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Introduction

虾养殖在孟加拉国在扮演着重要角色e national economy since the early 1990s. Shrimp farming is practiced in Bangladesh as monostock of the black tiger shrimp Penaeus monodon (Uddin and Kader, 2006). Before 1990s, shrimp farmers only stocked natural post larvae and harvested the marketable size of shrimps while the culture system was totally dependent on natural seed supply. At present, the entire culture of tiger shrimps almost depends on hatchery produced post larvae which left a ser ious environmental consequences through their waste water discharge into the aquatic coastal ecosystem. The fluctuation of water parameters such as temperature, pH, salinity, dissolved oxygen, alkalinity and hardness above the nor mal range may deteriorate the ambient water quality that leads to shrimp diseases and other problems. Besides, water quality maintenance is of the greatest importance in the growth and survival of larvae, post larvae and brood sto c k. The p h ysico-chemical pa r ameters including w ate r tempe r atur e , salinit y , dissol v e d o xygen , pH and the micro n ut r ients li k e nit r at e , phosphate and silica pl a y a significant role in P. monodon hatche r y ope r atio n an d managemen t (Da s et al. , 1996) . If a n y on e o f th e ab o v e pa r ameter s fluctuate s ab r uptl y , whole production m a y collaps e.

The role of physico-chemical parameters suchassalinityandwatertemperatureinrelation tosurvival,growthandproductionofP. monodonfrywasstudiedbymanyresearcherselsewhere (Chen,1985;Chakraborttyet al.,1986andBoyd,1989).Furthermore,Wickins(1976)andLaw(1988)reportedthatmortalityrateofP. monodonpostlarvaewas high at pH <6.0 and the recommendedvaluewas7.5to8.5.Theeffectofammoniaonpost larvaeofP. monodonwasinvestigatedbyWickins (1976),Chenet al.(1986)andLaw(1988).Their studies revealed that nitrite and nitrate may effect thegrowthofP. monodonlarvae.

Therefore, successful hatchery operation ofP. monodondepends on good water supply from the adjacent water environment and good quality management i.e. keeping the essential water parameters within suitable ranges. The present study was undertaken to assess the present status of water quality parameters at Kalatoly coast and a shr imp hatcher y namely Shahajalal Shr imp Hatchery at Cox's Bazar which could be valuable to understand the water quality of the hatchery zone where these types of studies are practically poor.

Materials and Methods

The present study was carried out in Kalatoly, Cox's Bazar coastal area and inside aP. monodonhatchery Shahajalal Shrimp Hatchery (Fig. 1). The hatchery drawns saline water from the sea, which is filtered through sand filters and chlorinated and dechlorinated before distribution to the larval tanks. Raw sea water and water samples from two points in the hatchery such as UV treated water and water from tanks rearing larval stages (PL7-PL15) were collected from 30 cm below the surface using sterile bottles. Sampling was done from January 2008 to June 2008. Sampling was accomplished twice monthly, and a total of 12 sampling trips were done and 36 samples were collected out over 6 months of study.

Surface water temperature, salinity and pH were measuredin situusing a mercur y thermometer, refractrometer and a digital pH meter respectively. Total dissolved solid was measured by TDS meter of HANNA instr uments products (Singapore, Model No. HI-98302). Alkalinity was measured by alkalinity test kit (HANNA instrument product, Singapore, Model No. HI-98737). The micronutrients such as NO-N, NO-N, PO, and Fe weredeterminedfollowingthemethodofChattopaday (1998). DO was deter mined byWinkler'smethod(Barnes,1959).

Figure 1. The investigated area (•) where the present study done. Click here to view figure

Figure 2. Temperature variation of different water samples. Click here to view figure

Figure 3. Salinity variation of different water samples. Click here to view figure

Figure 4. Dissolved oxygen of different water samples. Click here to view figure

Figure 5. pH value of different samples during the investigation Click here to view figure

Figure 6. Total dissolved solids at different sampling time Click here to view figure

Figure 7. NO -N values at different sampling period. Click here to view figure

Figure 8. NO -N at various sampling point. Click here to view figure

Figure 9. PO values at different sampling period. Click here to view figure

Figure 10. Different Fe values during the investigation Click here to view figure

Results and Discussions

W ater is the most vital component among the natural resources, and is crucial for the survival of all living organisms. Managing of water quality during the larval rearing phase is very important due to the sensitivity of larvae to the fluctuation of water parameters. The lowest temperature was observed in January (24.5°C) and the highest (28°C) in May for sea water (Fig. 2). There were major differences between the sea water and UV treated water (25°C-28.5°C) but the variation was not so remarkable in the water of LRT (29°C- 30.5°C). This occurrence is probably due to indoor larval management practice of the P. monodon . The water salinity varied from 28.0 ppt to 32.5 ppt, 29.0 ppt to 32.0 ppt and 30.0 ppt to 31.0 ppt in sea water, UV treated water and LRT water respectively. The higher water salinity (32.5 ppt) was observed in April and lower (28 ppt) in middle of June for both sea water and UV treated water (Fig. 3). Moulting and consequent gr owth of postlar vae are highly stimulated by high salinity (Chen et al. , 1989 and Ferraris et al. , 1986). However, salinity was almost uniform for the water of LRT (30-31 ppt) due to larval rearing practices of P. monodon in hatchery. The present water temperature and salinity were found nearer to the range of 29°C to 30.5°C and 29 to 30 ppt for LRT water which coincided with the finding of Deb (1990) and Chang and Liao (1986).

Dissol v ed oxygen ranges from 3.10 to 3.72 ppm, 3.90 to 4.72 ppm and 3.82 to 4.80 ppm for sea water, UV treated water and LRT water respectively and fluctuated through out the study period (Fig. 4). The variation of DO could probably due to the effects of temperature, salinity, atmospheric pressure and other activities. Usually, DO is used by plants and animals for their respiration whereas, photosynthesis by phytoplankton and other aquatic plants is the primary source of DO in water. Chenet al.(1986) recommended DO level to be between 4.0-8.0 ppm forP. monodonpost larvae rearing. The water pH were almost constant and varied from 7.5 to 8.2, 7.4 to 8.2 and 7.8 to 8.4 in sea water, UV treated water and LRT water respectively (Fig. 5). There were no remarkable variation for pH in sea water, UV treated water and LRT water. Generally, the variation in pH occurs due to the use of different artificial feed in larval rearing tank. The pH value also depends on the amount of carbon-di-oxide present in water and the addition of fresh water into the LRT. Chenet al.(1986) and Wickins (1976) reported that pH level between 7.17 and 8.34 was suitable forP. monodonpost larvae and this coincides with the present findings.

T ota l dissolved solids (TDS) varied from 21.28 to 24.20 ppt, 17.65 to 20.25 ppt and 22.87 to 25.12 ppt in sea water, UV treated water and LRT water respectively (Fig. 6). Present study showed that the TDS values were higher in sea water and LRT water compared to the UV treated water. This variation could probably due to the existence of naturally suspended mater ials and feeding movement both in sea water and LRT. The total dissolved solid (TDS) was similar to the findings of Chen and Liu (1988).

The water NO - N varied from 0.17 to 0.55 ppm, 0.15 to 0.45 ppm and 0.38 to 0.82 ppm for sea water, UV treated water and LRT water during the study period, respectively (Fig. 7).These findings are slightly higher than the result of Daset al., (1996). The nitrate value in LRT water was higher than the sea water and UV treated water. The nitrification processes and the release of hydrogen ion during the hatchery rearing activities probably influence this phenomenon. Water NO₃ - N varied from 0.006 to 0.12 ppm, 0.005 to 0.100 ppm and 0.018 to 0.21 ppm in sea water, UV treated water and LRT water respectively (Fig. 8). There was no distinct variation in sea water and UV treated water b ut the variation was clear in LRT water probably du e to nitrification and other microbial activities occurring in the LRT water. Jayasankar and Muthu (1983) reported that an acute level of NO - N forP. indicuswas 0.18 mg/l. Wickins (1976) reported that when nitrite nitrogen increased in the water of rearing tank, survival rate ofP. monodonlarvae gradually decreased. The highest safe level of NO₂ -N recommended forP. monodongrowth is 1.28 mg/l (Law, 1988) but the present findings are much lower than this. The concentration of PO4 was more or less similar in both sea water and UV treated water but slightly differed on LRT water during the month of May (Fig. 9). The present phosphate level was recorded 0.55 to 0.90 ppm in LRT water which is similar to the findings of Chenet al.(1986) and Daset al.(1996). The concentration of phosphate increases due to excretion of metabolites and heavy load of excess food, including both live phytoplankton and artificial diets. The concentration of Fe was more or less similar in both sea water and UV treated water but slightly differ on LRT water during the study period (Fig. 10). However, the introduction of fresh water may influence the concentration of Fe in LRT water atP. monodonhatchery.

Acknoledgements

The financial support for this study was given by "Research Cell", University of Chittagong, grant no. 4929/2007. Laboratory suppor t was provided by Institute of Marine Sciences and Fisheries.

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