Mangrove conservation in lagoons: lessons from Sri Lanka
Nearly all of Vietnam’s mangroves grow along coastlines in the Red and Mekong Deltas. But those in central Vietnam are mostly found in estuaries and lagoons. In 2010, Mangroves for the Future (MFF) awarded two small grants to Centre for Community Research and Development (CCRD) and Hue University of Agriculture and Forestry (HUAF) to conserve mangroves through improved community-based management in two lagoons in Thua-Thien-Hue and in Quang Nam respectively.
In 2009, MFF reviewed the results of five years of mangrove planting and conservation in Sri Lanka’s estuaries and lagoons. A feature of the lagoons in both Vietnam and Sri Lanka is that they are micro-tidal, meaning that the difference between high and low tides in small, typically less than 1 meter. By comparison, the tidal difference in the Red and Mekong Deltas s 3-4 meters.
The small tidal range of most lagoons limits the area over which mangroves, which grow in the inter-tidal zone, occur naturally. Because currents inside the lagoon typically have a flow speed of less than 1 meter/second, sediment that is carried into the lagoon is not carried out to sea. Sediment deposition is accelerated whenever, as is often the case, the connection between the lagoon and the ocean is blocked or impeded, for example by a bridge or build up of garbage. These barriers also block the recruitment of early stages of fish and shrimp into the lagoon during the wet season, resulting in the loss of native species and the introduction of exotics.
Since some mangrove species, especially Rhizophora spp., trap sediment very effectively, planting mangroves in Sri Lanka’s lagoons has often increased sedimentation and reduced the area of open water. In some cases, this has resulted in the collapse of the local fisheries and encouraged encroachment into the lagoon.
In 2011, MFF published the results of its analysis in a report entitled An Appraisal of Mangrove Management in Micro-tidal Estuaries and Lagoons in Sri Lanka. Following the 2004 Indian Ocean Tsunami, when mangroves were seen as having reduced loss of life and property, there was substantial international funding for mangrove restoration in Sri Lanka. The report concludes that most of this work was “a completely inappropriate, futile exercise with possibly negative consequences,” and that “it is necessary to move away from belief-based good intentions to science based procedures for ecosystem management.”
The purpose of this note is to highlight recommendations relevant to the MFF-funded projects in Thua-Thien-Hue and Quang Nam. These include:
- Mangrove planting should be the last resort and only done where mangroves have existed before. The objective should be to connect the lagoon to the sea’s tidal influences by removing barriers and permitting a more natural water flow. This will allow mangroves, which recover easily, to regenerate naturally and avoid the buildup of sediment in the lagoon and the consequent loss of fish habitat.
- Replanting should only be considered where natural regeneration is not possible (e.g., where seeds are no longer available). In India, MFF recommends a fish-bone “canal bank planting” system for mangrove restoration in low tidal amplitude areas.
- If mangroves are planted, it is vital to provide adequate post-planting care. Many planting projects in Sri Lanka lasted just six months. The report recommends a 10-year time-frame. In Sri Lanka, this means that the NGO or other implementing organization should focus on a small number of sites, enabling their staff to build solid relations with local communities and access the appropriate technical expertise.
- Pay attention to the protection of non-mangrove ecosystems. In Sri Lanka, recent research indicates that sea grass beds, submerged and rarely noticed, play a more important role in fisheries productivity than mangroves.
- The focus should on conserving the remaining mangroves rather than planting new ones. In Sri Lanka, as in Vietnam, mangroves are still being cleared for shrimp farming, settlement, tourism, and other purposes. Stopping further mangrove clearing must be the top priority.