Talking Point

The mud crab, Scylla serrata, also known as Mangrove Crab is a portunid crab, that is, it is a member of a group of swimming crabs which has the last pair of legs flattened for swimming. Scylla serrata is the largest portunid crabs. Mud crabs are large crabs with a smooth, broad carapace.  They have nine even sized teeth on each side of their eyes.  In the most common form, the colour varies from very dark brown to mottled green.  The other, generally smaller form has a deeper body and is reddish brown.

In India, the mud crabs have come into prominence since early eighties with the commencement of live crab export to the South East Asian countries which has created a renewed interest in the exploitation as well as in the production of mud crabs through aquaculture. The importance of live mud crabs as an export commodity has opened up great opportunities for crab farming. It has high demand and price in the export market.  

 

Habitat and diet: Mud crabs inhabit sheltered estuaries; the tidal reaches of some rivers, mud flats and mangrove forests, although females carrying eggs are present in deeper waters up to 50 kilometers offshore.  These crabs favor a soft, muddy bottom, often below low tide level. These crabs prefer salinity range of 10 to 34 ppt. and temperature range of 23⁰C to 30⁰C. Although many live in the intertidal zone, the majority lives subtidally, where they bury in the mud during the day and emerge at night and forage for food at night. Juvenile mud crabs eat planktonic animals, benthic molluscs and crustaceans of various types. Adults feed at night on a variety of slow moving bottom dwelling bivalve and gastropod molluscs including mussel, small crabs and polychaete worms.  Mud crabs are also attracted to dead fish and meat in traps.  They rarely eat fish under natural conditions since they lack the ability to catch them. The mud crab’s large claws are used for crushing and cutting their prey.  If they lose a claw, they may grow another one in successive moults. They are attracted to a wide variety of baits, including fish.

Life cycle: Mud crabs live for up to 3 years. The females reach sexual maturity at a size of about 12 cm in S. tranquebarica and 8.5 cm in S.serrata in the brackish water.  Both the species are continuous breeders with peak breeding seasons, which vary from place to place. Each animal spawns once in two months. Mud crabs reach sexual maturity at between 18 and 24 months and mate in the warmer months. The mature females (known as ‘jennies’) release a pheromone, which attracts the males (known as ‘bucks’). The successful male picks up the female and carries her around for several days until she moults. Mating occurs only when the female mud crab is in the soft-bodied condition following moulting.  The male deposits a spermatophore, or packet of sperm inside the female’s reproductive opening where it is stored till the developing ova are ready to be fertilized.

Following mating, the female mud crabs migrate offshore to spawn. They lay 2 to 5 million eggs in each spawning. Female crabs incubate the eggs for 2 to 4 weeks under their abdominal flap.  On hatching, zoea larva emerges which are sensitive to high temperatures and low salinities, and hence require marine conditions. There are four zoeal stages, which lead to the megalopa stage.  At this stage they resemble a small elongated crab (3mm) with a well developed abdomen projecting backwards like a tail. Appendages on the abdomen also develop by this time, which ultimately help the megalopa to swim back to the estuaries. Larval life lasts about a month. Once back to the estuary, the megalopae metamorphose into juvenile crabs and settle down in sheltered mangrove areas.

To be continued …

The world’s oceans have been fished nearly to the limits of their sustainable yields. With the current state of fisheries, additional production of seafood will have to come mainly from aquaculture. In recent times, mariculture has got a tremendous boost globally owing to technological developments in the field of cage culture and related areas in Norway, other Scandinavian countries, Chile, Japan and Australia. The Norwegian technology has helped countries like Chile to make tremendous foray in the field within a short period of ten years.  

Read more about mariculture, a specialized branch of  aquaculture involving the cultivation of marine organisms, in this multi-part series.  

Candidate species: The aquaculture of high value finfish species, such as groupers, is of increasing importance throughout the Asia-Pacific region, including Australia. The development of large and affluent markets for live reef fish, particularly in Hong Kong and southern China, has increased pressure on wild stock resources. In many areas the demand for live reef fish, and the profitability of this trade, has encouraged overfishing and the use of destructive fishing practices. One such practice is the use of sodium cyanide to ‘stun’ reef fish for capture by divers. Because of the high toxicity of sodium cyanide, many more fishes are killed than are captured live and coral reef areas are devastated. The aquaculture of high value reef fish species can potentially supply product to the live reef fish markets, as well as other regional and domestic markets. The development of aquaculture technology for these species will not only support an economically beneficial aquaculture sector, but will also contribute to reducing pressure on wild stocks. Currently, the major bottlenecks to increased aquaculture production of groupers are the generally poor, and highly variable, survival in larvi culture, and the limited sources of trash fish for grow-out. The important candidate species in our context may be Asian sea bass, Groupers, Snappers and Eels.

Sustainability of commercial mariculture: The major problems pertaining sustainability of mariculture are as follows.

• Nutrient and waste loading of the aquatic environment
• Depletion of marine resource by way of consumption of fish meal and fish oil

Considering the above constraints, R&D efforts are on to meet these challenges and the present developments in this regard are as under.

• Formulation of HND fish feed with FCR of 0.83 and protein sparing effect has made it possible to cut down on the consumption of feed to 44% of what it was in 1972 and brought down nitrogen loading from 180 kg per  ton of salmon produced in 1972 to the present level of 30 kg.
• Fish meal content of fish feed has been reduced from to 35 percent today from 70 percent in 1972. Further  reduction is being attempted using alternate feed stuff such as soya, rape seed oil and corn gluten. Chinese researchers are working on a yeast-based protein supplement that could reduce use of fish meal by 50 percent. There is a possibility that Spirulina could provide the vital input in this context.

What needs to be done: As regard mariculture of fin fish in inshore and offshore areas is concerned, there are number of  gaps in development which needs to be addressed.

1. Formulation of regulatory framework

If these activities are to be increased on a commercial scale, suitable policy framework needs to be developed for leasing of sea bed and marine areas for marine cage fish farming on a sustainable basis without encountering conflict with other resource usages. The need for carrying out aquaculture in environmentally sustainable, socially acceptable and in harmony with principles of common resource use, has led to the formulation of integrated coastal zone management plan by the coastal states as a follow up of 1992 United Nations Conference on Environment and Development, in Rio de Janeiro, Brazil. Such coastal zone planning, based on assessment of holding capacity, nutrient loading from all coastal activities and common resource use needs to be worked out with clear enabling conditions for setting up such ventures.

As regard the formulation of regulatory framework involving studies related to carrying capacity assessment and site selection, suitable agencies needs to be identified with experience and facilities for under taking oceanographic studies. Institutes like National Institute of Ocean Technology (NIOT), National Institute of Oceanography (NIO) and IITs in the coastal states could be entrusted with such responsibilities on the line of country wide assessment done in Norway. 

i. Identification of potential mariculture sites based on seabed conditions, shoreline and hydrographic characteristics, etc.

ii. Assessment of carrying capacity of each site based on coastal zone planning on the pattern of Norwegian ‘lenka system’ or such other models.

iii. Monitoring of physico-chemical characteristics of water and nature of benthos of the marine sites in relation to predetermined standards should form an on-going activity while mariculture operations proceed on expected lines.

2. Assessment of potentials or holding capacity

i. Inshore cage farming: In this connection identification of marine sites, assessment of carrying capacity of the sites, framing of leasing policy of sea beds, etc. have not been carried out. However, based on general knowledge of the Indian coastline it is believed that the Andaman & Nicobar Group of Islands in the Bay of Bengal and Lakshadweep Group of Islands in the Arabian Sea would offer suitable conditions for undertaking such projects. However, a pilot study in this regard needs to be carried out if necessary by utilising services of overseas technology transfer agents, a multilateral funding agency, fisheries research institutes within the country and Indian corporates.

ii. Offshore cage farming: The potentials in this regard needs to be assessed if necessary by engaging overseas consultants. Technology in respect of offshore cage culture involving submersible cages is neither available nor any attempt has been made to develop the same within the country. The Farm Ocean cage system or SARGO^TM technology could be useful in this context. A pilot study in this context is also necessary.

3. Technology related to marine cage fish farming

The technologies for inshore cage fish farming and that for offshore areas using submersible type cages (farm ocean cages / solid wall cages) are at present not available within the country. Since the technologies in this connection are very exacting and capital intensive, only big corporate houses may be able to master the resources to undertake these activities. It may not be possible to develop in-house technology within a reasonable time frame considering the high degree of automation and the high cost / sophisticated nature of R&D involved. A pilot study with external assistance under bilateral funding could be considered for transfer of technology. The possibilities of supporting adaptive research involving our research agencies of repute and appropriate technology transfer agent could be explored. An appropriate overseas technology transfer agent in this regard needs to be identified for the purpose. Such technology transfer could involve the following exercise to start with.

 i. Identification of appropriate technology for cage fish farming in marine inshore and offshore areas and foreign technology transfer agent for the same.

ii. Identification of suitable species for cage culture in marine conditions and a suitable technology transfer agent for the same. May be sea bass will meet the requirement of the sector.

iii. Identification of suitable research institutes within the country in the field of mariculture, which could conduct an adaptive trail with collaboration with the foreign technology transfer agent that would enable verification of the end results as well as adoption of the technology and assessment of the production parameters. Probably ICAR has the requisite mandate in this regard and could associate for the marine cage fish farming trials.

 iv. Identification of a suitable research institute within the country, which could take care of the technical know-how relating engineering aspect of site selection, cage design, fabrication and installation. This could ultimately facilitate indigenization of the cage system and its manufacture by an Indian manufacturer. Since NIOT, Chennai, IIT Kharagpur and IIT Chennai are engaged in research and development related to aquaculture engineering and has several departments with requisites expertise in the related fields, these institutes may be best poised to undertake the job.

v. Before any step is taken in the above direction, it may be pertinent to conduct a meet with the industry to find out its response to the technology transfer program.  A national meet with the policy makers and research agencies to discuss the various aspects of the program and the need to develop suitable policy frameworks in this context is a basic requirement.

The world’s oceans have been fished nearly to the limits of their sustainable yields. With the current state of fisheries, additional production of seafood will have to come mainly from aquaculture. In recent times, mariculture has got a tremendous boost globally owing to technological developments in the field of cage culture and related areas in Norway, other Scandinavian countries, Chile, Japan and Australia. The Norwegian technology has helped countries like Chile to make tremendous foray in the field within a short period of ten years.  

Read more about mariculture, a specialized branch of  aquaculture involving the cultivation of marine organisms, in this multi-part series.  

The farming systems: The five cage fish farming systems have been discussed here considering the past and the present developments in the industry.

Traditional farming system using conventional feed and feeding mechanism (System 1): Considering the Norwegian regulations, farm size of 12000 m³ has been considered with overall production limit of 300 tons. A wide variety of cage designs has been developed over the last 20  years; however, the common features are a floating collar, usually rectangular, a suspended net bag and a mooring system. The design of cage which is frequently used in Scottish and Norwegian farms, comprises a square or rectangular frame of cage superstructure with walkways in between rows of cages. A steel handrail about 0.75m high, is attached to the inside of the walkway, and from this, the cage bag is suspended.

Typically, the cages are connected to each other with ropes and shackles to form a floating rectangular raft of 15-20 individual cages with a central walkway. The raft is moored to the sea bed or to the shore using anchors, chains, ropes and shock-absorbing systems. In the present case 16 cages with cage volume of 800 m³ each have been considered of which 15 would be under production with one stand-by cage. Thus the effective production volume is estimated to be 12000 m³. Considering a stocking rate of 7.5 fingerling / m³, 85% survival and an average harvest weight of 4kg salmon, the final harvest is expected to be 300 tons. The feeding system is considered to be automatic feeder type based on a feeding program pre-set by the farmer to dispense dry pellet. A 12V battery and a control unit connected through cabling along walkways and cage superstructure can regulate feeding in number of cages adopting centralized feeding. Feeding is synchronized in all the cages using the timing device. In the traditional Farming System use of conventional feed with FCR value of 1.5 (‘N’= 8%; ‘P’ = 1%) is used.

Improved farming system using efficient feed and feeding mechanism (System 2): The farming system is similar to the earlier system excepting for the feeding system, type of feed used and the production program. In this system improved feed with FCR value of 1.0 (‘N’= 7.5%; ‘P’ = 0.9%) has been considered. A strict feeding regime envisages use of computer aided ‘Adaptive Feeding System’ that controls automatic feeders by accurately matching feed delivery to fish appetite. The Aquasmart AQ1 with advanced sensor, communications system and software is one such system with which an entire farm can be monitored and controlled from a centrally located PC. This enables minimising feed loss to the extent of 4% of total nutrient supply. The production program is based on the production potentials of such cage systems under Norwegian conditions as described by BjÃrndal (1990). Accordingly a cage volume of 12000 m³ is expected to produce 370 tons of fish, stocked at the rate of 10 smolt/ m³ with 15% survival and average weight of 3.7 kg/fish.   

Improved farming system using HND feed and efficient feeding mechanism (System 3): The farming system is similar to the earlier system excepting for the type of feed used.  In this system high energy nutrient dense diet (HND) with FCR value of 0.83 (‘N’= 7.2%; ‘P’ = 0.9%) is used. The low-protein, high-fat composition of these feeds with protein sparing effect is expected to reduce protein catabolism and utilize the same for anabolic process thus improving FCR while reducing excretion of nitrogenous products.  A strict feeding regime is envisaged using computer aided ‘Adaptive Feeding System’ as in the case of the earlier system. The production program is also similar to the previous system.

Farm ocean cage system (System 4): The Farmocean offshore cages encloses some 6000, 4500 or 3500 cbm of water depending on the net design. The cage itself consists of a tubular galvanised steel structure mounted on a hexagonal pontoon incorporating six ballast tanks. In its normal operating mode, the farm floats in a semi-submerged position with the pontoons three meters below the water surface. The upper platform and the feed silo are in this position located three meters above the surface. The design is made to withstand offshore conditions and it has been approved and verified by Veritas of Norway. The net is kept in place even in strong current by a sinker tube outside the bottom of the cage. Due to its submerged pontoons the cage stay calm in heavy sea decreasing the risk of losses of fish or damages to the cage. The system has been exposed to more than 10 meter waves without any damages to the cage.

The Farmocean cage is equipped with a computer controlled feeding system permitting feeding of the fishes continuously during preselected periods of the day. This give as a result a better growth and a reduced feed conversion factor . The fish will also be fed during days when the farmworkers stay ashore due to bad sea conditions. Feeding is also continuously matched to water temperature, wave conditions and biomass growth. All normal work is carried out from the upper platform around the feed silo. Normally this means that one person board the cage for the daily collection of data from the computer regarding feed, wind, waves and temperature. For inspection, treatment, harvest, etc. the farm can quickly be deballasted to surface position. A gangway running around the pontoons provides a safe platform.

Solid Wall Cage Farm (System 5): MariCulture Systems’ proprietary SARGO™ solid wall fish rearing system offers much significant technological advancement over the traditional near-shore open net pen cage methods currently used to grow marketable finfish. Both technologies, as compared in the following figure, are used to grow fish from their juvenile stage through their adult harvest stage.

Many of the key SARGO™ features are so advantageous, when compared to current net pen technology, that it is believed that the system may set new environmental and regulatory standards for commercial fish farming facilities. Environmental conflicts resulting from, farm waste and nutrient loading, escaped cultivated fish competing for food and habitat with native fish, and the visual impact of sprawling large-scale farming operations are quickly resolved. 

The system consists of one or more modular pods. Each pod is composed of four, rigid wall, floating fish reservoirs. Each pod also has a centralized service platform fitted with required pumps, feeding equipment and sensor control systems. Ideal, deep source water is pumped into the reservoirs continuously, replenishing oxygen levels for the fish and providing a water current “raceway” for the fish to school naturally. One hundred percent of the solid fish waste and other organic matter is collected for disposal in a waste management and optional treatment system. In a typical farm, there are multiples of pods, each of which provide the fish raising ability of over 40 net pens. A SARGO™ farm, physically equivalent in site size to the largest net pen farms currently in operation, would produce approximately 8,000 metric tons of salmon per year. The production capacity of the comparably sized net pen operation is only 1,000 metric tons per year.

To be continued …