The effect of sublethal concentration of DimilinR (TH6040) on the larval development of the mud-crab Rhithropanopeus harrisii (Gould)

Sublethal DimilinR (insecticide) concentration significantly decreased the percentage survival of larval R. harrisii . This decrease was marked at low salinities, when it may have become toxic. This was true for both 25 and 30 C. Duration of larval development did not seem to be affected by sublethal DimilinR concentrations even in extremely low or high salinities. High temperature, however, shortened the time of development. No anatomical abnormality was observed. These findings are only tentative as more replicates of the experiments are needed before definite conclusions can be reached. It will be desirable to work with more salinity-temperature combinations to get a complete picture of sublethal effects under different environmental conditions.

Impact of stocking density on growth and survival rate of mud crab (Scylla serrata Froskal)

A 10-weeks culture trial of mud crab, Scylla serrata in brackish water earthen pond was conducted in different stocking densities. The aim of the experiment was to identify a suitable stocking density for optimum production. There were three treatment as 5000 crab lings/ha, 10000 crab lings/ha and 15000 crab lings/ha of each with three replications. The initial mean weight of crab lings were same (5.5 ± 0.13 g). The experimental month was June '95 to August '95. The size of each pond was 500 m². To maintain good water quality water was exchanged in every spring tide. The salinity during the experiment were 2-18ppt. Prepared feed of about 32% protein consisting fish meal, MOC rice bran and wheat flour was used at 5% of their body weight. In terms of production, survival rate, growth and carapace width, the stocking density having 10000/ha showed the best (P

Effects of cowdung application on the production of mud crab (Scylla serrata Forskal) in brackishwater pond

A culture experiment of mud crab for 84 days was conducted in earthen pond at Brackishwater Station, Bangladesh Fisheries Research Institute, Paikgacha, Khulna. The aim of the experiment was to study the effects of cowdung application on crab production and water quality. There were three treatments as without cowdung (T1), 500 kg cowdung/ha/fortnight (T2) and 750 kg cowdung/ha/fortnight (T3) with three replicates for each. The result was evaluated on the basis of growth, production, survival rate etc. The experimental months were mid-April '96 to mid-July '96. To maintain a good water quality, water was exchanged in every spring tide. The range of salinity during the experiment was 8-19 ppt. Trash fish and fresh shrimp head were used as feed on raw basis in every alternate week at a rate of 8% body weight of crab at the same time rice bran and wheat flour were used at a rate of 2% body weight as a source of carbohydrate. The production of T1, T2 and T3, were 720.35 kg, 862.16 kg and 669.19 kg/ha respectively. Though the effects of cowdung on production of crab is insignificant but in terms of production, survival rate and growth, the study suggest that the application of cowdung in addition to feed can be recommended for mud crab culture at a rate of 500 kg/ha/fortnight.

A mixed culture trail of mud crab (Scylla serrata Forskal) with tilapia ( Oreocromis niloticus L.)

A ten-month study on mixed culture of mud crab Scylla serrata with tilapia (Oreochromis niloticus) was performed in brackish water earthen ponds using live tilapia fry as the only feed for crab. The monthly growth rate varied from 9.07-19.llg among four treatments. Treatment T 1, cw: 0.68cm (±0.72) and bw: 19.11 gm (±12.97) showed highest performance which was followed by T 3, cw: 0.62cm (±0.60) and bw: 13.42 gm (±10.51), T4, cw: 0.65cm (±0.64) and bw: 13.20 gm (±9.89) and T2, cw: 0.36cm (± 0.25) and bw: 9.07 gm (±8.05). Highest survivability of crabs was also recorded in T 1 (21.5%) which was followed by T 2 (15.65%), T4 (14.95%) and T 3 (14.15%). In terms of survivability, significant differences (p<0.05) were observed among the treatments whereas these were recorded as insignificant difference (p<0.05) in final weight, weight gain and production of crabs and tilapia. Mixed culture of mud crab with tilapia could make more rewarding than crab monoculture but the study suggests that only tilapia fry can not fulfill the feed requirement of crabs in respects of survivability, final body weight and weight gain. Besides, existed salinity level of 4-12 ppt during experimental period might be the another key factor for low survivability and weight gain.

Feasibility of bi-culture of mud crab (Scylla serrata) and shrimp (Penaeus monodon)

A study on the feasibility of bi-culture of mud crab (Scylla serrata) and shrimp (Penaeus monodon) in brackishwater earthen ponds (0.1 ha each) was carried out for a period of five months (March-August). Nursed shrimp juvenile (ABL:· 3.36±0.23 em and ABW: 0.26±0.04 g) and crab juvenile (ACL: 2.61±0.22 cm, ACW: 4.63±0.11 cm and ABW: 43±2.64 g) were stocked following the experimental design of shrimp 2/m2 (Treatment-1), shrimp 2/m2 and mud crab l/m2 (Treatment-2) and shrimp 2/m2 and mud crab 0.5/m2 (Treatment-3). Crabs were fed with chopped trash tilapia @ 10~5%, while shrimp were fed with Saudi-Bangla shrimp feed @ 3~5% of biomass twice daily. Significantly (p<0.05) higher specific growth rate (SGR) of shrimp and mud crab was 1.86% (g/day) in T2 and 0.83% (g/day) in T3, respectively. The survival of shrimp and mud crab also varied significantly (p<0.05) with a higher mean value of74.63% in Tl and 51.04% in T3, respectively. The production of shrimp (424.09 kg/ha) was significantly (p<0.05) higher in Tl and that of mud crab (568.80 kg/ha) in T2. Significantly (p<0.05) highest total production of 871.29 kg!ha was in T2 followed by 708.52 kg/ha in T3 and 424.09 kg/ha in Tl. The results indicate that mud crab can be cultured at a stocking rate of 1/m2 together with shrimp at 2/m2 •

Some bacteria isolated from marine plankton and mud

57 bacterial strains isolated from marine plankton and mud from the inshore waters of Cochin, are described. The flora of the plankton was dominated by chromogens while mud sample and failed to produce chromogens. The different strains are divided into five arbitrary groups for the convenience of description. The flora is similar to that of Mandapam (east coast) while it differed from the flora of Calicut (west coast) in the paucity of Bacillus and other spore formers.

The propagation of the mud crab Scylla serrata (F.) de Haan

The mud crab Scylla serrata is an important commercial species found in many brackish areas in the Philippines. During spawning and hatching, the berried females migrate to the sea. Seeds for pond stocking are obtained from the wild. Because of the unpredictability of seed supply, there is a need to propagate the species artificially. Thus, spawning, larval rearing, maturation, and rematuration of the species are being studied. The first attempts at hatching S. serrata were successful with rates varying between 75% and 90%. Two out of three trials on larval rearing yielded a few megalops. The first zoeal stages were fed diatoms, rotifers, Artemia salina, and bread yeast. Overfeeding programs were implemented during the critical premolting periods to prevent weakening of the larvae and lessen cannibalism. Larval weakening during the premolt makes them susceptible to attacks by fungi like Lagenidium and ciliates like Vorticella. S. serrata larvae survived salinity levels as low as 15 ppt until the 14th day of rearing. Other larvae were able to survive in salinities of 30-32 ppt for 8 to 13 days. Zoeal molting was hastened by lowering the salinity to 25-27 ppt. Artificial broodstocking of juveniles and adult crabs has been made possible using a simple refuge system made of three-compartmented hollow blocks. This system has been helpful in minimizing fighting among crabs. Remarkable growth rates have been observed with feeds like mussel meat and trash fish. Average growth increments of 11 mm carapace length and 20 . 35 g body weight have been observed every fortnight. A newly spent spawner could gain additional weight of 22 . 5 g in only 6 days. Feeding rates of juveniles and adult crabs have been established based on the average body weight from an experiment using mussel meat. Crabs feed more at night. In another experiment, eyestalk ablation was found to be effective in inducing growth and mating. Aside from hastening the molting process, copulation is induced even among the small crabs (average carapace length = 55 mm). Natural mating lasts about 26 hr. A copulation which lasted for seven days with a break in between was observed.

Polyculture of milkfish (Chanos chanos) and mud crab (Scylla serrata) at two stocking densities

Mud crabs (Scylla serrata) were cultured singly and in combination with milkfish (Chanos chanos) to compare growth, survival and production rates. Net production of crab was higher in polyculture than monoculture, but the reverse was observed for milkfish.

Culture of the mud crab, Scylla serrata (Forskal) at different stocking densities in brackishwater ponds

The mean weight percentage survival, relative growth increment, net production and feed conversion value of S. serrata stocked in monoculture at different stocking densities are presented.

Fishery of mud crab, Scylla serrata (Forskal) from Karwar waters

Catch data of Scylla serrata obtained in different gears from Karwar region during December 1983 — November 1984 indicated a sum of 9608 kg of landings of crabs. Majority of the crabs (89.57%) was found to be caught in backwaters by line fishing and cast nets. A considerable (10.43%) quantity of catch was also contributed by gill nets and shore-seines from inshore waters. A maximum of 1207 kg of crabs were landed in December, 1983. The size structure and sex-ratio of S. serrata varied with gear type and period.

Effect of stocking density on survival and growth of mud crablings, Scylla sp. in laboratory conditions

Mud crablings, Scylla sp. were reared, for a period of six weeks in fiberglass aquarium under laboratory conditions, to determine the effect of four different stocking densities of 1, 2, 3 and 4 crabling/l of water on their survival and growth. Salinity of water was maintained at 25 ppt throughout the rearing period. Stocking rates of 1, 2, and 3 crabling/l resulted in a similar (p>0.05) survival rates of 75, 74, and 83.5%, respectively, that of 4 crabling/l resulted in significantly lower (p<0.05) survival rate of 56%. No significant difference was observed among different stocking densities in average growth of carapace length (CL), carapace width (CW) and body weight (BW).

Mud crab hatchery and nursery operations

A brief account is given of mud crab (Scylla spp) farming activities in the Philippines. The expanding market for mud crab is the cause of intensified collection of wild juveniles. To counter the threat to wild population and ensure the sustainability of mud crab farming, there is a need to produce juveniles in hatcheries. Hatchery and nursery operations, and investment costs/returns are outlined.