By Rex Delsar B. Dianala
The lush foliage of mangrove forests are now the pride of their host communities – a badge for environmental conservation, a natural protection from storm surges, and a potential tourist draw. This is a far cry from the times mangrove areas were seen as unproductive wastelands and were soon cleared to make way for aquaculture ponds.
While aquaculture appears to be the main culprit for the destruction of mangroves in past decades, they are not mutually exclusive. Farming of fish, shrimps, and crabs within mangrove areas – termed aquasilviculture – may be done to combine the benefits of coastal protection, ecological productivity, and livelihood for nearby communities.
Considering the natural ebb and flow of tidal waters within mangrove areas and the obstruction of trunks and roots, aquasilviculture is not as straightforward as pond culture where the water level may be controlled. However, farming amphibious mangrove crabs, also called mud crabs, is seen as the best option for aquasilviculture.
“Mud crab culture in mangrove pens is the most lucrative and environment-friendly mangrove-friendly aquaculture system,” says Dr. Jurgenne Primavera, scientist emerita of the Southeast Asian Fisheries Development Center (SEAFDEC), referring to the grow-out of crabs, also locally known as alimango, a prized commodity with year-round demand in the market.
Growing ‘alimango’ in mangrove pens
According to a manual authored by SEAFDEC Aquaculture Department chief Dan Baliao, growing alimango among mangroves mainly entails stocking wild or hatchery-sourced crablets within a manageable area of between 2,000 square meters to 1 hectare enclosed with nets or bamboo slats. The shape of the area will depend on the topography and distribution of trees and roots.
Because some crabs tend to dig, enclosures should extend up to 70 centimeters beneath the soil. Meanwhile, the top should be not less than 30 centimeters above the highest high tide. Crabs should also be prevented from climbing over the pen using a 30-centimeter net overhang or plastic lining on top of the fence.
Another important component is ditches that will serve as refuge for the crabs. These should be able to hold water during the lowest low tide and cover between 20 and 30 percent of the enclosure area. While minor roots of mangroves will likely be affected by the digging, cutting main roots should be avoided.
Monosize crablets weighing between 30 to 50 grams each may be stocked at 5,000 to 10,000 pieces per hectare. Crabs are fed with chopped trash fish, animal entrails, mussel and snail meat, whichever is locally available and economical. These are broadcast daily beginning at 10 percent of biomass before tapering to 5 percent towards the end of a culture period that may last between 45 and 60 days or when they reach at least 200 grams.
Based on a pilot-project in Palawan, a 4,000 square meter pen which was stocked with 2,040 pieces of crablets yielded 1,767 market-sized crabs weighing an average of 275 grams. The crabs were partially harvested beginning the third month until the sixth month.
In all cases, mangrove crabs are marketed alive. With prices now upwards of P500 per kilogram, crab culture among mangroves might just be another reason for coastal communities to protect their mangrove resources.