Mangrove Ecosystems of Southwest Madagascar:
An Ecological, Human Impact, and Subsistence Value Assessment
by Radhika Dave, MESc 2006
Introduction ly in southwest Madagascar, which is home to
the nomadic Vezo ﬁshing communities.
The importance of mangrove forests in My summer research focused on the man-
maintaining crucial ecosystem functions such grove ecosystems of Baie des Assassins, a small
as nutrient ﬁltering, supporting coral reef ﬁsh- bay harboring about 25 km2 of mangrove forest,
eries, and providing storm buffers has become approximately 185 km north of the port city of
more evident after the tsunami that ravaged Toliara. I aimed to explore the dynamics of the
parts of Asia in December 2004 (Alongi 2002; forest structure, to obtain a measure of the
Mumby et al. 2003; UNEP-WCMC 2006). human impact on the forest, and to understand
However, these vital tropical ecosystems in the the subsistence value of mangroves to the local
coastal inter-tidal zones, covering about community. The indigenous Vezo populations
181,000 km2 (Spalding et al. 1997), continue that reside in ﬁshing villages around and north
to be under immense threat from a variety of of the Baie des Assassins use these mangroves for
human actions. Over the last twenty years, timber, fuel wood, and ﬁshing. However, the
approximately 35% of the world’s mangrove intensity and periodicity of use and level of
forest area has been lost (Valiela et al. 2001). dependence on the mangrove forest is undocu-
Nevertheless, activities that contribute to this mented. The region around the Baie des
depletion continue. These activities include Assassins may potentially form part of a multi-
timber and fuel wood extraction, urban devel- ple-use conservation site, thus necessitating a
opment, and the expansion of shrimp aquacul- clear understanding of the forest structure and
ture, which is by far the greatest cause of man- its signiﬁcance to local communities.
grove loss (Valiela et al. 2001).
Mangrove ecosystems in many countries Background
face a combination of these pressures. This is
true for the island nation of Madagascar as well. Nearly 98% of Madagascar’s mangroves lie
Along with some of the most remote and biodi- along the west coast of the island facing the
verse coral reefs, 3,270 km2 of mangrove forest Mozambique Channel (Roger and Andrianasolo
lie within Madagascar, supporting numerous 2003). The mangroves here are part of the Indo-
coastal communities and the nation’s economy Paciﬁc domain. But like other African man-
(Cooke et al. 2003). There is limited documen- groves, Madagascar’s mangroves exhibit less ﬂo-
tation of these forests in Madagascar, particular- ral and faunal diversity than those of South East
Asia (Roger and Andrianasolo 2003; Gaudian et
Radhika Dave is originially from India and has
spent the better part of the last decade studying al. 1995). A total of eleven species of mangrove
and working in the U.S. She obtained her BS in trees are known in Africa, out of which eight
Biology and Environmental Sciences at the College species have been recorded in Madagascar.
of William & Mary and worked with Conservation Baie des Assassins lies at 22°11'S and
International in D.C. before coming to F&ES. In the 43°15'E within a region that is important for its
future, she hopes to continue with ecological
coral reefs and mangroves. About half of the 50
research and working to integrate conservation
within development. km2 of mangroves in the region lie within the
Tropical Resources Bulletin 7
Baie des Assassins (Cooke et al. 2003; Gaudian To evaluate the direct use value of this man-
et al. 1995). Mangroves on the west coast are grove forest in the daily lives of Lamboara resi-
important breeding grounds for several com- dents, I conducted semi-structured interviews
mercial ﬁsh species such as mullet (Mugilidae), with 30 households in the village, accounting for
sickle fish (Drepanidae), and pony fish about one-third of the population, with the aid
(Leiognathidae) (Cooke et al. 2003). These of a translator who was also my research assis-
mangroves also provide local communities with tant. I asked questions to assess the level of
timber for construction and fuel. dependence and the variety of human services
Madagascar is currently in the third phase provided by the mangroves. When available,
of its National Environmental Action Plan, a information on the source site of mangrove
program that prioritizes marine and coastal wood and other products was also recorded. I
ecosystem management. The government aims collected data from the end of June through
to expand the nation’s terrestrial protected area mid-July, which coincided with the seasonal
coverage from 15,000 km2 to 50,000 km2 and northern migration of many ﬁshermen in search
its coastal and marine protected area coverage of better ﬁshing. Hence, most of the respondents
from 2,000 km2 to 10,000 km2 within a period for my social survey were women and older men,
of ﬁve years (Conservation International 2003). essentially, those who remained in the villages.
It has taken a series of steps in consultation with To assess species composition and level of
various organizations to meet its aim of tripling human impact in the mangrove forests, I select-
Madagascar’s protected area network and several ed four sites to be sampled ranging in distance
new Sites des Conservation, or conservation sites, from the village of Lamboara (Figure 1). The
are in the planning stages. Baie des Assassins ﬁrst site (Site A), is located adjacent to the village
and its surrounding region may be included in on the same land mass. Site B is a small island
one such site; hence, it is important at this stage across the water channel from the village. Sites C
to inform the ofﬁcial management and planning and D are on the fringes of the mainland of
process with the relevant ecological and socio- Madagascar, also across the water channel from
economic data on mangrove use values and the village, but not connected to Site B. Site A is
threats. While shrimp aquaculture increasingly the closest to the village followed by Site B, Site
is threatening mangrove habitats in the north- D, and ﬁnally, Site C. Within each site, three 10
west part of the country and tourism has nega- meter by 10 meter plots were laid out to record
tively impacted the mangroves of Toliara, the the mangrove species composition, to collect
Baie des Assassins mangroves are yet to witness diameter (according to CARICOMP 2001) and
large-scale destruction brought on by these height measures for mature trees, and to record
external factors. the number of cut tree stumps as a measure of
human impact. Randomly placed subplots were
Methods and Data Analysis set within each plot to record the species type
and number of seedlings and saplings as a meas-
My study focuses on the peninsular region of ure of regeneration (CARICOMP 2001).
the Lamboara village and its surrounding man- The relative density of the ﬁve species
grove forests within the Baie des Assassins. This recorded was calculated using the formula
land mass, though accessible by foot from the worked out by Cintrón and Schaeffer-Novelli
mainland at low tide, is essentially an island at high (1984). Multivariate analysis of variance was
tide. The village is comprised of the Vezo ﬁshing conducted on square root transformed data for
community and has a population of approximate- height and diameter for mature trees—those
ly 600 people, of which 60% are children. with diameter greater than 2.5 cm at breast
8 Volume 25, Spring 2006
Mangrove Ecosystems of Southwest Madagascar
height—to test for differences in these variables viduals of Sonneratia were found, one each in
as a function of differing site locations. These Sites B and D. Site A, which is closest to the vil-
sites are presumably differentially impacted lage, is used mainly for collecting fuel wood
because of distance from village and stated use and occasionally pole wood,1 with Ceriops
by respondents. I analyzed the human impact exhibiting the highest relative density of 51%
index in the form of ratio of stumps to mature for mature trees.2 Ceriops showed a similar high
trees for variation between the four sites. I used relative density when compared to the other
diameter size class distribution of trees to detect four species in Sites B (51%) and C (72%), but
differences between sites in species rejuvenation. was relatively less abundant in Site D (21%).
Finally, I performed paired t-tests to detect sig- Among sites, Ceriops had the highest relative
niﬁcance in the difference between the mean dominance of 31% followed by Rhizophora at
abundances of mature trees and juveniles. 28.2%.3 Ceriops also showed the highest rela-
tive density across sites at 49.6%, while
Results and Discussion Rhizophora and Bruguiera had similar overall
relative density of 21.6% and 21.9%, respec-
Five species of mangrove trees were found tively. Based on the analyses performed on the
in the mangrove stands surrounding Lamboara. data for mature trees, the diameter and height
These are (Malagasy common names in paren- of mature trees does not vary signiﬁcantly at
thesis): Avicennia marina (haﬁhafy), Rhizophora sites with differing distances from the village.
mucronata (tangandahvy), Sonneratia alba Diameter size class distribution for all the veg-
(songery), Bruguiera gymnorrhiza (tangampoly) etation at each of the four sites showed the
and Ceriops tagal (tangambavy). However, only reverse-J shape characteristic of species with
Rhizophora, Ceriops, and Bruguiera were found good rejuvenation (Lykke 1998).
in plots across all four sites and only two indi- Regeneration, measured as the total num-
Figure 1. Location of Lamboara village in Madagascar
Tropical Resources Bulletin 9
ber of seedlings and saplings found at each site, tion. Site C, which is farthest from the village
and the number of mature individuals showed and is used for its relatively taller wood, had a
similar variance in abundance across the four highly variable human impact index in its plots.
sites. This was true for all the sites except B, One of its plots exhibited a 2:1 ratio of stumps
where the total number of overall seedlings and to mature trees (Figure 3).
saplings is markedly less for Ceriops tagal than its Houses in Lamboara are made from pole
adult abundance (Figure 2). A paired t-test con- wood that originates from the surrounding
ducted on the total number of mature individu- mangrove forests. In addition to using pole
als and total regeneration indicated that there wood and collecting dead wood for use as fuel,
was no signiﬁcant difference (at p value of 0.05) Lamboara residents also collect crabs in the
in means of mature and juvenile tree numbers (p mangroves during neap tides and shrimp when
of 0.094). The next step would be to perform in season. Occasional use of Avicennia marina to
another analysis of variance to determine any sig- treat stomach ailments and fevers was also
niﬁcant difference between species and groups observed. Shellﬁsh (Murex sp.) are harvested
(juveniles and adult trees) for each of the sites. from the mangroves for consumption. Their
It is also worth noting that consistently shells are burned to produce lime. When mixed
high stump abundance equal to the abundance with sand it is called sookay and is used as
of mature trees was found in Site B (Figure 3). cement in constructing the walls and ﬂoors of
Site B is a small island and experiences a mixed some houses or sold to other villages upon
use pattern of fuel wood and pole wood collec- request. A considerable amount of pole wood
Figure 2. Variation in abundances of mature trees and regeneration (total
number of seedlings and saplings) across sites
10 Volume 25, Spring 2006
Mangrove Ecosystems of Southwest Madagascar
Figure 3. Variation in abundances for mature trees and stumps across sites
collected from the mangroves is used to burn the The mangrove species preferentially har-
shells, make drying lattices, walls, fences, and vested for pole wood in the Lamboara region
torch lights for night ﬁshing of sea cucumbers. are Rhizophora, Ceriops, and Bruiguiera. This is
No charcoal production takes place in the Baie also a preference seen in other parts of Africa,
des Assassins region, and most people collect such as in Kenya’s Mida Creek region
dead branches for fuel wood. Women are gener- (Dahdouh-Guebas et al. 2000). Rhizophora,
ally the ones to collect ﬁre wood and crabs, Ceriops, and Bruiguiera grow long and tall and
while pole wood collection and ﬁnishing is a have different properties that make them valu-
predominantly male activity. Occasionally, some able for constructing different parts of houses
women collect crabs for sale within the village to or fences. While Bruguiera gymnorrhiza is well-
augment their income from ﬁshing, selling each suited to constructing roofs due to its strength,
palm-sized crab for about 200 ariary (equivalent Rhizophora mucronata is considered ideal for
to $0.11 at the time of research). building walls and especially for use as thicker
Cutting pole wood and producing sookay corner poles because it can withstand saline
is also a means of earning additional income for conditions better (Dahdouh-Guebas et al.
some people in the village, who sell them to 2000). Ceriops tagal yields thinner poles and is
people from other coastal villages north of the used for the interweaving structures of the walls
Baie des Assassins. Pole wood fetches varying or towards the construction of fences or small
prices depending upon the quality and size— sheds for poultry.4
pole wood for constructing houses is sold for According to Kairo et al. (2002), ﬁeld
anywhere from 600 to 1,200 ariary for poles 8 observations show that in a mixed stand of
cm in diameter and 3 to 4 m long. Similarly, a Rhizophora and Ceriops, there is a tendency for
sack of sookay (the size of a 50 kg rice bag) will natural regeneration to favor Ceriops, irrespective
be sold for 1,000 ariary ($0.53) to others with- of the harvested crops. This has implications for
in the village and to outsiders if it is in demand. the long term species composition of the
Tropical Resources Bulletin 11
Lamboara forests as well. If the forest structure to determine which species are under the most
shifts to a predominantly Ceriops forest, then a harvesting pressure. The next steps will be to
more socially desirable forest of Rhizophora may disseminate these results to the Lamboara resi-
give way to an inferior one from a local econom- dents in order to produce a community-initiat-
ic viewpoint (Kairo et al. 2002). ed management plan to govern the use of man-
groves in this region of Baie des Assassins.
The local communities surrounding Baie I would like to thank the community of Lamboara
des Assassins are highly dependent upon the and Andavadoaka in Madagascar for their support
mangroves for a variety of services and subsis- and hospitality, the staff and students at the Institut
tence needs. Small-scale, selective extraction of Halieutique et des Sciences Marines (IHSM) at the
individual mangrove trees may have little effect University of Toliara, the Wildlife Conservation
on the entire mangrove ecosystem, but it Society (WCS) staff at Toliara and Antananarivo,
removes individual trees. Rhizophora species do and the Blue Ventures staff in London and
not sprout again following cutting, while Andavadoaka. Special thanks to Hajaniaina
Avicennia do (Ellison and Farnsworth 1996). Rantsoavina and Joelson Rakotoson for their invalu-
Since the forest is composed of, at most, ﬁve able assistance and friendship during my ﬁeld work
species of trees supporting a variety of faunal at Lamboara and Ampasilava. WCS staff, Francisco
assemblages, drastic reduction in a single in Andavadoaka, Mr. Rijasoa in Toliara, Mr.
species can potentially reverberate throughout Bemahafaly Randriamanantsoa, Dr. Simon Harding,
the ecosystem. However, ecosystem functions and Dr. Helen Crowley deserve special thanks for
of each mangrove species is not yet quantiﬁed their technical guidance and logistic support. I
in ways that can inform management decisions would also like to thank my former colleagues at
(Ellison and Farnsworth 1996). Conservation International in Washington D.C. and
In the case of the Lamboara mangroves, Antananarivo for their insights and support. I am
human use of the forests is not leading to out- grateful for the advice I received from Dr. Oswald
right loss of forested area; however, a shift in Schmitz throughout this project, to Dr. Jonathan
species composition and forest structure may be Reuning-Scherer and Dr. Amity Doolittle for their
changing the forest qualitatively. While the eco- guidance, and to Monisha Gangopadhayay for her
logical impact of shifts in dominant species is edits to this paper. Thanks also to Tendro
difﬁcult to estimate from the data gathered and Ramaharitra (F&ES ‘05) for his guidance. This
from other studies (Kairo et al. 2002), the sub- research was made possible in part by funding from
sistence function of the forests may be changed the Tropical Resources Institute and the Yale School
or weakened more perceptibly. All four study of Forestry and Environmental Studies Summer
sites exhibit higher regeneration by Ceriops tagal Research Fund.
in comparison to Rhizophora and the other
species (See Figure 2). Ceriops also has the high- Endnotes
est relative density. Further multi-year data col- 1 Based on interviews with village residents.
lection on forest structure and human use can 2 Relative Density = (Number of individuals of a
yield conclusive information to determine if the species/Total number of inividuals) x 100.
forests surrounding the village are undergoing a 3 Relative Dominance = (Total basal area of a
species shift. species/Basal area of all species) x 100.
Ecological data should be augmented with 4 Based on interviews with village residents.
information collected during household surveys
12 Volume 25, Spring 2006
Mangrove Ecosystems of Southwest Madagascar
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Tropical Resources Bulletin 13