EXAMPLES OF MANGROVE USE - Most mangrove forests in Malaysia

Most mangrove forests in Malaysia are found along the meandering coastline of Sabah (350,342 hectares) followed by those in Sarawak (172,792 hectares) and in Peninsular Malaysia (123,482 hectares). Sustainable extraction of wood for charcoal and construction materials is a major and widespread activity. It is possible to maintain a sustainable yield with appropriate management; for example, in the Matang Mangrove Reserve an average felling cycle of 30 years is practised, and in Johore there is a 20-year cycle with a thinning cycle of about 10 years. Sustainable fisheries are also of major importance, with traditional equipment including gill nets, barrier nets, bag nets, cast nets, traps, hooks and lines. About 30 species of fish and 9 species of prawn have been obtained by using cast nets in the Kapar mangroves, which gives some idea of diversity. In Peninsular Malaysia, the preponderance of mangrove forests along the west coast rather than the east is strongly reflected in the fisheries statistics as shown in the table. They are in accordance with the belief that there is a positive correlation between mangroves and fisheries production.

An interesting fishery is that for jellyfish, which are routinely harvested in the Rejang estuaries of Sarawak between February and June each year. They are brined and processed for export markets in Japan and Taiwan. There are highly productive cockle culture sites on some mangrove foreshores in Peninsular Malaysia, and other sites have floating net cages for the culture of sea bass, grouper and snapper. The Pulau Ketam mangroves support well over 2000 of these floating net cages.

The non-renewable conversion of mangrove swamps for aquaculture is within the limit of a total of 20 per cent of existing mangroves in a given district mentioned in national guidelines on the use of mangrove ecosystems. In 1990 a total of 1552 hectares of brackish water ponds were converted from mangrove swamps in Peninsular Malaysia for the culture of prawns and fish, and fattening of crabs. Other examples of uses which deplete the mangrove habitat are shown in the photographs below. In contrast, many mangrove forests in Malaysia are wildlife reserves, as indicated on the map. Such areas conserve the natural mangrove habitats, and so contribute to the conservation of a number of threatened species such as the salt-water crocodile, milky stork and proboscis monkey.

HUMAN USE OF MANGROVES

Sustainable uses
The mangrove ecosystem has a utilitarian value based on both ‘goods’ (products) and ‘services’ of benefit to humans.

Products from mangrove trees include logs, fuelwood, charcoal, wood-chips, paper pulp, scaffold poles, piling and construction material, stakes for fish traps and fishing platforms, railway sleepers, wood for furniture making and carvings, material for roof thatching, bark for tannin, medicinal products, sugar, alcohol, acetic acid and dyes. Exploitation to obtain products for domestic use has occurred since the beginning of history. Modern exploitation has increased to industrial levels, and the application of proper forest management practices based on cutting cycles and specially designed tree-extraction systems has been necessary in order to maintain a sustainable yield.

In addition to plant products, the mangrove ecosystem can provide a sustainable yield of fish and shellfish if it is not over-exploited.
Services include:

• stabilization and protection of shorelines;
• filtering and trapping of water-borne pollutants;
• provision of nursery and feeding grounds for numerous species of finfish and prawns, and habitat for crabs and molluscs;
• provision of nesting sites for sea and shore birds; and l provision of resources for tourism and recreation.

Non-sustainable uses
Non-sustainable uses lead to loss of the mangrove habitat, and associated losses of shoreline, organic matter production and species dependent on the habitat and mangrove-based food chains. Mangrove forests may be felled for uses such as aquaculture ponds, salt pans, agricultural use including rice fields, airport and road construction, port and industrial development, resettlement and village development. Canalization and changes in drainage associated with these uses modify the natural water supply and may be detrimental to remaining mangroves.

From the viewpoint of oil spill response, some of the above activities are vulnerable to oil and need to be considered along with the mangrove habitats.
Aquaculture and the use of salt ponds might be particularly affected. Appropriate responses are also needed for port and harbour facilities and associated navigation channels. Aquaculture ponds for fish and crustaceans (prawns and shrimps) have traditionally been excavated in mangrove areas in some parts of the world, notably south-east Asia and South America.
During recent decades, increased aquaculture of crustaceans has been an economic success worldwide, representing, nevertheless, a major problem for mangrove conservation. This is because of the large scale of habitat conversion, with associated changes in the natural tidal flow patterns and the generation of acid conditions (by oxidation of the disturbed sulphide-rich soils) which eventually affect the larvae. These problems can lead to a vicious cycle of mangrove clearing and abandonment called ‘shifting aquaculture’.
Brine evaporation ponds required for salt crystallization are developed mainly by clearcutting mangroves on arid or semi-arid coasts, and significant areas have been destroyed in this way.

In very dry regions, solar evaporation is sufficient to cause crystallization, but under more humid climatic conditions the brine has to be boiled over fires using mostly mangrove wood as fuel, which increases the pressure on the local mangrove resource.
The establishment and expansion of deep-water ports and associated waterways has led to direct loss of mangroves, and erosion. In particular, port facilities located upstream on tropical rivers can have a strong influence on downstream mangrove areas through the permanent dredging activities (and associated dumping of dredge spoil) required to maintain the navigation channels. Dredge spoil may physically smother mangroves, and there may also be an increase of acidity due to oxidation of the sulphide-rich sediment.

ECOLOGY OF MANGROVE - The term mangrove refers to salt-tolerant species

The term mangrove refers to salt-tolerant species of tree or shrub which grow on sheltered shores and in estuaries in the tropics and some sub-tropical regions.
There are about 60 species which occur exclusively in this habitat, and many non-exclusive species. Mangroves are outstandingly adapted to growing in sea water, which they desalinate by an ultrafiltration process. Mangrove roots typically grow in anaerobic sediment and receive oxygen through aerating tissue which communicates to the air through small pores (lenticels) on the aerial roots and trunks.

Mangroves may occur as narrow fringes on steeper shores and river banks, or as extensive forests on flat delta-land. Within any area of mangroves there may be zones or mosaics of different biological communities, depending upon many factors. These include height of sediment surface relative to tidal water movements, and salinity and nutrient supply (which in turn are influenced by freshwater inputs from the catchment area).

Mangrove forests in optimum conditions are one of the most productive ecosystems; for example a net primary productivity of 23.3 tonnes/ha/year and litter productivity of 10 tonnes/ha/year was measured for a 15-year-old stand of Rhizophora at Matang, Malaysia. The litter (such as fallen mangrove leaves) is broken down by bacteria, fungi and herbivores, and the resulting detritus supports food webs including large populations of invertebrates and fish.

The calm waters in the forests are ideal breeding and nursery grounds for young fish and shrimps, while the aerial roots, lower trunks and mud surface usually support a varied fauna of oysters, snails, barnacles, crabs and other invertebrates. The upper part of the mangrove trees is an essentially terrestrial environment with a fauna of birds, mammals and insects. Mangroves are affected by the freshwater and nutrient supply which they receive from their catchment area, and on the other hand have a strong influence on the adjoining coastal waters and associated ecosystems such as coral reefs, seagrass beds and tidal marshes. For example, they trap and stabilize sediment which might otherwise limit the growth of corals.