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Alongkorn Kanokboriboon, SCG Chemicals

Thai Government Hatchery Utilizes Rotomolded Parts to increase populations of the Blue Crab

Ocean’s natural resources have been dramatically depleted in recent decades due to over-fishing and indiscriminate catches. This causes a disruption in the natural life cycle of marine animals and leads to reduced marine life population. Without some type of human intervention, it is unlikely that nature can heal itself back to the same sustainable level.

In this issue, we will discuss a case study about a government hatchery center that utilizes rotomolded tanks and bins to increase the population of ocean blue crabs. This hatchery is located in Chantaburi, a coastal province in eastern seaboard of Thailand. It was found about 23 years ago by the vision of late King Rama 9.

Blue crab populations in the Gulf of Thailand have drastically decreased over the years.

Blue crab is a local marine life commonly found worldwide. It is quite a popular delicacy both in homes and restaurants. There is also a large commercial fishing of blue crabs for its meat to be processed as frozen seafoods. As a result, blue crab populations in the Gulf of Thailand have drastically decreased over the years.

In the natural environment, blue crabs take about 4-5 months to grow and mature into adults. One mother crab can produce about 300,000-700,000 eggs but less than one percent (or 3,000 crabs) will survive and grow into adulthood. In the past,

Figure 1: Blue crab with external eggs

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local fishermen would keep all the crabs (including pregnant crabs with external eggs) they could catch even though these mature eggs are not edible. As a result, blue crabs never had a chance to reproduce and replenish the ocean— causing huge reduction in blue crabs population in the Gulf of Thailand.

Sea Farming Demonstration Unit in Kungkrabaen Bay in cooperation with a Pack Foods Public Co. Ltd. started a program called “Crab Eggs Loan” to borrow pregnant crabs from local catches, harvest their eggs, and return these crabs to the fishermen. Collecting these eggs is done by hand by using nylon brushes to gently scrub the bottom-side of pregnant crabs with external eggs, which can vary from orange to dark brown. Orange color indicates that the eggs are still young and will take a few days to hatch while dark brown mean these eggs can catch within hours.

These harvested eggs are entered into Circulation and Separation Hatching Technique (CASHT), which consists of seven (7) blowmolded HDPE 150liter tanks connected in series, a water circulating pump, and oxygen pump. At the end of the line, there is a rotomolded cone-shape bottom hopper, where separation of crab eggs and baby crabs take place. Air is added into the tank to cause baby crabs to rise to the top and crab eggs to sink to the bottom. Crab eggs are suctioned back to Tank Number 1 and recirculated continuously until they hatch. Baby crabs, which float to top, will overflow to an open-mouth rotomolded rectangular bin, where they are caught in a receiving net. Remaining water is also circulated back to the system. These tiny crabs are then collected to be released into open water at night between 9 p.m. and 3 a.m. for highest chance of survival.

The rotomolded separation unit was manufactured by Ocean Plastics Co. Ltd. (Thailand) with general-purpose grade medium-density polyethylene (0.932 density and MI 4.0). Originally, it was designed to be a beer dispenser.

Figure 3a: Circulation and Separation Hatching Technique (CASHT) process diagram

Figure 3a: CASHT in operation

Since the program has started, there is a noticeable rise in blue crab population in the Kungkrabaen Bay and nearby coastline. This indicates that the program has a positive impact in restoring local marine life as well as livelihoods of fishermen in the area. As a result, there is an easy cooperation from both local fishermen and surrounding communities to voluntarily donate pregnant crabs to the program. Since September 2012 until August 2020, it is estimated that the Center has processed 431,000 pregnant crabs, whose eggs of about 9.8 metric tons have been collected and released back to the nature.

In summary, simple rotomolded tanks and bins with the right design and accessories can be great affordable tools to restore the well-being of both marine life and local fishing community.

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Figure 3c: Receiving net to catch baby crabs