Introduction

Development of aquaculture in India is now centered on shrimp due to its high unit value realization and export demand.  With its rich natural resources in the form of brackish water bodies suitable for shrimp aquaculture, India is bound to emerge as one of the leading shrimp harvesters in Asia (Ghosh et al., 1994).  The major bottleneck in development of shrimp aquaculture is the non-availability of quality seeds.  The production of quality post – larvae in hatchery depends upon the healthy brood stocks collected from the sea.  Male brooders generally mature in captivity so that induced maturation mainly concerns females. Three basic approaches have been employed to induce ovarian maturation in penaeids – endocrine (eyestalk ablation), nutritional, and environment.  Hitherto, maturation in P.monodon has been induced only through eyestalk ablation although diet and environmental parameters may also enhance the reproductive performance (Primavera, 1978; Millamena et al., 1986).

In the present study, an attempt has been made to unravel the effects of two natural feeds, polychaete worms and crabs on the reproductive performance of wild- collected Penaeus monodon.

Materials and Methods

The present study was carried out in “Matha Prawn Hatchery”, 35 km away from Chennai.  Brooders of Penaeus monodon were obtained from Pazhayar, Nagapattinam District of Tamil Nadu.  Forty-five female brooders weighing between 90 and 290 g and 30 male brooders, weighing between 80 and 200 g, were used for the study.

The brooders were acclimated to ambient water conditions at the hatchery in a quarantine room. Brood stock quality was assessed based on physical condition of the animals. They were stocked at a density of 5 animals per square meter, at 1:2 male: female ratio.  The water parameters maintained in the tank throughout the experimental period was salinity 28 to 30 ppt; temperature 27 to 30⁰C and pH 7.8 to 8.2.

The brooders were conditioned for a period of ten days before the eye stalk ablation was carried out.  Brooders were divided into three groups.  The group I was fed with polychaete worms, the group II with crabs, and group III with polychaete worms and crabs. Polychaete worms were used in fresh live condition, after rinsing in clean seawater in a sieve.  Crabs were cleaned individually by removing appendages, pinchers, gills and exoskeleton cut into 2 to 4 pieces, and stored in the deep freezer, until used for feeding.

Individual weights of brooders in each group were recorded and the total biomass of each tank was calculated.  Each feeding group consisted of 15 females and 10 males.  For all the three groups the individual weights were taken and the total biomass in each tank was calculated.  Each group was fed with 12% of its total body weight at a frequency of 4 times/day.

The water in the maturation tank where newly ablated female and male brooders were stocked pre-treated with 4 ppm antibiotics.  This was to ensure that the wound of the crushed eyestalk would not be infected.  The antibiotic treatment was given for 3 consecutive days.  Water level was maintained at 60 cm.

In maturation tank, aeration was turned off for maximum visibility.  Sourcing of gravid females was undertaken with the help of underwater flashlight held perpendicularly against the breeder’s abdomen illuminating the part where ovary was obviously visible.  Females were checked individually, and those in stage IV were singled out using scoop net.  Females could be easily identified due to their one-eye compared to males having both.  All the stage IV female brooders that were collected were distributed to tubs/basins, with aeration, after they were first disinfected in 300 ppm formalin for 5 minutes.  Subsequently, they were washed by dipping in pure clean seawater, and then placed in the spawning tanks.

Prior to stocking of spawners in spawning tank, 10 ppm of EDTA was added to the water.  The spawning area was kept dark throughout the night and was free from any unnecessary disturbances.  No feeding was given when the spawner was in the spawning tank.

Early in the morning of the following day each spawning tank was inspected for a pinkish to orange scum attached to the sides of the tank, indicating that spawning had occurred.  The animals were then removed individually from the tank, using scoop net, and each were checked for spawning condition.  Unspawned ones were, returned to the maturation tank from where they were sourced.  The spawned brooders were separated to a newly prepared maturation tank.

Detailed egg evaluation was carried out after 10 a.m. in order to allow complete development of eggs to nauplii so that fertile and unfertile eggs could be easily identified.  The eggs were observed under a compound microscope.  Fertilized eggs could be easily identified by the symmetrical nature of cell divisions.  Frequently, appendages and setae were also visible.  Unfertilized eggs appeared as opaque or dark brown sphere, with very irregular cell divisions.

The water level in the hatching tank with eggs, to be estimated was maintained up to a mark of known volume.  Using a 5 ml inverted pipette a 3 ml sample was obtained from the tank, and released into a petridish.  The eggs were counted with naked eye against a white background.   Three samples were counted from each harvesting bucket and the average was calculated.

The total number of eggs in a hatching tank was calculated using the formula:

The nauplii hatched out in about 12 hours, and reached the 6^th sub-stage, N6, within 36 hours. The counting of nauplii was carried out using the similar method adopted for egg counting.

Results and Discussion

The summary data relating to body weight of the brooders, total number of eggs spawned, number of nauplii hatched with percentage of hatching in each group and the statistical analysis of the above are presented in Table 1. A comparison of the number of eggs spawned and number of hatched eggs in each feeding group is shown in Figs. 1 and 2.

Table 1: Spawning and hatching of eggs from brooders reared with three types of  feeds.

Note: ¹There is no significant difference among the mean body weights of the three groups; ANOVA: F: 0.36; P = 0.7.

²The means of Group I and Group III do not differ significantly; The mean of Group II is significantly lower than the means of Group I and Group II; ANOVA: F: 12.24; P < 0.001; Q-test: Significant difference at 0.05 LS, 1.1.

³The means of Group I and Group III do not differ significantly; The mean of Group II is significantly lower than the means of Group I and Group II; ANOVA: F: 11.68; P < 0.001; Q-test: Significant difference at 0.05 LS, 1.2.

There is no significant difference in the mean weights of the brooders of the three groups. Brood stock response in terms of fecundity i.e., total number of eggs spawned, is significantly higher in group III fed with polychaetes and crabs and group I fed with only polychaetes than in group II fed with only crabs.  Likewise the number of nauplii hatched was higher in groups I and III than in group II. The results of the present experiments thus clearly indicates that the reproductive performance in terms of number of eggs spawned, number of nauplii hatched and the hatching percentage is significantly better in brooders fed with polychaete worms (group I) and in those fed with polychaete worms and crabs (group III). The better reproductive performance may be attributed to the high nutritional value of the polychaete worms.

The importance of nutritional quality of the diet on the reproductive performance and larval survival has been experimentally demonstrated in lobsters, prawns and shrimps.  For instance, ovarian maturation and spawning could be induced in captive P. setiferus when these diets were supplemented with lipids rich in C₂₀ and C₂₂ polysaturated fatty acids (Middleditch et al., 1979).  High survival rate could be achieved in the juveniles of lobsters (Conklin et al., 1980) and P. japonicus (Deshimeru, 1981) when soybean lecithin and phospholipids were incorporated into their diet.  Reproductive erformance, in terms of total number of spawnings, eggs and nauplii production, average hatch rate of eggs and larval quality of pond-reared P. monodon brood stock, increased ignificantly when the diet contained lipids from different sources (Millamena et al., 1986).  The best performance was recorded when the diet contained lipid in the form of cod liver oil, followed by soybean lecithin.  However, a combination of cod liver oil and soybean lecithin gave only the poorest overall response.  Thus cod liver oil supplemented diet was found to be a suitable diet for successful maturation and spawning of pond-reared P. monodon.

In contrast, certain diets such as frozen adult Artemia biomass fed to the peppermint shrimp Lysmata wurdemani (Lin and Zhang, 2001) and moist artificial diet fed to wild caught P. monodon (Gey et al., 1997) did not improve the reproductive performance of the respective species.

Figure 1   Click here to View figure

Figure 2   Click here to View figure

The results of the present study showed that a diet of polychaete worms is significantly effective in enhancing the reproductive performance of the wild-collected, hatchery-reared P. monodon.  The polychaete worms are probably rich in nutrients that promote the reproduction of the prawns.  Further investigation on the biochemistry of the polychaete worms, especially the qualitative and quantitative aspects of their lipid contents, would help to understand the efficiency of these worms as brood stock feed.

Acknowledgements

The authors are grateful to the management of Matha Prawn hatchery for the permission and Chief Technician. Mr. M. Laxmanan and Mr. V. G. Karthick for their kind co-operation to conduct the experiment.

References

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