The stone crab (Menippe mercenaria) is a promising aquaculture species due to its fast growth and good market acceptability and price. However, a stone crab aquaculture operation requires significant capital input for both the breeding and grow-out phases. Our stone crab farming will require expertise in husbandry of crustaceans, water quality control, trench management, nutrition, processing and marketing.
Stone crabs currently have good export potential. But the development of new stone crab products, both for domestic and overseas markets, will also create new much larger economic opportunities. One of these new products is bulk-packed flash frozen crab claws. Claws that are flash frozen at -50 degrees are suitable for bulk packaging and marketing in traditional grocery stores. Other products such as crab cakes and canned crab also have significant financial potential.
Bahamas Certified Seafood (BCS) will build and manage a stone crab hatchery. This hatchery will be part of an ongoing research program run by Dr Hector Cruz Lopez. This "controlled" operation is the basis for all Proforma financial projections. It will represent less than 1% of the total BCS breeding stone crab population.
BCS will maintain their fleet of traditional fishing boats for a period of time. During this initial period of time, all crabs that are caught in traps will be declawed and returned to the BCS tidal trenches as broodstock. Males and females will be place together. This breeding arrangement will consist of approximately 250,000 females breeding "uncontrolled" in the trenches. Constant monitoring of the "uncontrolled" breeding stock will be accomplished with the Humphrey Harvester. None of the offspring from the "uncontrolled" breeding have been factored into the Proforma projections.
A certain portion of the wild caught crabs will already be carrying egg sponges. Special care will be given to gently transfer these females to our special trenches for hatching. Due to this, the tidal trenches will immediately began producing crabs. This fact has not been factored into any of the financial projections of the BCS Proforma.
Mature female stone crabs kept in large tanks or in ponds under suitable conditions will extrude eggs. Each egg batch contains about 1 million eggs and a female may extrude up to 5 batches in a year. The eggs take 12-14 days to hatch, depending on temperature. BCS will have 30 tanks with 30 crabs per tank for a total of 900 breeding females in the "controlled" operation. This control operation will be part of the research facility monitored by the scientist. All proforma projections are based on these 900 breeding females with only 1 batch per year considered in financial projections.
After they hatch, the larvae feed on small planktonic animals. Rotifers or brine shrimp (Artemia spp.) are live food types used successfully in cultures. Hygiene during the egg and larval phase is critical to success through to post-larvae in the hatchery. Water quality parameters, such as temperature, salinity, pH, oxygen and ammonia, should be maintained at approximately open sea conditions throughout this phase. Crowding of larvae near the end of the cycle can lead to heavy losses due to cannibalism. The fully planktonic zoea larvae grow through 5 stages over 2-3 weeks at 27-28°C before changing into a megalopa. Using water directly from the tidal trenches assures the highest quality environment in this phase.
The megalopa has claws, resembling a small crab with a tail, but still swims actively, voraciously feeding on juveniles, other larvae and fresh and artificial diets. The megalopa stage lasts about a week before metamorphosing into the first crab stage, which is about 4mm wide.
Hatchery-produced megalopa or first-stage "crablets" are typically reared in nursery systems to a more advanced crablet stage, 10-40mm across, before seeding into a growout system.
During the 3-6 week nursery period, the crablets are benthic and cannibalistic but can be successfully reared in shallow trenches, at densities greater than 50 per square meter. Density levels will be maintained by separation panels in the trenches
The crablets are initially fed a diet of artificial formulated feeds, but very quickly, will readily consume minced fresh diets such as mussel or fish meat. Water quality is less critical at the crablet stages compared with the hatchery phase.
Growing crab is a common practice in the far east, but in the US, it has not yet progressed beyond pilot-scale operations. In aquaculture farms, post-nursery crabs reach marketable size and maturity in 12 months compared with 24-36 months under natural conditions.
Our stone crab growout system requires a substantial area of earthen trenches with access to large quantities of water. Farm design requirements for stone crabs are very similar to that for marine prawn production. Advances in high-density recirculating production systems will in the future remove some of the constraints on site area required and site availability, but this method of production will be capital-intensive due to construction cost of the trenches.
During growout in traditional pond systems, crab growth and survival are linked strongly to stocking rate. High stock losses due to cannibalism occur when crabs are grown to market size at greater than 3 per square meter. A stocking rate of around 0.5-1.5 per square meter may give the highest productivity and economic benefit in simple earthen ponds.
Water Quality and Temperature
Juvenile and adult crabs are far more tolerant of temperature and salinity change than the larvae. Nevertheless, temperature should be kept below 32oC to avoid water quality problems and above 20°C as growth is greatly reduced at temperatures below this level. This demand for high water quality will be accomplished with the tidal gate system
Salinity does not appear to be important to survival provided it stays above 10 parts per thousand (ppt) and does not exceed 45ppt. The ideal salinity range for growth is about 15-25ppt. Our crabs will totally be grown in their natural environment.
In the natural environment, stone crabs eat mainly shellfish and crustaceans. This is difficult to achieve in aquaculture conditions, so alternatives are used. Overseas it is common practice to feed trash fish or fish waste or other waste streams; however, this can be inadequate for maximum growth. Feeding with raw animal material is also prone to problems, with water quality deterioration due to fouling. Humphrey Harvester can be used to solve this problem.
For this reason, most aquaculture operations predominantly use formulated dried pelleted rations. Diet development work is underway to find a cost-effective formulation specifically for stone crabs. Good results have been achieved using pellets designed for marine prawn aquaculture. However we will be feeding our crabs, fish grown in the same trenches, along with the crabs. . Humphrey Harvester will be used to resolve this issue by the ability to constantly monitor food levels in the trenches and make adjustments. This will greatly reduce cannibalism as well.
Crabs grow by molting. They shed their exoskeleton and expand the new, soft shell by inflating it with water and then harden the new larger shell. During molting, the crab is vulnerable to attack by hard-shelled crabs. This molt-related cannibalism is an important limitation on the density at which crabs can be grown in traditional pond systems.
Traditionally providing shelter for vulnerable crabs
reduces the risk of attack and cannibalism during the short time their shells
are hardening. Traditionally, onion bags or shadecloth, masonry blocks, short
lengths of pipe or car tires were used to provide shelter. Providing shelter in
the pond environment improves survival and productivity; however, it also complicates pond
management. The amount of shelter in the pond is controlled by practical
limitations on the maintenance of pond quality, labor and harvesting. For this
reason the Aquaculture Tidal Trench System is far more superior than traditional
pond systems. Our system allows for protection from predators during the molting process.
The maximum economic density at which crabs can be grown also depends on the crab size at harvest. The smaller the size, the greater number produced. Segregation of the crabs with regard to size and sex is also beneficial in reducing cannibalism. Humphrey Harvester will be used to resolve this issue by the ability to constantly grade the crabs for sex and size.
This plan is based on a Aquaculture system unlike any other currently in the world. It uses 2 patent pending technologies that play a very significant part in the success of this plan. The first is a trenching system where water quality is created and maintained through a "sea gate" Each time the tide rises the trenches are filled and the gate is closed. When the tide reaches low level, the gates are opened and up to 35% of water is released from the trenches. Gates are then closed and when tide reaches high, the water is replaced with new fresh sea water on a daily basis.