Document
Metadata
Title
Effets des Aires Marines Protégées sur la structure, la régulation et le potentiel de récupération des assemblages coralliens de Madagascar
Authors
. RANDRIANARIVO Mahery H
Research Topic
Abstract
Like most coral reefs around the world, those of Madagascar have been impacted by numerous
natural and anthropogenic disturbances, which have increased in frequency and intensity over the
past five decades. This decline has considerably reduced the goods and services provided by these
coral reefs to human populations. Many initiatives have sought to address limiting the impact of these
disturbances and increasing these ecosystems’ resilience. In this context, Marine Protected Areas
(MPAs) are one of the most widely used management and conservation tools. Studies on coral reefs
in Madagascar began in the 1960s, with the inventory of reef communities in the Toliara (Tulear)
region. Subsequent studies have focused on the spatial structure and the impact of disturbances such
as coral bleaching events. However, these studies typically concern a single site and use different
methodologies, thus limiting comparisons between the regions of Madagascar. Furthermore, recovery
and resilience capacities remain largely undocumented at the regional scale. While there are
demonstrated positive effects of MPAs on small-scale coastal fisheries, the effects on coral
communities have not been rigorously addressed. In this context, this PhD research program was
initiated to examine the effect of MPAs on the spatial structure, regulation processes, and recovery
potential of coral assemblages in Madagascar. The multi-criteria and multi-scale approach, with
numerous coral descriptors across site and regional scales, is one of the original features of this work.
A total of 18 sampling stations were distributed in three regions: Masoala in the northeast,
Nosy-Be in the northwest, and Salary Nord in the southwest. In each region, six stations were
established, three in fished areas and three in unfished areas (MPAs). Three randomly replicated belttransects of 10 m2 (10 1 m) were surveyed at each station between March and October 2020 to
determine taxonomic composition, generic richness, Shannon diversity index, and colony abundance
for juveniles and adults, distinguishing between the four main life history traits (competitive,
generalist, opportunistic, stress tolerant). The percent cover of scleractinian corals (and the hydrocoral
Millepora), macroalgae, turf algae, and crustose coralline algae (CCA) was estimated at each station
using three line intercept transects (LIT) of 10 m, placed in the middle of the belt-transects. Biomass
of herbivorous fishes was estimated at each station using underwater visual censuses on three belttransects of 250 m² (50 5 m). We also estimated the substrate rugosity with a visual assessment of
reef topography, graded from 0 (no vertical relief) to 5 (exceptionally complex with numerous caves
and overhangs). Cover of algae, herbivorous fish biomass, rugosity, fishing protection level, and
geographic location were recorded to estimate their potential influence on the spatial variation of
coral assemblages (“explanatory variables”). Data on coral diversity, abundance and cover, algal
cover and herbivorous fish biomass, coupled with those on thermal stress (temperature anomaliesover the last 40 years), allowed the calculation of a Recovery Index (RI) using the TOPSIS
(Technique for Order Preference by Similarity to an Ideal Solution) method.
The results showed a strong spatial heterogeneity of all coral descriptors, at site and/or
regional scales. At Masoala, high colony abundances were recorded, particularly of stress-tolerant
and competitive taxa such as Acropora and Pocillopora. Coral assemblages at Salary Nord were
characterized by lower abundances, especially of competitive taxa. At Nosy-Be, high diversity,
abundance and coral cover (with values between 42 and 70%) were recorded. Linear mixed effects
models showed that the spatial structure of coral assemblages was mainly influenced by herbivorous
fish biomass, rugosity, and CCA cover. The results also showed a positive effect of MPAs on all
descriptors except taxonomic composition. Concerning the regulation processes, we found that for
most of the dominant genera, the spatial variation of juvenile and adult densities was positively
correlated, potentially underscoring the "recruitment-limitation" regulation model, though other taxa
seem to be regulated by post-settlement events instead. Except for Porites, spatial variation in juvenile
density was not influenced by MPAs, whereas such an effect was found for adult colonies of Acropora,
Montipora, Seriatopora and Porites. Strong variation in RI was observed at site and regional scales,
with a higher potential for recovery at Masoala, probably related to less frequent temperature
anomalies in this region, compared to the west coast where such anomalies are more frequent and
anthropogenic pressure is higher. Stations with the highest densities of juvenile corals generally have
greater RI, suggesting the important role of recruitment processes in the recovery of coral
assemblages. A positive effect of MPAs on RI was recorded in Nosy-Be, which harbors the oldest
unfished areas of Madagascar (since 1966), and where control measures are more effective compared
to Masoala and Salary Nord.
Our results plead for a reinforcement of management and conservation measures, with actions
focused on the protection during recruitment processes and of the recovery capacities of the coral
assemblages. Important aspects include enforcement and the establishment of locally managed
marine areas (LMMAs), which have proven effective in managing fisheries resources and appear to
be a well-suited conservation action for coral assemblages in Madagascar. As the effectiveness of
LMMAs increases with time, protection measures must be sustained. This work represents a solid
contemporary baseline to assist researchers and managers in monitoring the evolution of coral
assemblages and making effective conservation decisions for Madagascar's coral reefs.
Volume
1
Date of publication
March 10, 2023
Institution (University)
Institut Halieutique et des Sciences Marine Toliara
Page numbers (pp)
157