|Tsunami Assessments in Seychelles PART TWO
Discussion - Assessment of Tsunami Impacts on the Marine Environment of the Seychelles
Major patterns of tsunami damage to reefs
Two major patterns in coral reef damage were noted, controlled by the geographic location of each island and exposure direction of each site, and reef substrate. The northern islands clustered around Praslin (including Curieuse, La Digue, Felicite and the rocks of Isle Coco and St. Pierre) showed very high levels of damage (approaching 100%) on carbonate reef substrates. By contrast, sites around Mahe showed much lower levels of impact. The limited damage on Mahe is due to the shelter provided by the outer northern islands, and energy dissipation of the tsunami traveling over the greater distance of shallow water from the outer edge of the banks to Mahe.
Figure 2. Benthic cover status at study sites on Mahÿ, Praslin and La Digue, separated by granitic or coralline substrate type.
Granitic reefs suffered less damage than reefs with a calcium carbonate substrate (fig. 2). Granitic surfaces were either immovable as they form the bedrock of the islands, or in the case of boulders and rocks, are too dense and of a compact shape to be displaced by the force of the tsunami. Even on carbonate rock surfaces that were consolidated and firm, attached corals showed little breakage and mechanical damage or overturning. However the majority of true coral reef sites in the granitic islands have a reef framework that is loosely consolidated due to mortality during the 1998 El Ni–o and subsequent bioerosion. This reef matrix was not robust enough to resist the tsunami waves, either from direct impact of the force of water, or movement of rubble and rocks. In these areas significant reef rubble was moved by the wave and consequently associated live coral colonies were also displaced and damaged.
Thus these preliminary assessments suggest that extensive damage was done by the tsunami to all coral reefs in northern and eastern island groups that have carbonate frameworks. It might be that this is also the case for outer atolls and islands, and surveys should be prioritized to the eastern islands closest to the tsunami origin. Little direct damage from the tsunami is expected for all coral reef habitats in the central, south and western parts of the granitic islands, and all outer islands sheltered by the Mascarene plateau and Seychelles bank (e.g. the Amirantes, Aldabra and others). In these areas, some damage is possible for shallow corals on carbonate substrates, but little damage is expected for all deeper habitats and all sites with granitic substrate.
Comparing these two patterns numerically, these surveys documented > 50% of substrate damage and >25% of direct damage to corals in northern and eastern-facing carbonate framework sites), <10% damage in shallow carbonate substrate sites in central, western and southern locations, and < 1% damage on all granitic substrate sites. Given the importance of coral reefs to the economy and social structure of the Seychelles (e.g. all the damaged northern sites are prime tourist locations for the country) this provides a strong threat to the country and requires action for mitigation.
Shoreline sensitivity and importance of reefs
The vulnerability of the low coastal plains to wave damage is clear from the tsunami. While fringing coral reefs protect these shorelines during regular conditions, their protection was limited during this extreme event. The GIS datasets used by the Seychelles government include a shoreline ranking variable that characterizes the shoreline by substrate type (Annex 1, fig. 3; granite rock, coarse sand, fine sand, etc.). This provides a useful starting point for developing a shoreline vulnerability index to wave threats such as the tsunami and the consequences of climate change – sea level rise, northward migration of the cyclone belt and increases in storm frequency and intensity. A similar index should be developed that incorporates the wave-protection properties of the coral reefs (which enable beaches to accrete) versus granite shores, fine-tuned by vulnerability shown by shoreline damage to this tsunami event.
Since coral reefs are a primary shoreline protection asset for the people and developments on the coastal plains, their role in this, as well as for general tourism, biodiversity and fisheries, should be recognized, quantified, publicized, and incorporated into coastal and marine management regimes. Of particular importance to Seychelles vulnerability to climate change will be the ability of reefs to continue growing upwards with rising sea level. In many cases (such as the fringing reef off Anse Royale, which is dominated by algae rather than coral) this will require protecting and enhancing the growth of reefs traditionally considered of low value for tourism and biodiversity.
Specific damage to Curieuse Marine Park
Along with the high vulnerability of coral reefs in the northern islands, the Curieuse Marine Park suffered damage to its infrastructure (see also main UNEP Mission report). The wall protecting the mangrove forest and artificial lagoon was damaged, which will expose the high-diversity mangrove stand to erosion during the southeast monsoon. Additionally, infrastructure of the MPA was damaged, including boat engines, electrical equipment and physical facilities on land. An assessment of the damage to and repair needs of the causeway wall at Curieuse is urgently needed. This must be done before the change in monsoons results in wave damage to the previously sheltered mangroves. Mitigation activities will have to start by April 2005, if they are to meet their objectives.
Conclusions and Recommendations:
The impacts of the tsunami damage to coral reefs in the Seychelles were severe on the northern carbonate-framework reefs, but minor elsewhere. These damages, occurring while reefs were still recovery from 80-90% mortality of corals during 1998, point to a critical vulnerability of the coral reefs of the Seychelles. At this time, the primary reef carbonate frameworks in the granitic islands are relatively weak geological structures, consisting of attached and loose calcium carbonate pieces of varying sizes. These may become strongly consolidated by coralline algae growth over 5-10 years under good conditions (e.g. observation from Baie Ternay). The chemical and biological consolidation into a rigid reef framework, such as that found on some fringing reef sites (e.g. Anse Royale) may take hundreds to thousands of years. The El Ni –o in 1998 created extensive rubble fields from death and breakage of the fast growing branching corals (Acropora and Pocillopora) that dominate the shallow waters of Seychelles reefs. The impacts of the tsunami, 6 years later appear to have been exacerbated in these areas. Loosely consolidated reef frameworks were not able to resist the force of the waves, and loose rubble and rocks were carried by the waves.
In the short to medium term, mitigation activities will have to deal with the problem of loose reef frameworks and the long time needed for reef matrix consolidation, in order to promote coral reef recovery and growth. In the medium to long term, damage from the tsunami should be considered in the context of Seychelles as a Small Island Developing State. As such, it has a particular vulnerability to shocks and threats due to its small size, from natural disasters to economic and global political influences. While damage from the tsunami was not catastrophic on a wide scale to coral reefs, it significantly worsened the catastrophic impact of coral bleaching 6 years previously, with impacts focused on the most vulnerable, and most valuable, coral reef areas. On these reefs, the tsunami set back biological recovery of corals by 6 years. Because of the extensive physical damage to the reef matrices, however, the set back to overall reef recovery may be much longer than that.
The interaction of these two types of threats in the medium to long term will be particularly important for the Seychelles – physical exposure to extreme waves events, and their increasing severity due to climate change – rising sea level, northwards migration of the cyclone belt in the southern Indian Ocean, and increasing severity and frequency of major storms. While the occurrence of another tsunami cannot be predicted, the increasing severity of the threat from waves to the Seychelles is clear.
Broader principles reflecting the importance of coral reefs to Seychelles should be developed (see box for examples) to guide the long-term protection of coral reefs and associated ecosystems. Specific recommendations center on three main areas with respect to coral reefs:
Mitigation of current damage – are there any options for enhancing recovery of damaged coral reefs from this tsunami event?
Development of a targeted coral reef and environmental monitoring programme to provide clear evidence for decision making with respect to coral reef recovery and protection, and enhancement of the contribution they make to the Seychelles economy.
Capacity building (technical training and provision of resources) to facilitate recommendation 2, particularly through improved resources for Marine Protected Area management and wider Coastal Zone Management.
Short term recommendations (3-6 months)
Mitigation of tsunami damage and enhancing coral reef recovery.
Rehabilitation and restoration technologies for coral reefs are in their infancy, and no current methods are feasible financially or logistically at spatial scales necessary for ecosystem-level processes. Nevertheless, a feasibility study developed through local research institutions such as the SCMRT-MPA should be promoted, to identify potential restoration trials. The key factors needed to be addressed are:
substrate stability and the fixing or removal of mobile rubble from heavily impacted carbonate reef surfaces (e.g. Isle Coco, St. Pierre, etc.).
water quality and ecosystem process improvements through coastal zone planning and freshwater management, and impacts from pollution and overfishing.
Enhancement of natural recruitment and survival of small corals.
Assess Curieuse Marine Park wall
Assess and repair as appropriate the damage to Curieuse Island causeway. If this is to be done it will be essential to do it before the next Southeast monsoon in April 2005, when wave damage to the mangrove communities will intensify.
Replace lost Infrastructure of Curieuse Island Marine Park
Restoration of Marine Parks infrastructure (mooring buoys, patrolling and monitoring equipment) in order to maintain the ecological and tourist infrastructure (Annex 5).
Medium term recommendations (6 months to 2 years)
Implement substrate stabilization pilot projects for damaged coral reef areas, SCMRT-MPA
Establish trials for medium-scale stabilization of coral reef rubble substrates, focusing on low cost per area, technical feasibility and practical application.
Development of coral reef and environmental monitoring capacity, SCMRT-MPA
As a part of the institutional structure of SCMRT-MPA, existing plans for environmental and resource monitoring need to be refined, implemented and expanded. In particular, we identify the following areas for capacity building in the medium term (Annex 6):
An annual sampling plan, covering all priority sites within and adjacent to MPAs, be identified, with the frequency of sampling targeted at specific needs and activities, such as coral reef monitoring, use by divers, seasonal changes in fish populations, tsunami recovery processes, etc..
Staffing needs for the monitoring plan be identified and built into individual-MPA and headquarters workplans, covering all aspects of preparation and planning, fieldwork, data entry and archiving and report writing.
Based on current and projected staffing levels at SCMRT, the technical and managerial responsibilities for implementing the monitoring programme be split as part-time responsibilities. In both cases, dedicated funds for the monitoring programme should be used to guarantee a portion of their time to this work.
Technical responsibility, including liaising with external scientists and maintaining quality within the programme should rest with the Senior Researcher at SCMRT.
Coordination and scheduling of the monitoring programme can be delegated to a more junior position, and include all aspects of fixing workplans and sampling times with MPA staff and monitoring partners.
Additional monitoring equipment will be needed, such as underwater digital cameras, land cameras with zoom, Kayaks, VHF radios, etc.
In addition to the above systematic development of the monitoring programme, we identified the following areas in which changes can be made, to enhance rapid assessment, surveillance and reporting of monitoring activities:
Access to many coral reefs and sea grasses around Mahe is difficult and expensive with current boat and diving protocols. SCMRT-MPA would benefit from using sea-kayaks to allow easy and quick access to nearshore environments.
Large scale monitoring techniques are currently not being used in the Se ychelles. The use of manta tow surveys (English et. al 1997) would enable the rangers to monitor reef and seagrasses rapidly and around many islands of the archipelago.
Underwater digital cameras are essential for rapid data acquisition and communicating such physical impacts to marine habitats locally and internationally. This is especially important to the Seychelles as it is a relatively isolated archipelago.
Land cameras equipped with strong zoom lens would also aid in documenting coastal damage and would also increase capacity for surveillance and enforcement in the Marine Park, a difficulty borne of the large areas to be monitored and the overstretched resources of the Marine Park Authority. Recommendations for 4 cameras with 35-400 mm zoom.
Enhanced monitoring capacity in other areas should also be developed:
an accurate assessment of the impacts of the tsunami on Hawksbill Turtle nests is needed and tagging program should be continued to monitor population size and growth.
Impacts to the local trap and other fisheries may occur and should be monitored.
Euro 60,000 annually
Strengthen the Seychelles Coral Reef Network
Monitoring of coral reefs in the Seychelles involves a broad range of institutions and stakeholders, and has been built up over many years of short-term projects and institutions. The Seychelles Coral Reef Network, currently chaired by SCMRT should be built up to ensure complementarity among monitoring programmes of participating institutions and projects. In particular, the follow gro ups, and current and recent programmes, should be consulted and their collaboration and information obtained to improve monitoring and management of coral reefs:
Seychelles Marine Ecosystem Management Project (SEYMEMP. Completed 2004)
Living Oceans Foundation (Annelise Hagan)
Aldabra Marine Programme (Cambridge University)
Cosmoledo/Aldabra group (Island Conservation Society/CORDIO)
Global Vision International (volunteer-based monitoring, Cap Ternay (2004-2008?)
Euro 20,000 (2 years)
Development of a shoreline vulnerability model and planning capacity
Patterns of tsunami impact clearly demonstrated differential vulnerability of shorelines to waves impacts, and the role played by healthy coral reefs. Development of a shoreline vulnerability index (similar to the shoreline ranking shown in detailed maps below), as part of a comprehensive national policy on coastline deve lopment and use will be necessary to minimize any future impacts from tsunamis, and from wave damage related to climate change. Additionally, detailed coastal bathymetry should be developed to enable predictions of wave-height and exposure to waves along the coastline. The policy should designate no build areas, define buffer zones around vulnerable beaches, coast lines, and reefs, and include management measures for maintaining and enhancing shoreline protection structures such as coral reefs.
Development of a long term strategy and recommendations for coral reef monitoring to support protection and management.
A separate exercise within the medium term period should be dedicated to setting a long term strategy. Provisional goals should be set ast the start of the medium term period, with progressive revisions to form a long term strategy ready for implementation by the end of the period.
Long term recommendations (3+ years)
Specific long term recommendations will be identified under recommendation 8 above, as implementation during the medium term period will lead to the defining of a longer term vision for coral reef management and protection as a national asset for the Seychelles.
We would like to acknowledge the assistance and guidance given to us in the implementation of this consultancy, particularly by Mr. Rolph Payet, Principal Secretary of the Ministry of Environment and Mrs. Mary Stravens (Seychelles Center for Marine Research and Technology-Marine Parks Authority, SCMRT-MPA). The staff of SCMRT-MPA supported all of our activities, and provided data they collected from previous surveys, and wed like to thank Nigel Horeau, Jude Bijoux, Rodney Quatre, Kate Pike, Allan Cedras, and other staff and rangers that assisted with logistics. Finally, this report was developed as a part of a broader UNEP mission under the Asian Tsunami Disaster Task Force, and wed like to acknowledge our other team members: Mark Collins (Team Leader), Elizabeth Khaka, Robbert Misdorf and Mindert de Vries.