Human-induced stresses on mangrove swamps along the Kenyan coast

Author

P. A. W. Abuodha & J. G. Kairo

Abstract

Mangroves form important ecosystems in Kenya’s coastal areas. They produce goods and services that are of environmental, ecological and economic importance to human society. However, mangroves are under continuing pressure from anthropogenic disturbances. A particular concern has been the clearing of mangrove areas to reclaim land for other uses such as aquaculture, salt manufacture, agriculture and housing. About 10 000 ha of mangrove areas have been cleared for salt manufacture between Ngomeni and Karawa, while in Lamu, close to 100 ha of mangrove forest was killed by dredged-up sediment that was deposited during the construction of the Mokowe sea jet. 100 ha of mangrove area have been converted for aquaculture at Ngomeni. At Gazi Bay, about 100 ha of mangrove forests was [sic] cleared for fuelwood and in Makupa Creek, Mombasa, 10 ha of mangroves died due to oil pollution. The total area lost is therefore 10 310 ha which represents about 20% of the total mangrove forest.

In this paper, deforestation, conversion of mangrove areas for other land uses and pollution of mangrove swamps on the Kenyan coast are discussed and a call for sustainable use, and the government policies that will enable this, is made.

Main Results and Conclusions

• Many general ecosystem goods and services are listed for mangroves (cited from other studies).
• Kenyan mangrove ecosystem goods/services and threats:
  • Historically, the mangroves near Gazi and Ngomeni, Kenya provided the important ecosystem service of erosion mitigation: “At Gazi, substantial erosion of the coastline commenced immediately after the clear felling for fuelwood of Sonneratia in front of a coconut plantation in 1977 (Fig. 2)…Clear-cutting of mangroves causes hydrodynamic changes in inshore circulation that tend to increase erosion of shorelines. Also at Ngomeni, much erosion has occurred after mangrove areas were converted to shrimp ponds. Ten meters of the coastline have been lost since 1970 giving a rate of loss of the coastline as 0.5 per year” (258).
  • Kenyan mangrove forests were originally used for wood poles, charcoal and other products, but these goods resulted in extensive mangrove deforestation: “Mangrove areas are destroyed because certain tree species (especially Rhizophora, Heritiera, Bruguiera and Ceriops) have strong, attractive and durable woods for firewood, charcoal, poles for boats and housing, tannin for leather among others. It is these species, which should be considered seriously in the context of resource management… The mangroves have so much been exploited that in some areas they are threatened with extinction” (259).
  • Methods of deforestation became so damaging that the government stepped in to prevent further mangrove loss: “In 1975, the Government of Kenya imposed a ban on the use of mangrove pole for charcoal production” (258-259).
  • Other areas of mangroves have been cleared for forms of coastal development such as: “agriculture, aquaculture, housing and transportation networks…[and] salt production” (259). Additionally, “In Lamu, close to 100 ha of mangrove forest was killed by dredged up sediment that was deposited during the construction of Mokowe sea jet”(260).
  • Furthermore, oil pollution from five tanker accidents between the years of 1983-1993 affected 10 ha of mangroves.  Observations of the affected area are as follows: “…ten years after these spills, the effects of oiling are still visible both as a reduction of the area of substrate in the mangrove fringe and reductions of the cover of attached animals. The mangroves have not yet recovered”(260).
• A mangrove rehabilitation project at Gazi Bay was enacted in 1994. The best mangrove survival rates were seen where replanted mangroves experienced the greatest amount of protection from wave action (261).
• Mangrove coverage in 2003: “The Kenyan coast now has less than 50,000 ha of remaining mangrove forests and the extant forests need to be managed carefully and sustainably“ (261).
• Three recommendations:
  • Public participation: “The active participation of these people”—coastal populations—“is required for effective implementation”(262).
  • Groundwater protection: “Noting that the Kenya coastline is semi-arid, and that the major mangrove forest cover depends on discharges of groundwater, mangroves are dependent upon the sensitive management of groundwater aquifers and must be considered in water management decisions”(262).
  • Sustainable mangrove harvest: “Some extensive, fast growing mangrove species e.g., S. alba, A. marina, X. granatum and R. mucronata can be planted for fuel wood or timber as well as supplying fish and wildlife to nearby human population. For slower growing species e.g., C. tagal and H. littoralis exploitation must be slower”(263).