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A method for monitoring mangrove harvesting at the Mngazana estuary, South Africa
Abstract
The Mngazana estuary supports the third
largest area of mangroves, and probably the largest stand of Rhizophora
mucronata Lamk., in South Africa. The objective of this study was to
determine the extent of harvesting in the Mngazana mangrove forest, using
available aerial photographs and ground surveys. At 19 sites where harvesting
was evident the number of juveniles, adults and stumps were counted in three
replicate 25m2 quadrats.
GIS was used to generate maps, which
indicated that approximately 80% of the mangrove forest showed signs of medium
to high harvesting intensity, and that harvesting was taking place in easily
accessible areas, especially where Rhizophora mucronata was the dominant
species. The ESRI ArcMap density function was used to illustrate the number of
harvested stumps and adults within a specific area. GIS analyses classified the
mangrove area according to three harvesting intensity classes: low intensity
with a ratio of adults to stumps of 2:1, medium intensity 1:1 and high
intensity 1:2. According to the classes, 21% of the mangrove area was classified
as harvested at low intensity, 35.5% was medium intensity and 43.5% was high
intensity. The results indicate that areas dominated by Rhizophora mucronata
showed a high intensity of harvesting, while areas where Avicennia marina
was dominant showed a low intensity of harvesting.
The information from this study can
be used to make recommendations for the conservation and management of
mangroves in the estuary. In a management plan, access to areas where
harvesting is concentrated should be controlled so as to minimise impacts and
to allow regeneration to take place.
African Journal of Aquatic Science
2004, 29(1): 57–65
largest area of mangroves, and probably the largest stand of Rhizophora
mucronata Lamk., in South Africa. The objective of this study was to
determine the extent of harvesting in the Mngazana mangrove forest, using
available aerial photographs and ground surveys. At 19 sites where harvesting
was evident the number of juveniles, adults and stumps were counted in three
replicate 25m2 quadrats.
GIS was used to generate maps, which
indicated that approximately 80% of the mangrove forest showed signs of medium
to high harvesting intensity, and that harvesting was taking place in easily
accessible areas, especially where Rhizophora mucronata was the dominant
species. The ESRI ArcMap density function was used to illustrate the number of
harvested stumps and adults within a specific area. GIS analyses classified the
mangrove area according to three harvesting intensity classes: low intensity
with a ratio of adults to stumps of 2:1, medium intensity 1:1 and high
intensity 1:2. According to the classes, 21% of the mangrove area was classified
as harvested at low intensity, 35.5% was medium intensity and 43.5% was high
intensity. The results indicate that areas dominated by Rhizophora mucronata
showed a high intensity of harvesting, while areas where Avicennia marina
was dominant showed a low intensity of harvesting.
The information from this study can
be used to make recommendations for the conservation and management of
mangroves in the estuary. In a management plan, access to areas where
harvesting is concentrated should be controlled so as to minimise impacts and
to allow regeneration to take place.
African Journal of Aquatic Science
2004, 29(1): 57–65
