 9.84 Fin fishes 2 1 1 6 5 3 2 4 1 2 12 14 53 0.92 Other rnacrozoo 550 3 - 6 57 211 101 591 10 840 1298 1924 5591 96.565 _plankton 591 Total number 556 10 1 13 70 223 110 605 26 846 1333 1997 5790 100.00 Mother river P.monodon 1 1 - - 1 2 - - - - - 1 6 0.24 Other shrimp 2 5 7 3 6 11 7 8 16 27 8 11 111 4.46 Fin fishes 1 4 2 2 1 2 4 17 1 1 5 9 51 2.05 Other rnacrozoo 285 2 4 4 34 46 37 364 28 816 343 360 2323 93.25 plankton Total number 289 12 13 9 44 61 48 389 45 844 356 381 2491 100.00 *Operating a drag net (1.60 x 0.6 m) for about 10 minutes as a unit effort.

109 M.S. Islam et al. P. monodon PL was folll1d to occupy a very small portion of the zooplankton community, such as 0.06% in Ichamati, 0.33% in Coxali, 0.18% in Kalindi, 0.05% in Kholpatua and 0.24% in Mother river. Other shrimps ( P. indicus, Metapenaeus monoceros, M. brevicornis, Palaemon styliferus, Macrobrachiu1n rosenbergii, M. rudi and other palaemonid species) including fin fishes (Liza parsia, L. tade, Rhinomugil corsula, Lates calcarifer, Setipina phasa and Glossogobius spp. etc) occupied 6.75, 20.00, 11.90, 3.39 and 6.51% in Ichamati, Coxali, Kalindi, Kholpatua and Mother river, respectively. The catch composition and the extent of damage caused to macrozooplankton as a result of indiscriminate exploitation of P. monodon PL are presented in Table 2. It was fotmd that on an average in the total catch composition, P. monodon PLs occupied 0.17%, other shrimp 7.60%, fin-fishes 2.11% and other macro-zooplankton 90.12%. Considerable variations in monthly as well as yearly catch composition is in different rivers are evident (Table 1). The results obtained revealed that for catching a single postlarva of P. monodon, the fry collectors destroy about 45 other shrimp, 12 fin fish and 530 other macro-zooplankton. According to Mahmood (1990) for fishing a single "bagda" fry, 14 other shrimp, 21 fin fishes and 1631 zooplanktons were destroyed. The cause of such great variation was possibly due to use of nets having difference in mesh sizes. Mahmood (1990) used a rectangular nylon net with smaller mesh size (0.5 mm). The mesh size of the net used in the present study was 1.0 mm which is similar to that used by shrimp seed collectors. Thus, smaller zooplankton escapes through large meshes (1.0 mm) of net used in present shtdy. The tremendous loss of valuable larval fish resources was also reported by BOBP (1992) and Khan et al. (1988).

Table 2. Average catch composition of P. monodon, other shrimp spp, fin fishes and other macrozooplankton in five different river-eshmries of Satkhira Major taxa Yearly average catch Number of other species (%) destroyed for each P. monodon PL collection P. monodon 0.17 Other shrimp spp. 7.60 45 Finfishes 2.11 12 Other macrozooplankton 90.12 530 Total 100.00 587

Observation on the number of seed collectors/Km, length of the river, number of boat and hour of engagement reveals that 0.56 million man days/year are involved in shrimp fry fishing activities in Satkhira district. Variable information on the same due to the variation in place and season/time was referred by several authors. Ftu1egaard (1986) stated that

110 Colossal/ass of larval population during fry collection 20,000-25,000 people were engaged in shrimp fry collecting in Satkhira district while according to Chowdhury (1990) about 75,000 fry collectors were observed only in Satkhira district. It was formd that about 11.60 million P. monodon fry were collected in Satkhira during 1996 while it was more (18.0 million) in 1992 (BFR11997). Ftmegaard (1986) reported that about 2000 shrimp fry /net/ day were collected by catchers of Satkhira district in 1982 which was formd to decline at 200 fry /net/ day in 1986 (Alam 1990). So, the findings of the above studies and the present study reveal that there has been a trend of gradual reduction in the abtmdance of different kinds of PL of crustacean, fin­ fish larvae and other zooplankton in neritic and inshore waters which may be due to over harvesting and indiscriminate fishing of zooplankton that hinder the usual recruitment pattern to the original mother stock, and it is a great threat to the nah1ral biodiversity protection mechanism

References Ahmed, M. K., 1981. Behundi net fishery in brackishwater area of Satkhira with emphasis on bionomics and mortalities of shrimp. Freshwater Fish. Res. Stn., Chandpur, Bangladesh. Research Report No.3: 42 pp. Alam, M. M., 1990. Study on the colossal loss of shell-fish and fin-fish post-larvae for indiscriminate catch of Penaeus monodon post-larvae along the coastal waters of Cox's Bazar and Teknaf. M. Sc. Dissertation. Institute of Marine Science, University of Chittagong, Chittagong, Bangladesh. 45 pp. Begum, S., 1984. Temporal and spatial distribution of ichthyoplankters and other zooplankters in the Mathamuhuri estuary, Bangladesh. M. Sc. Dissertation, Institute of Marine Science, University of Chittagong. 49 pp. BFRI (Bangladesh Fisheries Research Institute), 1997. Survetj and assessment of shrimp fry resources of Bangladesh - Annual Report. In : Research Progress Report (1991-96). BFRI, Mymensingh. (unpublished) BOBP, 1992. The estuarine set bagnet fisheries of Bangladesh. Bay of News, 47 : 1- 10. Chowdhury, M. R., 1990. A study on biological and social aspects of shrimp fry collection in the . In : Seminar on Environmental and Policy Aspects of Shrimp Cultivation, BAAS, Bangladesh. pp. 20-29. Elias, M. S., 1983. Zooplankton of the Mathamuhuri estuary with special reference to shrimp and fin fish larvae. M. Sc. Thesis, Institute of Marine Science, University of Chittagong. 172 pp. Fischer, W. and P. J. P. Witchead (eds.), 1974. FAO species identification sheets for fishery purposes. Eastern Indian Ocean Fishing Area 57 and Western Central Pacific Area 71. Rome, FAO, Pag-Var., Vol. I-IV. Funegaard, P., 1986. Shrimp seed any to sell? Come to Satkhira, Bangladesh. News, 22 : 2-6. George, M. J., 1969. Prawn fisheries in , II. Systematics-taxonomic consideration and general distribution. Bull. Cent. Mar. Fish. Res. Inst., 14: 5-48.

111 M.S. Islam et al. Hossain, M. A., 1984. Studies on zooplanldon commtmities of the Mathamuhuri river estuary with special reference to shrimp larvae duriltg south west monsoon. M. Sc. Dissertation. Instih1te of Marine Science, University of Chittagong, Bangladesh. 3£ pp. Islam, M. M., S. L. Rahman and M. A. Lati( 1996. On the damage of different shell fish and fin fish fauna in collection Penneus monodon post-larvae in Meglma estuaries of Bhola. Universihj ]. Zool., Rajshahi Univ., 14 and 15: 71-75. Khan, M. G., M.S. Islam, Z.A. Chowdhury, and S.C. Paut 1988. Shrimp resources of Bangladesh - four stages of its exploitation and their comparative effect on the stock Marine Fisheries Survey, Management and Development Project, DOF, Bangladesh. pp. 1-12. Mahmood, N., 1990. An assessment on the quantum of damage caused to the zooplankton while fishing tiger shrimp Penneus monodon fry in Bangladesh esh1aries. Proc. Seventh Zool. Conf Bangladesh. pp. 87-94. Motoh, H. and P. Buri, 1980. Identification of the postlarvae Penaeus (Crustacea, Decapoda, Penaeidae) appearing along shore water. Aquaculture Dept. SEAFDEC Trigbaun, Iliolio, Philippines, Res. Rep. 4 (2): 15-19. Muthu, M. S., 1978. Larval development : Specific identity of penaeid postlnrvae found in bmckishwater areas in coastal aquaculture. In : Marine Prawn Culh1re, Part-1 : Larval development of Indian penaeid prawns (ed. KNK Kartha). CMFRI Bulletin, 28: 86- 90. Nasker, K. R. and N.M. Chakraborty, 1984. Studies on the economic fauna from the Sundarbans delta in .]. Indian Soc. Coastal Agricl. Research, 2 (1) : 56-62. Rahman, S. L., M. M. Islam, M. E. Hoq, G. C. Halder and S. U. Ahmed, 1997. A sh1dy on damage caused to crustacean and fin fish larvae during collection of Penaeus monodon (Fab.) postlarvae in the estuaries of Barguna, Bangladesh. Bangladesh ]. Fish. Res., 1 (1): 41-46.

(Manuscript received 22 December 1997)

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