Abstract: The present study was designed to find out the suitability of white shrimp, Penaeus monodon in the cages of Vellar estuary. In the previous experiments, the autoentrants and fouling were found to be disturbing the production and hence the present study deals with the controlling of this problem by changing the cages with fresh cages at regular intervals except control. Among the four uniform size rectangular cages (10X5X1.5 m) used, the first cage was treated as control, the second cage was changed at 15 days interval, the third cage was changed at 30 days interval and the fourth cage was changed at 60 days intervals. The culture was carried out for 120 days and the juveniles of size ranging from 3.3 to 4.0 g were stocked uniformly at the rate of 20 m-2 for all the cages. In the cage changed at 15 days interval, the autoentrants and fouling species were found to be less. The maximum growth of 25.5 g, survival rate of 96% and production rate of 491.52 g m-2 were recorded in the cage changed at the interval of 30 days. From the findings of the present study, it is suggested that the cage changed every 30 days regular interval is ideal for the higher production of P. indicus.
INTRODUCTION
Owing to the increasing demand for shrimps in world market, aquaculture is considered as extreme focus area by many countries. Much emphasis is also given to this fast moving commodity, because of its production potential, earning foreign exchange and scope for employment generation. Therefore last two decades, shrimp farming has witnessed tremendous technological advancements, the world over. Shrimps are cultured mainly in ponds, cages, pens and raceways. Out of which ponds and raceways are land based rearing methods and cages and pens are water based rearing methods. Pond and raceway culture are found suitable in coastal areas and brackish water areas. Cage and pens are suitable in bay, lagoon, back water and open seas. If shrimps are cultured in cages, autoentrants pose the most serious problem as they compete for food and shelter with the cultured species and even predate the culture species (Rabanal and Hasillos, 1957; Pillai, 1973; Bensam, 1982; Nandakumar, 1982; Beveridge, 1987; Shanmugam et al., 1994, 1995, 1998). Scientist did not suggest a clear-cut solution to overcome this problem. Eventhough the height of the cages were raised from 1 to 1.5 m only top portions of the cages were free from fouling. But on the sidewall, the fouling was worse even though cleaning was made at regular interval (Srikriahnadhas and Sundararaj, 1990; Shanmugam et al., 1994). Chien et al. (1989) reported that transferring system of farming can increase production by 30%. Based on this, in the present study, it is attempted to overcome these problems by changing the cage at different time intervals for the higher production of P. indicus in Vellar estuary.
MATERIALS AND METHODS
Cage Erection
The rectangular cage (10x5x1.5 m) was suitable for the culture of P.
indicus. So initially four such rectangular cages were erected on the bottom
soil substrate. Of which the first one was kept as such without changing the
cage and maintained as control. The second cage was changed by a fresh cage
at the regular interval of 15 days. The third and fourth cages were replaced
by the fresh cages at the intervals of 30 days and 60 days, respectively. After
replacing the old cages, they were dried and cleaned to reuse as fresh cages.
Seed Transportation
The seed purchased from hatchery was transported to culture site in Vellar
estuary by oxygenated polythene bag and were kept in styroform boxes. Before
transportation, the qualities of seeds were examined by taking the seeds in
a plastic container to ensure uniform size and good health. Postlarvae of penaeid
shrimps are small, fragile and are sensitive to change in water conditions.
So the postlarvae purchased from the hatchery were acclimatized to estuarine
condition before stocking to avoid heavy mortality. In order to increase the
survival rate and prevent the escape from the cage, the acclimatized seeds were
released into already erected small hapa of 2x1x1 m size in the estuary and
the seeds were reared for 10 days.
Stocking
After 10 days of culture in the hapa their average length and weight of
the postlarvae were recorded. Healthy juveniles of size ranging from 3.2 to
4.0 g were stocked at the uniform rate of 20 m-2 in all the cages.
Feeding
Initially 12% feed were provided to shrimps of all the cages and then slightly
raised and finally reached 14% because of higher autoentrant biomass in the
control cage. But it was gradually decreased to 3 to 6% for the shrimps of the
other cages, because of the lower autoentrant biomass. Feed was given at dawn,
mid-day and dusk. But the two fourth of the feed were provided to dusk since
shrimps are nocturnal.
Culture Period
The culture was carried out for 120 days. Periodical sampling assessment
of environmental parameters, harvest and assessment of autoentrants were done.
Assessment of Fouling
Immediately after harvest, the fouling organisms were removed from the respective
casuarinas poles and cages and were analyzed both qualitatively and quantitatively.
RESULTS
Growth
The higher growth rate (25.6 g and 131 mm) was recorded for the shrimps
reared in cage changed at 30 days interval, followed by 23.2 g and 129 mm for
the cage changed at 60 days interval, 22.2 g and 128 mm for control and 19.3
g and 128 mm for the cage changed at 15 days interval (Table 1).
Similarly, the average daily growth increment also revealed the same pattern.
| Table 1: | The production of P. indicus in relation to the control of autoentrants and fouling problems |
Survival Rate
The higher survival rate 96% was observed in the cage changed at 30 days
interval and the lower survival rate 60% was noticed in the cage changed at
15 days interval. In cage changed at 60 days interval and control the survival
rates were 93.5 and 82.1%, respectively (Table 1).
Production
The shrimps of the cage changed at 30 days interval yielded more (24.57
kg 50 m-2) than the others. The lower yield 11.58 kg 50 m-2
was reported for the shrimps of the cage changed at 15 days interval (Table
1). The shrimps of the control cage and cage changed at 60 days interval
yielded 18.22 and 21.69 kg 50 m-2, respectively. The production rates
of the shrimps of the cages changed at 30 days interval, 60 days interval, 15
days interval and control were found to be 4915.2 kg ha-1 or 491.52
g m-2, 4338.4 kg ha-1 or 433.84 g m-2, 2316
kg ha-1 or 231.6 g m-2 and 3645.2 kg ha-1 or
364.52 g m-2, respectively.
Environmental Parameters Salinity
The salinity inside the cages was found between 20 to 30 ppt throughout
the culture period. The fluctuations in the salinity were higher during the
initial period than the later period (Table 2).
Dissolved Oxygen
The dissolved oxygen level ranged from 4.1 to 5.3 (mg L-1) throughout
the culture period. Compared to the salinity, the higher dissolved oxygen level
were observed during the initial period of the culture and then slowly decreased
towards the later culture period (Table 2).
Hydrogen-Ion-Concentration
The pH values ranged between 8.2 to 8.7 during the culture period. Eventhough
the fluctuations were meager, the lower values were observed during the starting
period and the higher values were noticed during the last culture period (Table
2).
Temperature
The lower ranges of temperature were observed during the initial culture
days and the higher ranges of temperature were noticed during the later days
of the culture as in the case of salinity. During the culture period the temperature
of the surface water ranged from 27 to 32°C (Table 2).
Autoentry
The autoentrants was found to be higher (14.32 kg) in the control cage and
lower (1.43 kg) in the cage changed at 15 days interval. In the cages changed
at 30 days interval and 60 days interval the autoentants were found to be 2.91
and 6.74 kg, respectively (Table 3). In general, among shrimp
species, P. indicus, M. monoceros were dominant; among crab species,
P. pelagicus was dominant; among fishes Ambasis sp. was dominant
in all the cages.
Fouling
The total weight of the fouling organisms in casuarinas poles (supporting
materials) of the different cages were more or less similar (0.54 to 0.62 kg).
On the contrary, among the cages, the weight was higher for control. The cage
changed at 30 days and 60 days intervals showed 3.65 and 6.32 kg, respectively
(Table 4). In all the casuarinas poles Balanus amphitire
was the single dominant species. While on the cages, Crossostrea sp.
was the dominant species following by B. amphitire. The percentage of
B. ampiphitire was higher in all the casuraina poles and the percentage
of Crossostrea sp. was higher in all the cages (Table 5).
| Table 2: | Result of environmental parameters in range |
| Table 3: | Autoentrants (kg) at the change interval of the days |
| Table 4: | Macrofoulers in casuarina poles and different cages |
| Table 5: | Percentage of macrofoulers in casuarina poles and different cages |
DISCUSSION
The present investigation clearly showed that the cage changed at 30 days interval yielded a higher production rate (491.52 g m-2) then others in the order of 60 days intervals, control and 15 days interval (Table 1). The high and low autoentrants were found in control and 15 days interval respectively. The high autoentrants in control cage can be attributed to unchanging of this cage till harvest which paved the way for the continuous incoming of eggs and larvae of autoentranats and grow in size inside the cage cannot escape through the smaller mesh size. On the short duration (15 days) of cage changed automatically gave a chance to remove autoentrants regularly at every 15 days interval throughout the culture period than others.
The lower production rate in the control cage then the cages changed at 30 and 60 days intervals may be due to the high autoentrants, which not only competes for food and shelter but also predates the culture species. Shigeno (1975) observed several harmful fishes like porgy, black sea bream, yellow spotted grunt, trout let sand borer, flathead, halfbeak, gobies etc., in the culture ponds of western part of Sctonaikai and also he reported that all these fish infiltrate into the ponds through the screens in the form of egg or fry. Then they grow in the ponds and some of them not only share the feed provided for the shrimps but also feed on them.
The higher production in the 30 days interval may be the optimum to shift cage for the shrimps. The autoentrants (2.91 kg) observed in this cage was meager and may not pose any problem for the shrimps. Due to less competition and predation by autoentrants, the production rate was higher in this cage. Eventhough the autoentrants was very low (1.43 kg) in 15 days interval the production rate was lower than 30 days interval, 60 days interval and control cages. This low production may be due to the often shifting (15 days interval) of shrimps from old cage to new fresh cage, which causes handling stress to shrimps which inturn affected the production of shrimps. Rajyalakshmi (1982) started that along with other stresses, handling stress also would cause mortality.
Kurian (1982) reported that the larvae of predators, including perches and crustaceans may get into the ponds through the meshes and get themselves established. Perches are found to devour even large shrimps. However no satisfactory method is yet to be available for the complete eradication of these perches from ponds. According to Shanmugam et al. (1995) the problem created by the autoentrants are the excess density of shrimps than expected in the cage and the competition for food and shelter, resulting in the reduction in growth rate of culture species. Thus it could be assumed that the autoentry is an ecological factor of paramount importance, but which is unavoidable in the cages and pens. Therefore, the farmers should pay more attention on this aspect and to minimize it in order to maintain the quality of stocking shrimps. Constant checking and removing them may be helpful in small ponds (Kurian, 1982). Similarly, Shanmugam et al. (1994) also stated that, the problems of autoentry could be solved to a certain extent by the periodical removal of the autoentered fin and shellfishes using scoop net or hand net from the cages. As suggested by them, attempt was made to remove them by hand nets, but it was not effective.
Further Nandakumar (1982) also attempted to remove tilapia by operation gill nets and drage nets but without success in pond culture of shrimps. He found that even after removing tilapia and other fishes from pond before commencing stocking operations, young ones of tilapia in hundreds were noticed within a fortnight in the culture ponds. Similarly Pillai (1973) also stated that tilapia is regarded as one of the pests in culture ponds since it is a prolific breeder and complete for space and food. Apart from these, Rabanal and Hasillos (1957) observed that tilapia consume crustaceans under crowded condition in the absence of vegetable food. Nandakumar (1982) noticed the presence of gobiid fishes of size ranging from 10-25 mm in large numbers in the ponds and were observed to feed on plankton, thus completing for food with shrimps. In the presence study, eventhough tilapia and gobiid fishes were not recorded, other autoentrant of fish species were noticed in the cage creating same problems as discussed by the above researchers.
Bensam (1982) reported that in India, the predator fishes usually found in shrimp culture ponds are Lates calcarifer, Elopes jarbus, Epinephelus, Therapon etc. He also noticed crab predators such as Scylla serrata and Portunus pelagicus. In the present experiment T. jarbua was observed in the control cage and also S. serrata and P. pelagicus were observed but later one was predominant (Table 3). According to Bensam (1982) toxicants like derris powder, tea seed oil cake and mahuva oil cake may be helpful for the eradication of bony fishes. This method was not suitable even to bony fishes in the present study, since the cages were erected in the estuarine medium. Another way of preventing the entrance of predatory fishes into culture ponds is to mechanically obstruct their penetration by providing a series of fin-meshed nets across the passage of water (Bensam, 1982). This method was also found to be not suitable for cage culture operations as it paved a way for serious fouling problems.
In the present experiment the total autoentrants were found to be high in the control cage (14.32 kg), followed in the order of cages changed at the interval 60 days (6.74 kg), 30 days (2.91 kg) and 15 days (1.43 kg). In all the cages predominant autoentrant species were P. indicus, M. monoceros, P. pelagicus, S. serrata, Ambasis sp. and Mugil cephalus (Table 3). Shanmugam et al. (1994, 1995 and 1998) also found similar predominant autoentrant species with slight variations. From the findings of the present study it is obvious that the cage changed at 30 days interval showed better survival rate (96%) which could be attributed to the reduction in the autoentrant biomass which inturn reduces the competition for both food and shelter as well as predation by the autoentered species. Similarly, Venkatesan and Bose (1982) obtained good survival rates (<81%) in the absence of predators in the pond culture of P. monodon. This confirms the view of Krantz and Norris (1975) who observed a survival of 60-80% under suitable pond conditions such as absence of predators, sub-optimal temperatures and salinities. On the contrary, eventhough normal salinity, dissolved oxygen, pH, temperature (Table 2) and low autoentrant problems were observed at the cage changed at 15 days interval, only a low survival rate (60%) was observed than the others including control. This may be attributed to the handling stress, which causes mortality.
In the present study the biofouling was higher in the control cage (12.51 kg) followed by cages changed at the intervals of 60 days (6.32 kg), 30 days (3.65 kg) and cannot be identified for 15 days. The presence of biofouling in the cages were seems to be directly related with the time of existence of cage in the water medium. It was proved that the control cage existed continuously in the water medium and showed higher macrofouling community and the cage changed at very short interval of 15 days shared lower macrofouler population. It is interesting to note that for all the cages, the casuarinas poles were not changed. While changing the cages, the poles remains as such and the new cages were anchored with these supporting structures. Due to this all the casuarinas poles were existing throughout the culture period and they possess uniform macrofouling population (0.54 to 0.62 kg).
The macrofouler species found in the casuarinas poles (Table 4) used in the present study showed the predominant species of barnacle, Balanus amphitire (77.59 to 79.03), following by bivalve, Crossostrea sp. (13.79 to 15%). Similarly, Santhanam et al. (1983) also observed B. amphitire as a dominant species in the casuarinas poles and suggested that the appearance and dominance of only a single cirripedian barnacle species namely B. amphitire in the brackish water environment could be due to its wide tolerance in the euryhaline system. This species is also reported to have a continuous breeding activity in tropical estuaries resulting its level recruitment throughout the year (Dharmaraj and Nair, 1981). In all the cages except control cage, the dominant macrofouling species observed was Crossostrea sp. (66.61-76.99%) followed by B. amphitire (18.9-24.68%). Santhanam et al. (1983) also observed this species in the cages. The dominant Crossostrea sp. may be to the cages, which act as a suitable substratum for the attachment than the barnacles.
Srikrishnadhas and Sundarajai (1990) stated that a tropical country like India has to face the problem of biofouling when taking up mariculture. Though the fouling community form the basic natural food source for the shrimps, their overgrowth burdens the culturists. They suggested two ways to overcome this problem. One-way is the periodical cleaning of the cages, however in the earlier study an attempt was made to overcome this problem by transfer of the stocked animals to the next series of new cages and the old ones can be made ready (even by expose to air and used subsequently). Eventhough in the present study cages were changed at different periodical intervals (15, 30 and 60 days), in the cage changed every 30 days periodical interval the observed results were coincided with their statement and the old cages were also reused as suggested by them after thorough cleaning. Similarly, the view of Chien et al. (1989) also confirmed that the transferring system of farming (multiphasic system) can increase production by 30%. While the size of the cages was also a crucial problem when transfer the cages, proper further research will throw more light on all the aspects of economics. In the present study, 10x5x1.5 m cages were used, which was found to be suitable for transfer and it did not pose any problem. From the findings of the present study, it is suggested that the cage changed every 30 days regular interval is ideal for the higher production of P. indicus.