Abstract: A study was undertaken to evaluate the possibility of utilizing natural brackishwater (from Shibsa River) in the hatching operation and larvae culture of Macrobrachium rossenbergii. Three treatments each with three replications were tested. Natural brackish water diluting by rainwater (T1), natural brackishwater diluting by under ground tap water (T2) and natural brackish water with salinity range from 12 to 20 ppt. (T3) were considered. In case of dilution (in T1 and T2) salinity concentration range was maintain from 4-18 ppt as per requirement of the experiment and stage of brood and larvae. Significant higher survival (23.38%) for T1 was observed followed by T3 with 13.63% and T2 with 4.38% up to post larvae (PL) stage. Significant (p<0.05) difference in survival rate of larvae between T1 and T2 and T2 and T3 was observed. Study reveals that natural brackishwater diluting by rain water can be effectively used in the hatching operation and larvae culture of the freshwater giant prawn, M. rossnbergii.
Introduction
Macrobrachium rossenbergii is a freshwater prawn and in Bangladesh most commonly known as Golda chingri found in fresh water rivers with tidal influence. They are also found in ponds, canals, irrigation ditches having connection with tidal water. It is indigenous to the south-east Asia including Bangladesh. But in our country the availability of this species in nature is decreasing day by day demanding its culture practice. In recent time, the heavy demand of this prawn in both for national and international markets due to its good nutritional value as well as deliciousness, expand the culture system of the animal. To date, in our country monoculture of this species are being practiced at Khulna, Bagerhat, Piruzpur, Gopalgong, Madaripur and Narail districts covering a total area of about 9000 hector. Polyculture of prawn with carp in ponds, rice-cum-prawn culture is also being practices in our country (DOF, 1994, 1996). Moreover there is enormous potentiality of mixed culture of freshwater prawn M. rosenbergii with brackishwater shrimp P. monodon in coastal region of the country particularly in Cox’s Bazar and greater Khulna districts (FRI, 1994). But main obstacle in flourishing culture practice of M. rossenbergii is the scarcity of seeds from nature as well as from hatcheries due to various difficulties. One of the most practical difficulties of providing prawn seed from natural source is that, it is almost impossible to identify prawn larvae from other freshwater prawn species which prohibits the monoculture system of M. rossenbergii directly. Though in the recent year a considerable number of backyard prawn hatcheries started to produce prawn larvae in the country, but depending upon the location of the hatchery, source of saline water/brine and water management in hatchery operation, the production rate of prawn post larvae for most of this hatcheries are found below the standard rate (not more then 20%) which is again another important drawback for prawn culture development. So effort should be directed towards artificial sources with proper water management. Nowadays, giant fresh water prawn’s larvae are reared with the conventional techniques where artificial brackishwater of about 12 ppt. Salinity is prepared from seawater. The utilization of natural brackishwater in the hatching and larvae culture of giant prawn has so far received very little attention from aquaculturists. In present study the attempt was taken to utilize the natural brackishwater for hatching and larval rearing of giant prawn.
Materials and Methods
The study was conducted from mid April to late September 1998 at the experimental hatchery unit of Brackishwater Station, Bangladesh Fisheries Research Institute, Paikgacha, Khulna.
For dilution of brackishwater into different required concentrations, natural brackishwater from the nearby Shibsa was used. For removing dirt, suspended particle and microbial cells, collected natural water was at first filtered by primary filtration unit (filter formed by gravel) and stored into under ground water tank from where it was lifted up to overhead tank and passed through sand filtration unit and ultra violet (UV) unit and stored into ground tank then again it was lifted up to overhead tank and finally stored in cistern. Prior to use for the hatching activity and larvae culture, 60% chlorinated bleaching power at the rate of 12 ppm was added to this water for further purification and kept for 2 days for settling followed by treatment with sodium thiosulphate at the rate of 10 ppt. and allowed another 2 days for settle which was followed by agitation using air blower for 1 days. After agitation it was kept 1 days for settle down all waste material. Then supernatant was used for the experimental work.
All necessary equipment’s i.e. siphon pipe, bucket, bowl, net pieces etc were washed twice daily prior to use for siphoning and water exchanging by chlorinated water (60% chlorinated bleaching powder) at the rate of 12 ppm. Larvae produced were siphoned off and the number of larvae was estimated by the Aliquot method prior to stock in larval rearing tanks.
Three different water type (considered as treatments) each with three replications were used. The treatments were as follows: T1 for natural brackishwater mixed with rainwater; T2 for natural brackishwater mixed with underground tap water and T3 for natural brackishwater only.
The study was divided into three batches and the experimental treatments were set in randomized block design. In each batch 3 berried females were used as mother prawn. Three rectangular fiberglass tanks each of 152 L holding capacity were filled with the rearing medium three days prior to stocking.
Hatchlings were reared with varying densities as 44.70-66.82 larvae/L and fed with Artemia nauplii. Artemia nauplii were supplied after 48 hours of hatch at a rate of 5 naupli/ml of water and Artemia nauplii as feed were used up to PL stage. During rearing of larvae in the tanks aeration was provided at a rate of 10 L/min with 30 minutes break after continuous one-hour operation. To keep the tanks bottom clear, water exchange was done every morning and evening of the day at the rate of 90 and 50% respectively by siphoning. The rearing process continued till the attainment of about 95% of the stock as post larvae (PL) (Table 1).
Water quality parameters of each rearing medium such as temperature, dissolved oxygen, pH and ammonia were recorded twice daily. The total hardness of each rearing medium was also determined. The salinity of the rearing medium was checked once a week with a hand refractometer.
| Table 1: | Replication wise dates of hatching, stocking dates and density of hatchlings |
| Table 2: | Treatment wise average survival rate of M. rassenbergii up to the attainment of post larval stage |
| Table 3: | Duration of metamorphosis of M. rossenbergii hatchlings during this larval rearing under different treatments. |
Results and Discussion
Significant higher survival (23.38%) of the post larvae was observed in T1 followed by T3 (13.63%) and T2 (4.38%) with significant variation among the treatment average (p<0.05). The findings clearly represent the impact of water quality on larval survivability. Because, the maximum survivability in T1 might due to the better quality of rain water added during dilution than the under ground water, that contains higher amount of iron as well as hardness of water is maximum, those directly influenced larval metabolism and development. Due to variation in water quality and due to presence of some undesirable compound, Ong (1975) and Aniello and Singh (1980) made similar observation on survivability. Different in composition of the rearing media may consider as possible cause of difference in morality rate among the treatments (Table 2).
The other cause of low survival on identified were cannibalism among the members, improper acclimatization of the larvae before stocking in the rearing tank, high iron content of underground water and adhesion of the larvae along the inside wall of rearing tank due to jumping. Larval growth period to first day of metamorphosis was also found variable (Table 3). Data reveals that T1 had the earliest mean at day 28, followed by T2 and T3 at 31th and 31.66th respectively. The table also shows that T1 had the shortest mean larval cycle of 44.66 days, followed by T3 (46.66 days) and T2 (47 days).
The variation in length of larval cycle might due to the impact of temperature of culture environment that has direct effect on metabolism and growth performance of the animal. The course of study as per water parameter record, the T1 part was performed in a warmer environment than T2 and T3. During the operation of T2 and T3, due to occasional rain during the period, temperature went down and retards animal’s metabolism and growth. Similar observation on larval growth due to variation in temperature was reported by Gibson (1975), Suharto et al. (1980) and New and Singholka (1982). Authors also reported that temperature lower than optimum (i.e. below 24-26°C) required more larval rearing time.
In Bangladesh, generally proto type recirculatory Macrobrachium sp. hatchery with biofiltration unit does not acceded survivability 25% up to its post larval development and time length required for attaining post larvae 25-30 days (DOF, 1996) which is in both the cases more or less similar with the findings of T1 where clear rain water was used for alteration of the natural brackishwater up to the required level. So the findings of the experiment indicates that natural brackishwater diluted with rainwater can be effectively used for larvae culture of giant freshwater prawn.