Crab culture with a conscience

The rapid spread of aquaculture in recent decades has brought riches to some, ruin to many, exclusion to the poorest coastal dwellers, and environmental degradation. Scientists in the Philippines are adapting aquaculture to make it sustainable over the long term and suitable for small-scale, family-level operators. An innovative system of captive crab culture in live mangrove is being developed in the central Philippines and is now being verified and demonstrated on the southern island of Mindanao.

"Aquaculture needs to become more mangrove-friendly to be sustainable," argues Dr. Jurgenne H. Primavera of the Iloilo, Philippines-based Aquaculture Department of the Southeast Asian Fisheries Development Centre (SEAFDEC), an 11-member inter-governmental treaty organisation headquartered in Thailand. "That means developing aquaculture techniques that don't require clearing the trees."

Tantanang is one of many scenic bays in Alicia, a coastal municipality on western Mindanao's Zamboanga Peninsula. The region is known to the outside world mostly for its long-running Muslim insurgency, but Tantanang Bay is a local leader in environmental protection. Three-quarters of the residents of its 16 villages draw their livelihood from the bay, which includes a 5-hectare fish sanctuary.

Environmental and economic sustainability

To counter illegal fishing, the government of Alicia established in 1998 a federation of 10 community-based organisations and cooperatives, two of them Muslim, with a combined membership of about 400, half of them fisherfolk. Called by its Filipino acronym NAGMMATABA, the Tantanang Bay federation has introduced in four of its member villages an environmentally and economically sustainable method of raising mud crabs (Scylla serrata) in pens among the mangroves.

Central to the effort is Cris Batonghinog, a federation member and fishery technician who trained in sustainable aquaculture and coastal resource management at SEAFDEC/AQD in Iloilo in 1999. Municipal funding for Batonghinog's mud crab proposal came though in 2003, and the pens went into operation the following year. Measuring 2,000 square metres for each farmer, the pens are adjacent to each other to facilitate the sharing of labour during construction (funded by the Alicia municipal government) and joint protection from poachers. A community-based forest management agreement with the Philippine Department of Environment and Natural Resources lends the villagers renewable 25 year tenure over their pens, which are permitted to cover a maximum of 4-5 hectares, or up to a quarter of the total mangrove area of each village.

Unlike the SEAFDEC/AQD prototype, which divides the pens with nylon nets and requires elevated bamboo walkways for access, the Alicia pens are separated by low dikes with openings that allow the tide to flow freely in and out. This is essential for maintaining the health of the mangrove trees that shelter the crabs. To prevent the crabs from straying, nylon netting extends above the dikes. Half-metre-deep canals occupy 20-30 per cent of each pen to retain water for the crabs at low tide. As the tide comes in, farmers broadcast supplemental feed. They continuously stock the pens, allowing 1-2 square metres per juvenile crab. Selective harvests of mature crabs in June, August and October 2004 saw yield and income rise by 20 per cent.

Feed pellets

One problem is that farmers feed their crabs raw fish - so-called trash fish that is nevertheless valued and consumed by poor people. Aiming to reduce farmers' dependence on this depleted resource, SEAFDEC/AQD teamed up with the Bureau of Fisheries to study supplementing the crabs' diet with meal feed pellets. This study for developing sustainable aquaculture and fisheries of mud crabs - one of many supported by the European Commission through the Culture and Management of Scylla Species Project - was in Batan, a coastal community at the far end of Aklan Province from the resort island of Boracay. Taking into account the natural productivity of the mangrove pens and continued supplementation with fish, albeit at a lower rate, the experimenters used nutritionally incomplete pellets to hold down costs.

"The final harvest after five months showed little difference in growth or survival between feeding the crabs fish biomass plus pellets and feeding them only fish," reports Primavera. The somewhat lower survival rate and slightly higher cost of pellet feeding nevertheless depressed profit and return on investment by nearly half and extended the payback period from one year to four.

Sensitivity analysis showed that raising the crab survival rate from 37 to 50 per cent would raise profitability to nearly that of fish-only feeding and shorten the payback period to two years. "Increasing the culture area from 1,000 to 2,000 square metres would mean further improvement to 232 per cent return on investment and one year payback," Primavera adds.

The doctor of marine science insists that reducing the amount of trash fish fed to crabs is ethically vital. "Philippine fisheries are so degraded, fish supplies are so inadequate, and people are so poor that they need fish of all sizes and prices," she says. "Crab farmers should leave as many fish as possible for the low-income coastal communities that depend on them."

For further information visit http://inco-cams.seafdec.org.ph and www.mangroveweb.net