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8. The distribution and socio-economic aspects of mangrove forests in Tanzania

J. R. Mainoya, S. Mesaki, and F. F. Banyikwa

Mangrove vegetation is characteristic of sheltered coastlines in the tropics. Mangrove communities are extensive in protected shallow bays and estuaries, around lagoons, and on the leeward side of peninsulas and islands. In Tanzania mangrove forests occur on the sheltered shores of deltas, alongside river estuaries, and in creeks where there is an abundance of fine-grained sediment (silt and clay) in the upper part of the inter-tidal zone.

FIG. 1. Mangrove forests along the coast of Tanzania

The establishment of mangrove vegetation is governed to some extent by the degree of exposure to strong winds. The largest continuous mangrove areas are to be found on the coasts of Tanga district in the north, the delta of the Rufiji River in Kilwa and Lindi districts, and in Mtwara, where the Ruvuma River forms an estuary close to the Mozambique border (fig. 1). Thus, the mangrove forests stretch along coastal districts from Tanga to Mtwara and cover an area of 79,937 ha. Mangroves are also well represented on the coasts of the main islands, Zanzibar, Pemba, and Mafia. On Pemba mangroves cover an area of 12,146 ha, while on Zanzibar there are 6,073 ha under mangroves.

Flora of the Mangroves of Tanzania

Studies of the mangrove vegetation of the East African coast include those of Graham (1931) along the Kenya coast, and Walter and Steiner (1936) along the coastal district of Tanga in Tanzania. MacNae (1963) reviewed the literature on mangroves of eastern Africa and included a description of the various plant communities and their zonation. Graham (1931) described the species present in mangrove swamps in Kenya, and commented on their ecology and economic utilization.

McCusker (1971), working on the mangrove vegetation of the Kunduchi area near Dar es Salaam, recorded six principal mangrove species: Sonneratia alba Sm. (Sonneratiaceae), Rhizophora mucronata Lam. (Rhizophoraceae), Ceriops tagal (Perr.) C. B. Robinson (Rhizophoraceae), Bruguiera gymnorrhiza (L.) Lam. (Rhizophoraceae), Avicennia marina (Forsk.) Vierh. (Avicenniaceae), Xylocarpus granatum Koen. (Meliaceae). In addition, Heritiera littoralis Dryand (Sterculiaceae), an "associated species," has been reported from estuarine mangrove swamps in East Africa by MacNae (1968). Other "associated species" frequently encountered in Tanzania are Lumnitizera racemosa Willd. (Combretaceae), Arthrocnemum indicum Moq., Salicornia pachystachya (Bunge ex Ungernsternb), Sesuvium portulacastrum Linn., and Sueeda monoica (Forsk. ex J. F. Gmel).

The composition of mangrove vegetation has been influenced by a number of factors. For example MacNae (1963) noted that the presence of sand appears to restrict the growth of certain species, notably Bruguiera, Rhizophora, and Ceriops, and that Bruguiera gymnorrhiza predominates under conditions of fresh-water influence. MacNae and Kalk (1962) also noted that Ceriops and Bruguiera seedlings develop only in the shade of other trees. Though Rhizophora can germinate and grow anywhere in the upper intertidal zone, it will grow to maturity only in waterlogged areas. Moreover, Sonneratia alba is most commonly found in loose muddy sand, and Xylocarpus granatum is virtually confined to sandy soils with a low humus content (McCusker 1971).

It has been observed that the usual pioneer species in sandy habitats is Avicennia marina, and in muddy areas Sonneratia alba. Because of its high tolerance of dry conditions, A. marina has a wider distribution than S. alba, which is mostly found in silty waterlogged sites. Heritiera is much less tolerant of strongly brackish water conditions and is therefore found mostly around the upper limit of sea water in river estuaries.

Walter and Steiner (1936) described the zonation of mangroves in the Tanga region and observed that Bruguiera gymnorrhiza does not form a distinct zone but occurs interspersed with Rhizophora and Ceriops. They concluded that the occurrence of B. gymnorrhiza is not directly related to distance from the outer and inner limits of the mangrove swamp but to depth and salinity of the water and the texture of the substrate. The composition of the mangrove community is largely a function of tolerance to salinity of the water and waterlogging of the substrate (Chapman 1970). However, such factors as clearing, changes in amount and regime of rainfall, and changes in the pattern of sand bars at the mouth of a creek which limit the rise and fall of the tide in the mangroves can all lead to changes in the species composition of the vegetation. For instance McCusker (1971) noted that the clearing of mangroves in Kunduchi Creek resulted in an increase of Ceriops saga/ at the expense of other species in the regenerating plant community.

Socio-economic Aspects of Mangroves

Overview of Commercial Exploitation

Mangrove poles and bark have been exported from the eastern coast of Africa for a very long time. Trade between the Persian Gulf and East African ports dates back to a time when the Arabs secured a footing on the coast of East Africa. For centuries, chows from the Gulf visited Lamu, Mombasa, Zanzibar, and the Rufiji delta during the north-east monsoon (kaskazi), bringing with them dates, carpets, salt, and earthenware and taking back mangrove poles (boriti) and firewood on their return journey during the southwest monsoon (kusi).

In this study of the Zanzibar Empire (17701873), Sherriff (1971) suggested that it was probably such commodities as grain, mangrove poles, and charcoal that initially attracted and sustained the attention of Arabs to East Africa. The importance of mangrove poles as a building material in Arabia during the beginning of this century was well documented by Grant (1938), who showed that as late as the nineteenth century, the Sultan of Zanzibar retained a user right in the Rufiji delta, whence he derived free of charge a large number of poles and other building materials, even though the mangroves were then under German control.

Mangrove bark has been exported for tannin extraction. Buckley (1929) claimed that East African tannin extract was more plentiful than that from Malaya. In the 1930s the potential export income from mangrove ecosystems was considered to be of sufficient economic importance for the government of Tanganyika to experiment with mangrove silviculture (Grant 1938). Between 1923 and 1958 the mangrovepole trade was in the hands of private entrepreneurs, who employed local labour and overseers to exploit the mangroves of the Rutiji delta. As time went on, the situation proved unsatisfactory because the Forest Department was not receiving the full royalties due, and the local labourers were complaining of poor wages. Therefore, in 1958 two local co-operative societies, the Kishoka and the Rufiji Delta Co-operative, were formed to exploit the entire Rufiji delta mangrove area jointly. These co-operatives contracted out most of the cutting of mangroves but did not have much control over the marketing of mangrove poles, which mainly benefited the overseers. In an effort to overcome this problem, the co-operatives were reorganized in 1 960 to consist only of bona fide local mangrove cutters, and three new co-operative societies were formed: the Amini Cooperative Society, the Mbwera Co-operative Society, and the Kiasi Co-operative Society.

This trade (see tables 1 and 2) flourished up to 1960, when a decline set in, probably due to the economic changes that followed the discovery and large-scale exploitation of oil in the Arab Gulf countries.

In 1965 the pole trade went into the hands of the Coast Region Co-operative Union, which handled the trade on behalf of the three co-operative societies, but in 1976 all cooperatives were banned by the government and the mangrove trade was handed over to a para-statal body known as the Tanzania Timber Marketing Company, which still handles all the trade in mangrove poles with foreign countries. In 1979 alone, the company earned 4 million Tanzanian shillings in foreign exchange after selling 30,000 scores of mangrove poles (1 score = 20 poles), two-thirds of which were destined for Iran (Harnevik 1980).

Commercially Exploited Mangrove Species

There are seven species of mangrove of commercial importance to the local people:

1. Rhizophora mucronata, the most useful and abundant species of mangrove, is found as almost pure stands, or with scattered trees of Bruguiera and Ceriops, in muddy tidal areas. Under local conditions, it normally attains a height of about 8 m. It can be easily distinguished from other tree species by its aerial, bowed, stilt roots, many of which arise from quite high on the trunk and branches, and by its viviparous seeds. The bark is more fibrous, thinner, and rougher than that of Ceriops, and the tannin content is reported to be in the range of 24.8 to 43 per cent. The wood when seasoned becomes very hard and durable. The poles are used locally as telephone poles, and the wood is used as fuel.

2. Ceriops tagal is a shrub or small tree of variable size, growing to a height of 4 m. It is found
interspersed with Rhizophora and Bruguiera, and may be recognized by the angular character of its long viviparous radicle. Economically this species used to be the most important source of non-fibrous mangrove bark (average 40 per cent tannin content). The poles and wood are used locally for building houses and for firewood. Grain-sifting buckets (nyungo) are also made from the split stems of this species. The sap from the seeds is used for water-proofing fishing lines.

3. Bruguiera gymnorrhiza is less common than Rhizophora mucronata but is found interspersed with it. Under suitable conditions it will grow to a height of 20 m. The wood is used as fuel for lime burning and sometimes for building poles.
4. Sonneratia alba, a small, common tree, is one of the first invaders of mud flats subject to tidal inundation by salt water but does not extend to the higher levels. The wood is used as fuel for lime burning and occasionally for building poles, and its vertical shoots are used as floats for fishing nets. It is not an economic source of bark tannin.

5. Heritiera littoralis is a tall tree with slender, pointed germinating rootless that attain a length of at least 30 cm and become anchored in the mud. It is used locally as a source of fuel and building materials: poles and rafters. The bark is of little economic value for the extraction of tannin.

6. Avicennia marina is a common spreading tree, usually found on the higher levels of swamps. It is willow-like in general appearance and has a light yellowish green foliage. Vertically pointed pneumatophores arise in great abundance from the long, spreading, horizontal roots. The bark is smooth and greenish yellow when young, and variegated green and reddish in the older trees. The trunks are used for making small dug-out canoes, and the tree is generally used for building carts, for chow and canoe fittings and masts, for furniture such as bedsteads and chairs, and for fittings such as handles. It is also used extensively as fuel for lime burning. The tree is of no use as a source of tannin.

7. Xylocarpus granatum is a small tree, growing up to 6 m in height, that is scattered through the mangroves, especially in the higher parts. The large, round woody fruits, 15-20 cm in diameter, are of medicinal value. The wood is very hard and said to be like teak. It is used for carts, chows, furniture, and construction. Although the bark contains up to 32 per cent tannin, the tree is considered a poor producer of tannin.

TABLE 1. Number and estimated value of mangrove poles used locally and exported from Tanzania, 1950-1967

Source: Forest Department Annual Reports. a. Not recorded after 1959.

TABLE 2. Mangrove bark exported from Tanzania, 1923 - 1967

Source: Forest Department Annual Reports.

a. Value not recorded before 1950.

TABLE 3. Local uses of primary species of mangrove found in Tanzania

Source: Temu 1975.

Exploitation of Mangrove Products by the Local Community

In addition to the well-known export trade in poles, mangrove forest products are put to many uses locally (table 3).

Poles (boriti). Straight, round, barked poses between 8 and 16 cm in basal diameter and between 5 and 7 m long, cut mainly from Rhizophora, Ceriops, and Bruguiera species, are widely used in constructing houses. The mangrove wood, being tough and termite-resistant, is preferred for this purpose by coastal people. It is estimated that over 600,000 poles are used annually to build and repair ordinary Swahili houses in Dar es Salaam (fig. 2).

Mangrove bark. Tannin extracted from mangrove bark is used locally by leather-processing companies since the importation of tanning and dyeing agents has been restricted. In recent decades the question of establishing a tanninextraction factory to process mangrove bark locally has been discussed, but, although a mangrove-bark factory based on the Rufiji mangroves would be practicable and viable, the government has not yet made a decision.

Charcoal. Tanga, Dar es Salaam, Kilwa, and Zanzibar provide major markets for charcoal. Mangrove wood can be burned to produce excellent charcoal of high calorific value, but difficulties of transport to the centres of charcoal consumption have reduced the profitability and attractiveness of large-scale charcoal production from mangrove forests.

Fuelwood. Nearly all rural people as well as the urban poor in Tanzania depend almost exclusively on fuelwood, charcoal, and kerosene as energy sources for cooking and lighting (Nkonoki 1981). People in coastal villages use mangrove wood for firewood in their homes {fig. 3). Mangrove wood can also be used as fuel in coastal-village industries for the production of burnt bricks and tiles and in lime burning where alternative sources of fuelwood are not available.

Timber. According to Grant (1938), the best mangrove timber is Heritiera (msikundazi), which is the only species commonly found in large dimensions. But the use of large mangrove trees for timber has not been explored in Tanzania because timber from inland tree species is readily available.

Other uses. The coastal people of Tanzania use mangrove products for many other purposes as well. These include boat hulls, masts, and oars, fences for pig pens, and fish traps. In addition, they use Avicennia foliage as fodder for goats and cattle and Rihizophora leaf extract as a medicine for hernias.

The Importance of Mangrove Areas to Fisheries

Mangrove swamps support a resident fauna dominated by molluscs and crustaceans (Macintosh 1983). The fauna of Tanzanian mangroves has not been studied comprehensively, but it is known that the mangrove waterways and the areas submerged at high tide support important fish populations. The mangroves, especially around the Rufiji delta, may be seen as serving two fishery-related roles: as a habitat and as a nursery ground for many species of shellfish and fin fish that can be exploited commercially.

"Shrimp" of the mangrove and associated saline waters (Acetes sp. and Macrobrachium spp.) are not extensively exploited commercially. However, many women and children are involved in catching Acetes, which may be important to the rural economy.

Estimates of the total prawn potential of Tanzania, based on a survey by Japanese team (Marcotrade 1978) showed that up 2,000 tons of prawns could be harvested each year. This figure is probably an underestimate because the mangrove habitats north of Bagamoyo (Ruvu estuary), south of the Rufiji delta, and on Zanzibar and Pemba were not taken into consideration.

Sankarankutty (1974) identified six penaeid species at Kunduchi Creek: Penaeus indicus (Milne-Edwards), P. monodon (Fabricius), P. semisulcatus (De Haan), P. japonicus (Bate), P. latisulcatus {Kishinouye), and Metapenaeus monoceros (Fabricius). The commercially important species are P. indicus, P. monodon, P. semisulcatus, and M. monoceros.

In his study of the crabs of Tanzania, Hartnoll (1975) mentioned the crabs of the mangroves around Dar es Salaam. The common mangrove crab Scylla serrate is little caught in Tanzania, the estimated total annual catch being less than 100 kg (Sichone, personal communication). This small catch is from the mangrove stands close to the urban consumer centres of Dar es Salaam, Tanga, and Mtwara. There is, however, a great potential for crab fishery in the large mangrove stands of the Rufiji and Ruvu estuaries. For example, the Rufiji delta alone is thought to have a fishery potential of about 20 tons of live crabs per month (Marcotrade 1978).

The common mangrove molluscs of Terebralia spp. and Cerithedea spp. are not utilized at present in Tanzania, although MacNae (1974) reported that they form an important food source in some countries.

The Need for Defined Mangrove Conservation Strategies in Tanzania

Human activities are by far the most important factor influencing mangroves negatively in Tanzania today. One such activity is the clearing of land in and around mangrove forests to create salt evaporation pans (fig. 4). Tanzania produced 60,000 tons of salt from coastal salt pans in 1981.

Although saltworks have been in production for several decades around Kunduchi, north of Dar es Salaam, significant reduction of the mangrove vegetation in this area was not noticed until 1970 (McCusker 1971). By the end of 1981, however, there had been a large expansion of the saltworks, which reduced the once large Kunduchi Creek mangrove forest to a mere vestige (fig. 5). If this trend continues unchecked, the increasing demand for table salt in Tanzania is bound to produce a corresponding decline in mangrove habitats (Mwaiseje and Mainoya, in press).

Conservation of the mangrove habitats and resources they contain is very important. The island of Mafia and the Rufiji delta mangrove forest have the potential to be a very rich marine park area. The Rufiji delta supports dugongs and crocodiles, both of which are threatened species, and some islands around Mafia support the green turtle (Robertson 1968). In the Jozani forest on Zanzibar, the red colobus monkey, Colobus badius, has been observed to feed on the buds and young leaves of the mangrove Rhizophora mucronata (Mturi, unpublished observations 1 983).

Tanzania has an excellent reputation for wildlife conservation on land; it would be very desirable if the government took a greater interest in mangrove forest conservation. It has been shown that unplanned cutting of mangrove forests often leads to coastal erosion and sediment mobilization, and this adversely affects marine life dependent on the nutrients associated with the mangrove ecosystem. The destruction of the mangroves may reduce not only coastal fishery resources but also catches from off-shore fishing, far from the mangrove forest itself (Macintosh 1983).

Mangroves protect tropical shores from erosion by tides and currents, and Macintosh (1983) recommended that a mangrove strip at least 100 m wide should be left as a buffer zone on the more exposed shores. Although mangrove areas could also become important for aquaculture, it is fortunate that mangrove areas are not utilized for this purpose in Tanzania.

If silviculture is to be practiced in mangrove areas, greater consideration should be given to those mangrove species that have been over-exploited for poles, firewood, and charcoal - Rhizophora mucronata, Ceriops saga/, and Bruguiera gymnorrhiza (Grant 1938). Furthermore, because regeneration of mangroves is a slow process, thinning of mangrove stands could be carried out where necessary to reduce tree density and promote growth of the remaining trees.

The rapid growth of the human population in Tanzania is likely to increase the negative human impact on mangroves. The important role of mangroves in the preservation of coastal and offshore fish and shellfish stocks and the shore-protecting properties of mangrove vegetation make the preservation and sound management of mangroveclad habitats an urgent matter for developing coastal states like Tanzania.

FIG. 5. Vegetation and land use in the vicinity of Kunduchi Creek, 1963 (left) and 1981 (right)

Conclusions and Recommendations

1. Mangrove communities in Tanzania have not been studied in sufficient detail to plan their rational exploitation. There is an urgent need for further studies of these ecosystems.

2. Few integrated multidisciplinary studies of mangrove forest use have been made. Most studies have been of a reconnaissance nature, generally providing scanty information that cannot be used in land-use management plans. It is recommended that a more ambitious study of the mangrove ecosystems of the Tanzanian coast be carried out, with a view to providing a data base from which the government and other land-use planners can design their land-use programmes.

3. Because the trade in mangrove poles is very lucrative both for the internal market and for export, it is recommended that a plan for monitoring mangrove pole harvesting and utilization should be worked out. A strategy for growing the economically more important mangrove species could be investigated.

4. A body drawing members from the Ministry of Lands, Natural Resources, and Tourism, the Ministry of Housing and Urban Development, and the State Mining Corporation should be formed to monitor the rational utilization of mangrove areas as a natural resource.

5. The Tanzanian government should ensure that adequate records are kept with regard to the utilization and marketing of mangrove products.

Acknowledgements

The authors would like to thank the United Nations University for its financial support and keen interest in this project. The authors wish also to thank Dr. A. K. Semesi, Mrs. V. Kainamula, and Mr. L. B. Mwasumbi for their assistance and Mrs. Z. Juma for typing the manuscript.

References

Buckley, T. A. 1929. "Mangrove bark as a tanning material." Malay Forest Record, no. 7.

Chapman, V. J. 1970. "Mangrove phytosociology." Tropical Ecology, 11: 1-19.
- 1976. Mangrove vegetation. J. Cramer, Vaduz (Lubrecht and Cramer, Monticello, N.Y., USA).

Graham, R. M. 1931. "Notes on the mangrove swamps of Kenya." Journal of the East Africa and Uganda Natural History-Society, 36 (1929): 157-164.

Grant, D. K. S. 1938. "Mangrove woods of Tanganyika Territory: Their silviculture and dependent industries." Tanganyika Notes and Records, 5: 5-16.

Harnevik, K. J. 1980. Non-agricultural activities, Rufiji District: Survey, analysis and recommendations. BRALUP Service Paper 8012. University of Dar es Salaam.

Hartnoll, R. G. 1975. "The Grapsidae and Ocypodidae (Decapoda: Brachyura) at Tanzania." Journal of Zoology (London), 177: 305 328.

McCusker, A. 1971. "Ecological studies of an area of mangrove vegetation in Tanzania." Ph.D. thesis, University of Dar es Salaam.

Macintosh, D. J. 1983. "Wetlands of the world: Riches lie in the tropical swamps." Geographical Magazine, 60 (4): 184 188.

MacNae, W. 1963. "Mangrove swamps in South Africa." Journal of Ecology, 51: 1-25.
- 1968. "A general account of the fauna and flora of mangrove swamps and forest in the Indo West Pacific region." Advances in Marine Biology, 63: 73 270.
- 1974. Mangrove forests and fisheries. FAO/UNDP, Rome. (IOFC/DFVE/74/34)

MacNae, W., and M. Kalk. 1962. "The ecology of the mangrove swamps at Inhaca Island, Mozambique." Journal of Ecology, 50: 19 34.

Marcotrade. 1978. "Report on a biological study of crustaceans in the Kilwa and Rufiji areas." Marcotrade Tanzania fishing venture report (unpublished).

Mwaiseje, B., and J. R. Mainoya. In press. ''Mangrove habitats: Problems of conservation in Tanzania," In Proceedings of the International Society for Tropical Ecology (Silver Jubilee Symposium, Bhopal, India, 1981).

Nkonoki, S. R. 1981. The poor man's energy crisis. A report of the Tanzania Rural Energy Consumption Survey, 1981.

Robertson, l. B. J. 1968. "Marine parks and reserves." Report for the Ministry of Agriculture and Co-operatives, Dar es Salaam.

Sankarankutty, C. 1974. "Coastal and estuarine aquaculture: A case for introducing prawn culture in Tanzania." In Msangi and J. J. Griffin, eds., International Conference on Marine Resources Development in Eastern Africa, pp. 26-28. University of Dar es Salaam.

Sheriff, A. M. H. 1971. "The rise of a commercial empire: An aspect of the economic history of Zanzibar (1770 1879).'' Ph.D. thesis, University of London.

Temu, E. M. 1975. "Brief notes on the mangroves of Tanzania." In file no. FDI37117, Ministry of Natural Resources and Tourism, Dar es Salaam.

Walter, H., and M. Steiner. 1936. "Die Ökologie der östafrikanischen Mangroven." Zeitschrift für Botanik, 30: 65 191.

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